Good Question! Q&A From FOA Readers

And Some Tech Articles Covering Frequently Asked Questions

Reprinted From The FOA Newsletter Section "Good Question!"

FOA Guide

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Updated 11/21

Topics (Click on them to jump to that section)






Test & Measurement   


Where Are The Jobs In Fiber Optics?  


Manufacturing Guide?
Is there a guide published by FOA that provides insight as to the process of fiber optic manufacturing? It's my understanding that the guide stresses quality and controls to ensure performance and reduce product loss?
A: We do have a guide for manufacturers. It is mostly aimed at communications systems and components manufacture. Here is a link to download it.

Q: What is the importance of reflectance and all the other numbers in installing and trouble shooting a fiber circuit?
A: Reflectance has always been a secondary issue to connection loss but has some important issues that need consideration. There are two basic issues with reflectance, affecting with the output of laser transmitters and creating background “noise” in a fiber link.
Reflectance can interact with the laser chip itself, causing laser transmitters nonlinearities or random fluctuations in the output. The background noise is a secondary issue, but can be seen in ghosts in an OTDR trace. The light bouncing back and forth in the fiber that causes ghosts will be added to the signal at the receiver end, adding noise to the actual signal. Both these effects are more significant on shorter links, for example FTTH or LANs using PONs (passive optical networks). We always recommend using APC (angled physical contact) connectors on short SM links. And most short SM networks do use APC connectors.
FOA tries to stick to the definition that reflectance is the light reflected from a connection but some others call it “return loss.” Return loss has been defined generally as the combination of reflectance and backscatter from the fiber, and that’s how OTDRs measure return loss. Standards vary in the definition sometimes.
Here is a FOA Guide page on reflectance that gives the basics and explains how it is tested.

Electromagnetic Interference
Q: Is there and electromagnetic interference with optic cables?
A: The fiber is glass and the cable is plastic, neither of which are affected by electromagnetic interference. There is a cable used in electrical transmission lines called OPGW- optical power ground wire - that has fiber inside a wire conducting high voltage - doesn’t bother the fiber at all.

How Light travels In An Optical Fiber
Is there a generalised ratio between the length of an optic fibre and the length of the path actually taken by a light pulse inside that fibre? If yes, do OTDRs factor in such differences in any way? or they such sown the length of the actual path of the light pulses?
Each optical fiber has an effective independent of refraction. The index of refraction is the ratio of the speed of light to the speed of light in the material: n=c/v where n=index of refraction, c=speed of light in a vacuum and v=speed of light in the fiber.
For an optical fiber, the manufacturer measures the index of refraction which is usually in the range of 1.47. Corning SMF-28 singlemdoe fiber for example is specified at 1.4670 @ 1310 nm and 1.4677 @ 1550 nm.
So if you use the equation above, the speed of light in SMF-28 fiber for a 1310nm pulse is c/n or 300,000 km/s divided by 1.4670 = 204,500 km/s.
When an OTDR measures length, it actually measures the time its test pulse takes to go to the end of the fiber and return, so the distance is 2X the actual fiber length. The distance is speed x time.
If a fiber is 1 km long and the speed is 204,500 km/s, the  time forlight to travel the 1km is 1/204500 = 0.00000489 seconds or about 5 milliseconds.
OTDR will measure that fiber as 10 ms becasue its pulse has to go both ways, and it would calculate the length as i km, using that effective index of refraction of 1.4670.
Back to your original question, the index of refraction is the generalized number based on how light travels in the fiber.


Managing And Maintaining a Fiber Optic Cable Plant During Its Lifetime.
Q: Are there guides / recommendations for optic fibre cable life cycle management? (outside plant) including rehabilitation / replacement timelines together with factors that may alter those timelines ( such as seismic activity, extreme weather, human activity-induced fibre cuts etc) also including typical performance deterioration over the life cycle, and the performance levels at which replacement / rehabilitation happens. Or does it happen (and is it normally expected) that operators replace entire sections of fibre (say 400 km) as part of routine maintenance?

A: There is a saying here in the US that in fiber optics “the most common cause of failure is “backhoe fade” in underground cables and  “target practice” for aerial cables.” In other words, damage caused by humans. We know of many fiber optic cable plants that have survived natural disasters like earthquakes - in fact there is a lot of work today using regular cables used in communications to monitor for seismic activity. Fire can be a problem in remote areas, but often it’s because the poles are burned causing the cables to fall.

Over the years we have questioned cable manufacturers about the lifetime of fiber optic cable. They don’t like to make definitive statements but we have been told that based on the cables installed in the past that 40 years is a probable lifetime for most cables. There are certainly cables in use today that are over 30 years old already. The glass fiber is not a problem, it’s the protection from the cables that will eventually fail. Installation techniques can have an effect on the longevity. For example splice closures should be sealed properly to prevent ingress of moisture or dirt. Cables should not be installed with bends below the rated bend radius or with excess tension.

FOA has always told users that fiber optic cables do not need maintenance (, a response to some people advocating periodic inspection and cleaning of connections, for example. That’s just more likely to cause damage.

When an accidental break in a cable occurs, we have guidelines for restoration (, and planning for restoration when building the cable plant is very important.

Someday you will certainly want to replace cables, often well before the lifetime of the cable, but generally because you need more fiber or the older fiber will not support the network speeds you want for upgrades. Planning for more fiber by installing more cables can be eased by installing spare underground ducts when first installing cables - here in the US, we call this “Dig Once” ( Testing fibers for higher speeds is called "fiber Characterization” ( and is routinely done when speeds above 10G or certainly 100G are considered for older fibers.

Knowing that the lifetime of fiber optic cable plants are ~40 years, it makes sense to plan ahead for future applications, installing lots of fibers, leaving lots of open duct space and choosing network architectures that will not obstruct upgrades. See the article on Netly's network above.

Transmitting Multiple Data Types
How do you integrate fiber optic digital communications with other sensing and control systems and platforms?
A: Fiber optic networks generally have lots of bandwidth and sensors and control systems generally do not require much bandwidth. The mixing of data streams is generally done by multiplexing the data using electronics on each end, but one can also do it with wavelength division multiplexing.

Single Fiber DWDM
Can you do bidirectional links on a single fiber with DWDM? (Dense Wavelength Division Multiplexing)?
A: A company Called Edge Optical Solutions sells multiplexers for bi-directional DWDM on one fiber  by using adjacent wavelength channels for each direction. It is good to ~400km with coherent transceivers but cannot use fiber amplifiers for repeaters.

Identifying Users On A PON Network
How or what testing tool or technique can I use to verify whether there is a live customer w/ONT working on any fiber i may select @ a splice enclosure prior to getting further down the cable and  to the MST service terminal. All our fibers have light on them leaving the CO so when we go into a splice enclosure to pick a fiber to connect a drop to, to service a home, they are usually all lit up in that enclosure.
A: The simple answer for a tool or technique that can tell you if a customer is connected on an output of a PON splitter is “documentation.” If you know where each fiber is connected going downstream. Then the IT person who programs users into the system can tell you if that fiber is connected to a customer. There is a possibility that there is a test solution. Have you ever heard of a “fiber identifier”? It’s a gadget that can tell if there is signal in a fiber and some can identify the direction it comes from. What I don’t know if the unit can somehow indicate bi-directional traffic. Nobody we contacted seems to know either.

Slow Internet After Conversion From DSL To FIber
Could you please help me understand why I am getting a slow connection (the same as when I was using anADSL box and sometimes even a bit slower) while having a fiber optic connection to my home (FTTH then RJ45 between wall & iMac)?
I am using an old iMac from early 2009 but the cable needed is a regular RJ45, so I do not see why it should not take the high speed connection...
As a matter of fact, it still takes several seconds (3 to 6 or even 12) for some pages to load...

There are several possible reasons your Internet is slow loading pages.
The fiber optic link to your residence may have little or nothing to do with the speed you see. If you use a speed test to check the speed of the connection, it’s probably going to show faster speeds, but it generally only tests the connection to your ISP - Internet Service Provider - not to the Internet or a remote data center.  The actual connection to the data center sending you the pages you request may be hundreds or thousands of km long and through many switches, so that could affect the speeds.
The major problem we see is the speed of the connection of your ISP to the Internet. If they have many subscribers, the “traffic jam” is at their connection. This is generally easy to see over the time of day. In the evening when many people are streaming TV or movies, it sends to get much slower, just like automobile traffic during rush hour. At times when fewer people are online, speeds will be faster.
We have exactly the same problem here in Santa Monica. Our Internet over a cable modem tests at 100-200 Mb/s but pages are slow loading because so many people are on the network at once.
However, I also suspect your 12 year old iMac. The typical web page is more complex than a decade ago and may contain hundreds of files including graphics that have to be downloaded and assembled for you to see the page.Newer computers are much faster and software is more efficient at handling large pages.

Campus Network Expansion
Any advice for expanding a campus fiber optic network?
A: Here are some thoughts:
  • Building to building can more easily be done with indoor/outdoor cable to get past the 50’ code limit for OSP cable.
  • Many campuses have ducts but they are often crowded. Microducts or fabric ducts are often the solution even if you have to pull out an older cable to pull in new microducts into one old duct.
  • Microducts and blowing microcables are gaining lots of traction for their practicality.
  • Microducts and microtrenching can be what we call “construction without disruption.”
  • We are seeing more and more directional boring - works fine as long as you know where other buried utilities are! Many contractors need to learn more about underground location.
  • People are finally getting the idea about singlemode fiber - now it’s probably cheaper than multimode.
  • Passive optical LANs can save money. Biggest advantage is the upgrade from GPON to 10GPON is seamless - you can even run both simultaneously, e.g. for a student system and a faculty/research system.
  • High fiber count cables are tempting, but require special handling and lots of manhole/handhole space.
  • We’re working with a Corning “Pioneer” (retired engineer) on trying to educate installers about bend radius violations, esp when pulling large fiber count cables.
  • Most installations shortchange manhole/handhole space.
  • Aerial is sometimes used. Lashing to a messenger is probably best. We worked with a school in Canada last year trying to use short lengths of ADSS and it was not cost effective nor was their much applications support for short ADSS links.
  • Line of sight wireless (RF or optical) works across highways and may be cheaper than construction for fiber.

Minimum Link Length
I have a question regarding minimum fiber optic distances for horizontal runs. Is there a minimum distance for a horizontal fiber optic run? Any information regarding this would be greatly appreciated. Thank you for you help!
A: The answer to your question is for the most part no, there is no minimum distance for a fiber optic link. For example, fiber is used in offices, data centers, etc. sometimes connecting equipment on a single rack. And there are many fiber optic links used on platforms - aircraft, helicopters, ships, etc. - and in command posts.
Most of the Ethernet standards are based on a 2m minimum, but also most are defined by a maximum length. For multimode systems, the max length is mainly a bandwidth issue, so shorter links are no problem.
For singlemode links, the bandwidth is not an issue, it’s the power budget, limited by the transmitter power and receiver sensitivity, translated into the loss of the cable plant. But for receivers, often they have not only a minimum input power limited by their baseline noise but also a maximum power they can have before saturating and causing high bit error rates. See “Power Budget” on this page in the FOA Guide. So if a singlemode link is short, the receiver can be overloaded so an attenuator is used at the receiver.
There is a secondary problem with singlemode systems, reflectance. Reflections from connections can cause problems with both transmitters and receivers, a topic covered in the link given above. The reflectance problem can be solved with APC connectors.
The FOA Guide has many pages on links, networks, reflectance, testing, etc. that you may find helpful.

What Is A Ring Network?
If according to the TIA or ISO structured cabling standards the fiber optic campus backbone must be star-hierarchical type, how should a fiber optic "ring" be built? to always ensure connectivity on a LAN?
A: A “ring” network consists of a series of links connecting equipment (nodes) in series until the last one connects back to the first. Since the links are communicate in both directions, the network can still operate if any one cabling link or equipment fails. Today, survivability is usually ensured by using a “mesh” network; the architecture of data centers, the Internet or phones. In addition to having a series connection of nodes, there are other interconnections that provide for multiple alternative paths. See Networks in the FOA Guide.

Crossed Connections
If a FO connector is crossed connected i.e Rx connected Rx and Tx to Tx at both end, will it works?
I know in theory it will not due to light circuits arrangement, but is there SFP in the market can tolerate that? 
A: We do not know how a SFP could sense and change polarity unless it had an optical switch inside the module. A transmitter is a laser or LED and a receiver has a photodetector. Unless one could have the devices change function, changing polarity would be impossible.

Can you guide me some websites or pages where I can learn more about GPON Technology please?
A: FOA Guide has a big section on FTTH and OLANs using GPON technology. Follow those links, Also FOA has a new book, The FOA FTTH Handbook you can order from Amazon.

Cheating On Link Length
I have a fiber run for a camera starts at location A to location B it is 467 feet.  Location B jumpers through to location C which is 2060 ft at location C. Transceivers areSFPs ONLY GOOD UP TO 1800 ft, but this company only has a multimode system. Is there something i can do to make this work?
A: It might work as is, since electronics are usually quoted with conservative specs and will work farther than specified most of the time. If you have several SFPs, test the output power to see if it exceeds specs and choose the 2 ones with highest power. If that still doesn’t work, contact SFP manufacturers for higher power units.

Errors In A Data Link
: What is the significance of bit error, and what is the acceptable rate for communications and submersible vehicles?
A: On any data link, there is an acceptable amount of error that can be tolerated. If it’s a digital voice link, a BER 10E-6 (1 error in 1million bits) is acceptable without affecting voice quality. If it’s a link to your bank, the typical standard is a million times higher (10E-12). Link protocols usually have ways to determine BER, like attaching a checksum to the end of a data packet and having it checked at the receiving end. If a error is suspected, the packet will be discarded and retransmitted. Here is a tutorial on BER and an explanation of errors in a a fiber optic link.

What is normal Range for good power in an FTTH fiber?
A: The GPON specification for downstream power from the OLT is OLT transmitter power should be 0 to +6dBm and link attenuation in the range of 13 to 28dB, which says receiver power the ONT must be a maximum of 13 dB less than +6dBm or -7dBm and a minimum of 28 dB less than 0dBm or -28dBm, so -7 to -28dBm at the receiver.
Upstream, the similar calculation is ONT transmitter -4 to +2dBm  and the receive power at  theOLT is -11 to -32dBm.
See for the full specifications for GPON.

Reflective Events Causing Transmission Problems
I have a technical question about reflective events. I recently assisted to troubleshoot an intermittent SM fiber link for a customer. The cable was dug up a few years ago and a fiber contractor has (fusion spiced) a different chunk of cable into the link to repair it. When troubleshooting the link, I checked the cable with the otdr. I found that each of the 12 fibers had a reflective event at the fusion splice. This was only at the splice tube closest to me. The other fusion splices in the other tube were virtually invisible (as they should be). I'm a little puzzled as to why there are reflections at the fusion splices. I did a little research, but couldn't come up with a good answer as to what is a possible cause of the reflections. (The OTDR also showed a lot of ghosting on every fiber tested) (in some cases it recorded over 40 ghost events) Although I haven't been able to confirm that there is high Bit error rate due to the transceiver not providing these statistics, (except for 3 out of 10 pings fail) I am suspecting that High reflectance is possibly the cause of their unreliable fiber link.
A: Reflectance is a big problem in SM links, especially short links. If you are seeing lots of ghosts, I suspect the link is very short. Fusion splices can have reflectance if the splicer is improperly set and the fusion is incomplete or has bubbles. Those splices should have not only have reflectance but higher loss. The solution is to open up the closure, use a VFL to find the reflective events and redo the splices.

POTS over Fiber
I would like to know if there is information on your website that explains "POTS OVER FIBER"?
A: POTS - the acronym for “plain old telephone service” - is digitized to transmit over fiber. In the early days (late 70s and 80s) it was simply T-carrier with a fiber converter. By the end of hte 80s it was ATM and SONET. More recently, it’s all going to carrier Ethernet since 99%+ of the traffic is data not voice or PONs (passive optical networks) for fiber to the home.

Can I Build A GPON Network With "Taps"?
Q: Can I build a GPON network where I do a drop to one subscriber then continue to the next subscriber for another drop and so on?
A: There have been examples of this type of “tap” drop proposed, for example in rural areas for drops to  widespread subscribers on a longer network than is typical for FTTH. It’s just a version of a cascaded splitter network. with taps that just do a 2 way split. The taps used are typically 90/10 taps, where 10% of the power is tapped off for the drop.
There are some important issues to consider - Since you are dropping 10% of the power at each tap, you are limited by how many drops you can have.   If you calculate the loss budget - after the first tap, you have 90% power left less the excess loss of the splitter (~0.3-0.5dB). The tap power is down ~10.3 dB and the through power is down ~0.6 dB. At the next tap, you  use the same formula plus you add the loss of the fiber to that tap and so on until you reach the GPON limit.  It’s a pretty complicated process to design, but you can see that with these power losses you will not get a large number of drops in a GPON network with 28dB max power budget. We did a rough calculation and 20-24 drops may be possible depending on the fiber lengths.
This network will probably be much more expensive and more distance limited than simply running a cable with many fibers and dropping fibers from that cable with midspan entry. Couplers are expensive, fiber is cheap. We also do not know the issues with the large differences in transmission times between the first connections and the last ones, which depends on the length of fiber in the systems. That may require some programming at the OLT.

Do We Need Repeaters For 30 Mile Link?
Q: I need to design a 30 mile (~50km) link. Will regeneration like a fiber amplifier be necessary?
A: It depends on the comms equipment but I doubt you need regeneration. 30 miles is 50km, only 10dB of loss for the fiber at 1550nm, maybe 10 splices at <0.1dB adds only 1dB loss and another dB for connectors on each end. I think you probably can find equipment that runs on 12dB loss budget. That said, most new high speed systems (>10G) have 20km versions then go to expensive long haul coherent systems. So talk to the communications equipment manufacturers and see what they say. If you do need a EDFA, they are not that expensive but the site is expensive and requires power (+ backup). See if it’s possible to put the EDFA in the end facilities to get enough power for the whole run.

Connecting WiFi Access Points In a Passive Optical LAN
Q: If we install GPON passive fiber optical LAN in a new hotel, would one need to run fiber to every AP? Since every hotel room needs an AP this gets expensive. Any suggestion on the simplest and less expensive  way of connecting Fiber Cable to an AP in the hotel room?
A: You do not need a fiber to every wireless AP in a GPON passive optical LAN (POL). The AP needs a UTP (Cat 5e/6) cable with Gigabit Ethernet and  POE (Power over Ethernet) capability. The POL fiber  should terminate in a multiport switch that has a fiber input and then 4 or more UTP/POE outputs for the wirelses APs. That’s the cost saving architecture of a GPON POL. See this page in the FOA Guide:

You should also check out the APOLAN website ( for more information on hospitality applications with POLs.

Wireless Infrastructure
Q: I am wondering how the landscape will change as the nation moves from 4G-LTE to 5G. Will it use the same network as currently, or will the network need to be updated or replaced? To what extent will 5G be dependent on wireless vs fibre optic? Will the infrastructure nationally move more toward an underground wired one, rather than a Radio Access Network?
A: The wireless network is totally dependent on fiber optics for it’s communications backbone. The “wireless” part is the connection from an antenna to the mobile device. From that point, the network is cabled, mostly fiber already and soon to be all fiber.
4G/LTE and soon 5G in urban areas is moving to “small cells” with about 10X as many cell sites covering much smaller areas. Every small cell site needs a couple of fibers. Metro backbones will require very much larger fiber counts, especially with C-RAN (centralized radio access network) architectures now being implemented.
An example is Santa Monic, CA where we live. It has about 200K citizens, 8.9 square miles(about 23 sq km), but has planned for 600 small cell sites, spread over multiple service providers.

At FOA we see wireless as one of the most active areas for fiber, along with data centers.See

Fiber Ports Or Media Converters?
Should I Buy A Switch With Fiber Ports Or Use Media Converters?
A: I’m assuming you are thinking of using a switch with copper Ethernet ports and a media converter instead of a switch with fiber ports. The downside is that it adds complexity and increases the chance of failure. My analogy is something my primary flight instructor told me many years ago - multiengine planes are not safer because having two engines doubles your chance of having an engine failure. IBM still says that most network problems are cabling problems. Using media converters adds more electronics, more power supples and more cabling connections.

Passive OLANs in Hotels And Resorts
Are passive OLANs a good choice for hotels or resorts?
A: Passive Optical LANs are enterprise networks based on fiber to the home (FTTH) technology not Ethernet over structured cabling. The FTTH network is usually using GPON standard equipment over one singlemode fiber with passive optical splitters that provides basic Level 1 and 2 network functionality. This is not Ethernet but carries Ethernet over the GPON protocols at 2.5G downstream and 1.25G upstream.
Passive OLANs offer several advantages over conventional Ethernet switches and structured cabling, including much less cost  (~50% capital expense and ~20% operating expense), much lower space requirements (see the link to the library photos below and note the two racks of equipment that support 4000 drops), longer distance requirements (to 20km), easy expansion (these are systems designed for hundreds of thousands of users) and easy management (when you have hundreds of thousands of users, that’s important.)
For hotels, convention centers and similar facilities, the ease of upgrading to a passive OLAN is a big advantage - one fiber goes from the computer room to a splitter where it can serve 32 switches of 4 ports each. That’s right, one fiber can support 128 users! It can support anything that a network can - wireless access points, security cameras, secure entry systems, VoIP phones or POTS phones - anything that will run over a conventional network.

Duplex Communications Over One Fiber
Is true duplex over a single fiber possible, or is more like a shared time-domain technique in a quasi-duplex mode? I would guess that true duplex would lead to interference problems.
A. Bidirectional links are widely used - that’s how FTTH PONs work. They use splitters to combine/split the signals and one wavelength downstream and another upstream. See Fiber Optic Datalinks and for FTTH FTTH Architectures.


Fiber Optic Color Codes Reference Chart
Q: Has anyone made a fiber optic pocket reference chart that has cable color orders, frequencies, or other commonly used info on it?
A: The FOA has a page on its Online Guide that covers color codes ( It is the most popular page in the FOA Guide! It works great with a smartphone.

Where In The US Do Contractors Need Licenses For Fiber Optics?

We often get asked where in the US do contractors doing fiber optic installations need licenses. We found a good website for that information, the NECA -NEIS website. You might remember NECA-EIS, as they are the partner with the FOA in the NECA/FOA 301 Fiber Optic Installation Standard. NECA is the National Electrical Contractors Association and NEIS stands for National Electrical Installation Standards. They have a very easy to use map and table that gives you data on every state in the US, so mark these pages for future reference.

NECA/NEIS (See “State Regulations”) (all electrical licensing)
Low Voltage:

Good Technical Website For Installers
American Polywater ( has one of the best technical website for cable installers. Check out their website, especially “Videos,” “Engineer’s Corner” and  “Calculators.”

Underground Utilities Location
Q: From an OSP engineer: Is there a resource for underground utilities that we could use on our engineering designs? I know some counties offer this info but is there a single resource for all?
A: If you are in the US, the Common Ground Alliance (  , is a resource for designers and contractors looking for information on underground utilities. Their “CGA Best Practices” ( is the best reference for damage prevention.
Otherwise, the local authorities and utilities are the best source. The department that issues permits is usually the place to start.
Even with that information, it is recommended that the contractor do their own search using underground locating equipment before digging.
You may find this page in the FOA Guide on underground cable construction useful. ( )

Fiber Optic Safety Poster
We've had numerous requests to reprint our guidelines on safety when working with fiber optics, so we have created a "Safety Poster" for you to print and post in your classroom, worksite, etc. We suggest giving a copy to every student and installer.

Splicing Pigtails On A Cable
I seem to be having an issue finding fiber protection sleeves that can slide over the 3mm patch cable.  I bought a sleeve that said it with made for “single fiber fusion” but the thru hole which I would side the cable thru prior to fusion is too small for the patch cable.  When I try and look on-line for specifications for the thru-hole size, prior to fusion final melting of the glue in the sleeve, all I find are post-melting diameters, none which are even close to being able to handle the 3mm patch cable.
A: Splicing pigtails involves splicing the fibers only and the cables are secured separately. The usual method of splicing on pigtails is to splice the fibers and use the heat shrink tube to seal the splice and the fibers from the outside air and protect it from stress. The splice is placed in a splice tray. On either side, there is 2-3 feet of fiber exposed from the cables being spliced. The splice tray has clamps for all of the cables being spliced on the edges of the tray and the fiber to the splice is coiled neatly on the splice tray. The jacket of the pigtail is clamped at the edge of the splice tray but  ends there, so only fiber is coiled in the tray. If you try to coil fiber, the bulk of the cable can get to be a problem where it’s coiled with the bare fiber. You can get heat shrink protectors for fibers of 250 to 900 micron diameter buffers, but not for jacketed cables.

Gel Leaking From cables
Q: We have several instances where gel from inside the fiber optic cable has leaked into the splice closure. I have seen some information about sealing the ends of cables so that this doesn’t happen but cannot find a specific method or procedure for this or what to use for a sealant. Is this something that is common practice for outside plant cables? The gel creates a mess and definitely makes reentry for additional splicing more difficult. If there is a way to prevent or minimize this I would like our technicians to start implementing it.
A: When you install the cable, after inserting the cable in the splice closure and/or the budder tubes in the splice trays, seal the end with silicone RTV adhesive. It needs some time to cure but that should prevent the gel leakage. Or next time, order dry water blocked cable which will not have this problem.

Interpreting Customer Spec For Cable Plant Loss
Q:  I am currently being challenged by my customer on some testing parameters and  was wondering if I can receive  your input regarding the EIA/TIA-598B standard.
A: The customer spec: l. Rated attenuation: 0.35dB/km and 0.25dB/km at 1310nm and 1550nm, respectively
As we read the customer spec, this section “L” below refers to the cabled fiber attenuation coefficient of the cable as supplied to the contractor, not the installed fiber after splicing and termination. Thses are typical specifications for today’s high quality fiber. It is also slightly lower than specifications noted in most standards today, e.g. 0.4 dB/km at 1310 nm is more common. See in the table at the bottom of the page, noting OS!/G.652 fiber specs.
After installation, splicing and termination, the total loss of a fiber link includes the losses from splicing and termination, plus any passive devices like PON splitters that may be installed in the link. When calculating the loss budget ( to compare to actual test test results, splices may be calculated at 0.15-0.3 dB each and connectors at 0.3-0.75 dB each, adding to the end to end loss and affecting the calculation of dB/km.
For example, 10km fiber with 4 splices and two connections (the ends) would have a total loss of:
Fiber: 10 km @  0.35 dB/km = 3.5 dB
Splices: 4 @ 0.15 dB each = 0.6 dB
Connections: 2 @ 0.3 dB each = 0.6 dB
Total link loss = 4.7 dB
If you calculate dB/km for the installed link, it becomes 0.47 dB/km, even though we used the lowest loss specs for splices and connections.
As stated above, the spec you called out refers to the cabled fiber only and should be verified by the test documentation supplied by the manufacturer on the reel.
The loss of the installed link should be calculated in the loss budget for the link as designed using the given fiber spec plus reasonable specs for splice and connection loss.
Testing should be done with a light source and power meter (OLTS) and a second test with an OTDR per OFSPT-7 or FOA Standard FOA-1 (OLTS) and FOA Standard FOA-4 (OTDR).

Underground Utilities Location
Q: From an OSP engineer: Is there a resource for underground utilities that we could use on our engineering designs? I know some counties offer this info but is there a single resource for all?
A: If you are in the US, the Common Ground Alliance (  , is a resource for designers and contractors looking for information on underground utilities. Their “CGA Best Practices” ( is the best reference for damage prevention.
Otherwise, the local authorities and utilities are the best source. The department that issues permits is usually the place to start.
Even with that information, it is recommended that the contractor do their own search using underground locating equipment before digging.
You may find this page in the FOA Guide on underground cable construction useful. ( )

Digging Safely (Read the FOA Tech Topic)

Markers Required For Underground Fiber Optic Cables?

Q: Are signs required for underground cables like fiber optic cables? Are they required to have signage so people don’t dig them up or damage them?
A: In the US the answer is NO. There is no Federal or State law which requires marking anything other than hazardous liquids and gases. It is purely a business decision or a moral decision to invest in signs/markers to protect buried fiber. If a fiber gets cut it can disrupt 911 service and all kinds of vital communication related to hospitals, air traffic control, etc.

Distances Between Manholes
Q: W
hat is the standard or max distance between manholes and handholes for fiber optic cable?
There are no hard rules, but the distances are determined by a number of factors. In populated areas, the manholes or handholes would be situated where you need drops line in front of a building or a splitter pint for FTTH or conversion from underground to aerial or underwater cables. From a viewpoint of how far you can go, it’s determined by: 1. The length of cable on the reel (typically ~5km max, maybe further for smaller cables, shorter for higher fiber count cables.  2. The type of the duct, cable and method of installation for underground. That includes the type of duct, lubricant used, the number of corners passed, pulling equipment (pulled or blown)  and the tupe of cable - most limited to 600 pound tension. Cable manufacturers and American Polywater (lubricants) are good sources of information here. 3. Aerial cable can have quite long spans, esp. using the moving reel method, which can be limited by the length on the spool.

Gloves for Fiber Techs
I was wondering if as part of the safety rules, in addition to glasses, if it is recommended to use gloves.
If that the case, would you recommend a specific type of gloves.

FOA emphasizes the need for safety glasses because of the problem with fiber scraps flying around, especially when students in class are learning to strip fibers. Proper safety glasses have side shields that provide more protection than regular eyeglasses. For eyeglass wearers, prescription safety glasses are available at very reasonable costs that are much more comfortable to wear than wearing safety glasses over the user’s prescription eyeglasses.
We only recommend gloves when working with cables that have sharp metallic armor in them or some heavy outside plant cable. The metallic armor can cause serious cuts if one slips when splitting or removing it. The gloves to use are the kevlar gloves used to prevent cuts (they are also used for chefs working with sharp knives.)
Once the cable is opened and you are dealing with buffer tubes or bare fibers, gloves like the ones used for cables can make the work difficult because gloved hands are clumsy. Tight surgical rubber gloves might work for some, but still make working with bare fiber difficult and provide limited protection.  There we recommend bare hands and being very cautious. 

Height Of Aerial Fiber Optic Cables
Is there a code standard for how high from the ground a for a fiber optic cable running through a residential yard? if yes, please provide the standard or point me to the standard.
If we go by NEC 2020, the height is 8 feet,above roofs. with this qualifier. No driveways  just over grass. Art /section 770.44 B. Also 800.44 A 4 states 12 inches between electric service and Fiber optic cable. But service has to be 12 feet at house so I would say 11 feet above grass. If driveway is there, Residential 15 feet for service, electrical, so fiber at 14 feet.

Seal End Of Cable
For aerial OSP cable, are there any problems with leaving the end of the cable open or should it always be put into a closure of some kind?
The open end of the cable allows moisture to get into the cable and can be a problem.
I see several scenarios here. If the cable is installed and waiting for splicing, it could be a matter of time. If the work is to be done soon - a week or two - leaving it open is OK, but if the time is longer or you prefer being careful, just seal the end of the cable by wrapping it with plastic electrical tape. The end will be opened up for splicing;  about 2m of cable needs to be stripped to splice it, so a few days exposure is OK, but long term we’d recommend a simple tape seal, the way manufacturers do when shipping cable on a reel.

Installing Cable
Below are specs for an installation. We’ve never installed a Fiber Optic run this long. Please see below questions and info.
-Fiber Optic cable to be used is a 24 strand Single Mode application
-Length of run is 7200 m long
-Appears that all the Fiber is on one reel. However do you recommend having some junction points on pedestals along the way for testing-maintenance purposes or just one continuous run if possible?

FOA has lots of information to help answer your questions:
Re underground installation. See and in the FOA Guide.
There are other questions you need to ask:
Are there no intermediate connections or drops required? It’s just one straight fiber run? You should be able to install it continuously.
What is the installation type? Pulled in conduit or direct burial?
If pulled in conduit and you can pull in one try, that’s best. You should use a pulling capstan to limit tension, attached to the cable with a breakaway swivel pulling eye and use lubrication. Use the American Polywater guides ( for choosing lubricant and decide if you need an intermediate pull.
Direct burial is simple for a long run, just ensure you have the proper equipment.

October 2020's Newsletter article about the installation of a 6912 fiber cable in small conduit prompted a number of this month's questions on social media. And there were more too.
Installation of a 6912 fiber cable

Q: For this post, "Tight Fit: 6912 Fiber Cable Pulled in 1.25 inch Conduit”, he asks if they can see one end completely terminated?
A: It takes about 2 full racks of patch panels or one rack of splice trays. Sumitomo shows the splicing rack here Most systems using these cables will buy fully populated patch panel racks with a splice rack for the cable to splice to 6912 fibers terminated in the rack.
Q: And a second question:: How long does it take to terminate? And over how many panels?
A: A very experienced tech can splice one of these cables in ~75-100 hours using ribbon splicing.
Q: I assume that's smaller fiber like 80 micron cladding
A: All the fibers in the high fiber count cables are made with regular singlemode fiber - 9/125micron. TO make the cables smaller, the buffer coating diameter is reduced to ~200microns to make the fibers smaller.
Q: How was it prepared with the splice tray and ODF? It might require a dedicated panel and splice tray.
A: It takes about 2 full racks of patch panels or one rack of splice trays. Most systems using these cables will buy fully populated patch panel racks with a splice rack for the cable to splice to 6912 fibers terminated in the rack.
Q: Is this an actual photo or was the cable installed in a different type conduit.
A: We were told that is the actual size of the cable and conduit although not of the actual installation discussed.
Q: What is the minimum bend radius of that cable? What procedures did they use to maintain that bend radius through those 90 degree curves?
A: The minimum bend radius is 15X the cable diameter for that cable (diameter 1.14” or 29mm), about  17” or 435mm. The conduit bends had to be controlled to be larger than that radius. See the Fiber U MiniCourse Fiber Optic Cable Bend Radius

Preparing Cable For Splicing
Is there any standard on the preparation length of strip jacket upto the splice tray. Ideally its better to have a loop of buffer before getting into the tray if ever the closure has enough space for slack.. its also nice to put some hose to the buffer to add on protection. So far, i don't see any standard and can't support the remarks on what to follow. The practice was to take note on macrobend and have enough length of fiber to reach the machine.
There is a lot of variation in the size, shape and design of splice closures, so the length varies according to the closure and trays. For loose tube cable, the length of buffer tube from the entrance to the splice tray and the length of fiber needed in the tray are given in the directions for that splice tray. Similarly for ribbon cable, but the variations in ribbon cable designs often requires special handling and sleeving for the ribbons. Most manufacturers have specs available online.

Fiber Splicing Cost
What is the standard of costing for fiber splicing and terminations? Is it per core / per splice or per each cable end irrespective of the number of cores?
That is a very hard question to answer, other than to say ”it depends. ” The number of fibers is definitely a factor because each fiber must be stripped, cleaned, cleaved and spliced then placed in the splice tray.
It also depends on:

  • Single fiber or ribbon splicing?
  • Type of splice closure
  • Type of cable (loose tube, ribbon, flexible ribbon, high density, armored, ADSS, etc.)
  • Installation: aerial or underground
  • Location: urban or rural
  • Set up time (same for low fiber count cable as high fiber count cable)
Most contracts will be considering the number of fibers but also these factors, and probably they want to price by the number of fibers, but the price per splice will vary accordingly. We've seen quotes in the US for prices varying over a 10X range.

Lashing Aerial Cable With Cable Ties?
I am considering an electrical job installing fiber optic aerially on a messenger cable.
I have seen the cable tie method of lashing the fiber to the messenger. Would you recommend this method considering the cost of a lashing machine for a single project and if so what would be a good distance between ties for the proper support of the fiber to the cable.
A: The normal way to attach an aerial cable to a messenger is lashing the cable with stainless steel wire. If you use cable ties, you would need ensure the cable doesn’t droop and the cable ties are designed for outdoor use in the sun over a long time (stainless steel ones are available). How long is the span? If it’s more than 100 feet, I think I would go with lashing. If you don’t have a lasher, you can rent one. You will need a bucket truck anyway.

Restoration Time
Q: Do you have any statistical data on how long (on average) it takes for a utility network operator to detect and pinpoint the exact location of a fiber cut?
A: We don’t have any information on the average time it would take to find a fiber fault and like all averages, it might not have a lot of meaning.
Many fiber optic links today have alarms that indicate loss of transmission so they tell you immediately when the link goes down. Identify the link and the fiber connection.  Then it becomes a matter of troubleshooting and eliminating causes. Sometimes even when reconnected the equipment requires a system reset to get started.
First - check the power on the equipment
Secondly: determine if someone was doing something in the equipment area that might have caused a problem. We do know of a link that was brought down because an executive giving a tour disconnected a live link to show someone a fiber connector! If someone was working nearby, check that area first - patchcords, cables, etc. Don’t forget to check work records to see if a crew is working around the cable plant at that time. It’s possible a crew installing new cables damaged old ones. See the FOA newsletter for this month for what installers do on aerial cable plant or last month for underground
If it appears to be in the cable plant and nobody is working near it, OTDRs are generally used for troubleshooting. They get you into the area where the problem is and them it’s finding it manually. Underground it’s often contractors digging or boring, overhead it’s just poor workmanship - or as a guy from Bonneville Power put it, it may be “target practice” although animals damage aerial cables too.
FOA has a page on restoration and some of our instructors do seminars on it.
Time? If there are trained techs available, finding the problem can take less than an hour. If not, it can take a lot longer. Repair can be hours or days if the proper techs and equipment are not available.
If nonstop service is required, alternative fiber routing is the solution - build a mesh network.
Under any circumstances, having a restoration plan and repair materials should be ready. If tech personnel are not available, a contractor on call is needed.

Cable Installation Guidelines
I am trying to find information on the recommendations regarding fiber underground in conduit. I am looking for industry specific verbiage on the cumulative turn degrees before you need a handhole or manhole. I believe it is 180 degree cumulative but I can’t find it anywhere.
A: We’ve heard the 180 degree limit mentioned on some conduit but not for fiber optics. For any fiber optic cable pulling, the relevant issues are pulling tension and bend radius.
We know of no specific standards or guidelines on conduit bends for fiber optics. It has many factors, including conduit size and type - there are many types, length of the pull, radius of the bends, type of fiber optic cable and lubricants used, if any. For the cable, there are thousands of fiber optic cable designs that vary in diameter from ~3mm to ~30mm depending on the type of cable and number of fibers, the stiffness of the cable and the location and type of stiffer/strength members and the method of installation - pulling or blowing/jetting. And for locations as far North as you are, temperature can be an issue as cable gets stiffer when colder!
For any given installation, corners are generally accommodated by handholes/manholes and pulling done from handhole to handhole with figure-8ed cable pulling techniques to prevent cable damage by excessive tension or bending.
FOA has a section of our Guide on OSP construction: Outside Plant Fiber Optic Cable Plant Construction and in that is a section on OSP installation. For specific cables or conduit runs, we’d suggest talking to the application engineers at cable manufacturers who can give specific advice.

Construction Near Underground Fiber
What is the recommended distance for any new building construction to build near underground fiber duct channel?
A: We do not know of any standards or codes related to construction near fiber or other underground utilities. Common sense dictates that one stay far enough away to prevent accidental damage, so adding 5-6 meters(15-20ft) from the areas of construction makes sense.

Pulling Cable
: I’m having trouble finding much information on the matter. What type of swivel should be used to pull fiber and what would be the correct way to pull armored fiber.
A: Start  on the FOA Guide here and go here for types of swivel pulling eyes. with and here are sources It’s not common to “pull” armored cable since it’s designed for direct burial, but a kellums grip on the jacket will generally work.

Communications Cables on Utiity Poles
Is there a standard that service providers such as ISP, FTTH or cable TV should follow when installing their cables on existing electric poles. For necessary clearances etc. ?
A: The location of comms cables is in the “Communications Space.” At the top of the pole is the “Supply Space” for power conductors and between the two is a “Safety Zone Space.” It is There are guidelines of various types, mostly referring to NESC Rule 235. One of the best documents on this is from Nashville Electrical Service.  This presentation from Finley Engineering offers a good summary.

"Snowshoes" On Aerial Cable
For overhead installation, can snow shoes, or other service loop devices, hold two separate cables? 
Snowshoes are sized for different cable sizes and types. Some snowshoes are big enough for several cables, that’s no problems.

Fiber and Power Sharing Conduit
We are working on a project that has miles of underground 7 cell innerduct conduit with existing fiber already running through one of the cells. Is it possible to run electrical conductors through the 2 of the other cells? The conductors would be no larger than 1/0 AWG at 480V or 600V. Both the fiber and the electrical are being installed for the same use.
A:We questioned several people in the electrical side that also do fiber work. The opinion is that the electrical may use the other ducts. If the fiber cable has conductive members, e.g. armor, it must be properly grounded. And any cables spliced in manholes need separation and marking. The concern is over what happens with a dig up, but as long as the electrical is turned off before restoration begins, there should be no problem.


Fan-Out Kits Needed?

Q:I’m working on some MM fiber and am unsure if I need a fan out kit. Is there a way I can tell if I do?

A: Is it 250 micron or 900 micron buffer? Splicing or termination? Loose tube or tight buffer cable? Generally loose tube cable with 250 micron fiber needs no fan out kit for splicing - tubes go to splice tray and bare fibers are protected in the tray - but probably needs it to terminate if the fibers are exposed, for eample with SOCs - splice on connectors. Tight buffer - 900 micron fiber - does not need fan out kits.

Pull ADSS Cable In Ducts Underground?
Q: Our city is installing a 1.5 mile run, mostly aerial and we want to use ADSS cable. There are two or three road crossings where we want to go underground in conduit installed by directional boring. Can the ADSS cable be dead-ended, brought to the ground, figure-8ed and pulled through conduit then continue the aerial installation?
A: The answer is yes this is not an issue and is done all the time. It is standard procedure. (Thanks to Pat Dobbins, FOA, the expert on ADSS cables.)

Q: I recently read an article you wrote in April of last year about micro trenching..Currently, I am employed with an underground construction company. Something we have never been involved with is micro trenching and would like to possibly get some equipment and training scheduled in the near future. In saying that, it has seemed to be almost impossible to find numbers on the price per foot. Essentially, I am asking if you have any resources to some up with those numbers or models to maybe use for pricing purposes.
A: Microtrenching is becoming another tool that contractors are adopting because like directional boring is is less disruptive than regular underground construction. I’m working with one group that’s using microtrenching in CA cities, installing microducts and a 288 fiber   about the size of a #2 pencil. Cost is difficult to generalize other than “more than aerial and less than trenching.” Cost is very dependent on  where you are working and what the local geography looks like. We know one contractor who claimed to do 5 miles a day in rural Washington at costs near that of aerial. It’s especially good in areas with lots of base stone where trenching or boring is near impossible or cluttered utilities downtown.
Here are a couple of pages on the FOA website about microtrenching:
Outside Plant Fiber Optic Cable Plant Construction
Underground Cable Construction
Ditch Witch sells equipment for trenching and trains users. Condux has the equipment for blown cable and offers training several times a year.

Blasting Near Fiber Optic Cables
We have a project where blasting is planned near fiber optic cables. We find no standards for this. Is it safe for the fiber or should we treat it like other utilities like gas and water?
A: FOA recommends considering fiber optic cables to be similar to gas lines when blasting nearby. We know of no standards for this but there are some descriptions of projects requiring blasting near fiber optic cable installations. Here is a pipeline company's guidelines for blasting. The guidelines seems to focus on staying 5m from the fiber optic cable and using careful blasting techniques.

Markers For Underground Fiber Optic Cables
I have a general question about above ground markers for fiber optic cable in conduit.  Is there a recommended spacing for the markers?  Is there a standard to reference for this?
A: We asked some people who make them and they said the guideline is “line of sight.”  The rules for markers are mainly what information needs to be on them. Of course we also recommend adding marker tape about a foot above the conduit. I was curious if there were any legal issues and I found this interesting page from Cornell Law School:
So I might  add to line of sight any crossings of roadways, rail ways and some markers for bridge crossings.
We have a new section on the FOA Guide: Outside Plant Fiber Optic Cable Plant Construction that may be useful.

Terminate All Fibers Or Just Some?
We are currently running fiber which will be 12/24, my question is do we need to terminate every pair even if we aren't going to be using them or is there an alternative?
No you do not need to terminate all of them and leaving some bare fibers is often done when there is no planned use for the fibers or to save money. However, there are some other issues to consider. You do need some spare fibers ready to use, either in case of problems or for upgrades. For small fiber counts, the cost of terminating them all at once will be cheaper than having to come back to the site and doing it in the future. The economics are quite different if you have 144 or more fibers, of course. If you leave bare fibers, be sure to leave enough length to terminate or splice later - about a meter for termination and 2m for splicing. And protect them from damage so they can be used in the future.

Removing Old Fiber
I have several 1000 feet of old 62.5/125 armored fiber optic trunks under a raised floor that I am replacing/upgrading to 50u MM and SM trunks. Is there any guidance on ‘Best Practices’ to follow when cutting these trunks into more manageable lengths for removal?
A: Use a jaws-type cutter to cut the cable into reasonable lengths and remove it. There should be no danger in cutting the cable up as long as your workers only cut the right cable. Raised floors often have large numbers of cables - often including power cables - so its important to ensure the proper cables are being cut an removed.

Re-routing Old Fiber Optic Cables
; I have a questions about the re-routing of fiber optic lines that have been in place for a number of years.  Is it a standard transaction in the fiber optic business to have to re-route fiber that has been in service for a long period of time.  (e.g. >20 years)  If so, is there a best practice for removal from conduit for re-rerouting?
A; There is no way we would recommend removing and reinstalling 20-year old fiber cable. First of all, old cable may be damaged in removal. Then cable and fiber technology has improved over the years so you can get much better components today at greatly lower prices. (One industry analyst I know likes to say that fiber is cheaper than kite string and fishing line!) Today’s cable designs allow for much smaller cables with many more fibers (288 fibers in 9.7mm - just over 3/8”) and new conduit designs allow for more cables in a conduit (microducts and cloth ducts) and easier installation - blowing in cables and microtrenching are perfect for metro areas.
More fibers, especially in a big city, is a must. Smart cities, small cells, FTTH (fiber to the home), ITS (intelligent traffic systems), V2X (vehicle to vehicle, infrastructure, etc.) and many other services need lots of fibers.
Our recommendation is to pull it out and dump it. Install new ducts and the fiber you need (x10 maybe?) and have new ducts for future use. Are you familiar with “Dig Once”?

Why A Figure 8 Cables?
What is the reason for wrapping the cable in a figure eight?
When you need to do an intermediate pull, you have to pull the fiber and coil it on the ground. A simple coil will put a twist in the cable. Figure-8 coils put in twists of opposite directions on each side of the 8 making for no overall twist in the cable. See How To "Figure 8" Cable For Intermediate Pulls in the FOA Online Guide.

Maintenance of Fiber Networks
Can you guide me how to prepare Optical Fiber Cable Annual Maintenance Proposal?
A: Basically, the network needs to be installed properly, fully tested and everything carefully documented. Then no routine maintenance is required. Most problems with fiber optic networks occurs when techs are working with it, e.g. damaging cables or getting connectors dirty when testing, so leaving it alone is the best plan.
Electronic transmission equipment can be tested anytime to ensure proper data transmission, but that does not involve accessing the fiber.
We have several things which may be of help:
You Tube Video: FOA Lecture 39 Maintaining Fiber Optic Networks
Web page: Maintenance

Getting Old Cables Out Of Conduit
How do you get old cables out of a conduit when they are stuck?
Usually we are concerned about reducing friction when pulling cables through conduit, but sometimes you need to get them out. Here is a page from American Polywater the leading lubricant company with advice on the subject.

Designations For Fibers
I'm currently working on a project involving optical fiber with VMS signs, CCTV cameras and other ITS equipment. I was wondering if there is a standard or a good practice which describe the typical assignation for each optical fiber on a 6 fibers cable for example?  By that, I mean :
- fibers number 1 and 2 : VMS;
- fibers number 3 and 4 : CCTV;
- fibers number 5 and 6 : spares
There are several ways people assign polarity on duplex links. The way you suggest is the most common I believe. Use the odd number fibers to transmit in one direction, even number fibers to transmit in the other direction and document the color codes.

"Mining" Cables In Data Centers (Cable Removal)
Q: Is there any documentation out there on best practices for data center cable mining? Any help would be greatly appreciated.
A: This has been a topic in premises cabling since first proposed by NFPA  for the NEC twenty years ago. FOA has gotten this kind of question before for many different circumstances, but I must admit that I know of no recommended procedures or standards for the removal of abandoned cable.

Cable “mining” often refers to the removal of underfloor cable, may apply to cables in trays and is hard to get information online - search for “Cable mining” and you get links to sellers of cables designed to be installed in mines (coal, salt, minerals, etc.) underground!

The biggest problem with removing unused cables is identifying the cables to remove. Underlloor cables are particularly bad, it seems, since you have generations of abandoned cables, often poorly marked, sometimes mixed with power cables. With metallic cables, you can sometimes use an ancient telephone tool, a “toner” to trace cables, With fiber you just have to be careful that you identify the cable before you cut.
It’s always better to remove small sections, especially if you can tug on the cable and verify it each time before cutting. We’ve seen photos of an early data center with cable trays 2 feet deep by 4 feet wide full of about 4,000 fiber optic cables. After seeing that you understand why the new high fiber count cables (1728, 34456 and 6912 fibers) are desirable!

Why We Warn You To Be Careful About Fiber Shards

Fiber in Finger

Photo courtesy  Brian Brandstetter,  Mississauga Training Consultants


Fiber Optic Color Codes Reference Chart
Q: Has anyone made a fiber optic pocket reference chart that has cable color orders, frequencies, or other commonly used info on it?
A: The FOA has a page on its Online Guide that covers color codes ( It is the most popular page in the FOA Guide! It includes a print your own pocket color code chart and one for your smartphone or tablet - works great with a smartphone.

Multimode In Premises Cabling
Q: I wonder when/if single mode fiber will start invading the enterprise. There's a whole ecosystem, of course, in addition to physical fiber cabling.  Switches, server connections, protocols, etc. But I'm wondering if you see the industry moving towards some set of standards using single mode?
A: Today, singlemode transceivers are as cheap as multimode for 10G and cheaper at higher speeds. Indoor cell systems (DAS) use singlemode. FTTH PONs (passive optical networks using singlemode) are being used for LANs because they are cheaper too. Both technology and costs point to the advantages of SM. Multimode is the historical design and it's hard to change. But structured cabling standards (TIA-568, ISO 11801)  include singlemode and POLs (passive optical LANs.)

Storing Fiber Optic Cable On Reels
is there a "standard" for how to store a fiber optic cable reel?
A: This is another detail that has not in my knowledge ever been included in a standard. However manufacturers usually put a note on the reel to keep it upright - standing on the edges of the spool sides, not flat on one side of the spool.  If the fiber is to be stored for a period of time, it should be stored in a cool dry place and the ends sealed with electrical tape.
(Photo storing cable on reel)

Midspan Drop Cables
I am working on a project that has 5 sections, consisting of 5 miles each section, CCTV, detectors, DMS connected by 192 count fiber.  We were directed to use the consultants plans from the first section as a guide for uniformity for the remaining contracts.  The attached fiber detail shows a 4 fiber drop cable going to the ITS device.  I was thinking to take all 12 fibers to the device and back for redundancy?  Also, if we did use the 4 fiber drop cable, I didn’t understand why they would splice the other 10 thru cables and instead leave them intact? Is there a preferred method for a drop cable to a device or just preferences?
A: We are not sure why they do it the way they do. Perhaps the designer was not familiar with midspan access which would preclude having to make the other splices. Using a 12 fiber drop cable would be more expensive and perhaps unnecessary unless the device being connected is in a location where a small cell site might be located. They may also have uses for those other fibers that require a connection through the drop point.  We”d suggest to the designer that midspan access might allow saving the 10 splices at each drop.

Fiber Flexibility and Longevity
Q: When I think of glass, I think of a material that is not very flexible. If you try to bend most glass, it will break. So it is rather remarkable  that you can bend a fiber and not crack it, even though the strands are quite thin. Perhaps it's not a good idea to bend fiber too sharply? I was talking to the people who maintain the fiber network at the university here. They tell me they have a problem when fiber gets to be about 15 years old, it will start to become brittle. If you flex it, it will crack or break. Is this a common problem? How long can fiber be expected to last before it becomes brittle? Is fiber that is manufactured more recently have a longer life-span?
A: Fiber is quite flexible. One demo I did when I taught classes was to walk up to a large window and push on it, telling everyone to watch the reflections to see how the glass flexed. I can flex quite a lot. Like most materials, as it gets smaller, it can bend more easily because the stress is less across the cross sectional area. Consider a bar of steel 1” in diameter compared to a piano wire or banjo string. Most things break because either they are overstressed or there is some impurity in the material that focuses the stress and the crack propagates from there. Ever cut glass? You scratch it and stress it along the scratch line and its maps off - called cleaving - where the stress concentrates along the induced fault. Fiber is extremely low in impurities - a matter of how it’s made from raw materials, not melted sand like most glass - that’s part of what makes them have such low loss (efficient transmission) and high strength. A glass fiber is much stronger than steel of the same size. Corning explains it here.
 The brittleness of older fiber is due to the migration of moisture up the cable into the glass. The H2O becomes the OH radical which interacts with the glass to reduce its strength. Most cable companies say their cable today protects the fiber well enough that it should last 40 years, but where fiber is terminated or spliced and exposed to the air, it can get brittle and be hard to handle in 10 years or more. Older fiber had shorter lifetimes simply because we learned to make fiber coatings and cables better at sealing fiber from the ambient atmosphere.

Fiber Choice for LANs
Many manufacturers or suppliers worldwide emphasize the use of OM4 multimode optical fiber for the LAN. Does single-mode fiber not provide greater bandwidth than multimode? Do they imply that single mode optical fiber should only be used for long distance applications and not in LAN environments?
A: Multimode fiber is acceptable for LANs up to 10 gigabits/second and up to 550 meters depending on the type of fiber and Ethernet version. See this page for a complete list of network specifications.Higher versions of multimode fiber OM2-OM3-OM4 have higher bandwidth capability. OM5 is a version of MO4 that also supports wavelength division multiplexing with VCSEL sources in the extended wavelength 850-950nm range. OM1 is a earlier fiber with a different core size that has not been designed into new systems for almost 20 years. LANs can use singlemode fiber for all versions. Singlemode has longer distance capability (up to 40km) and virtually infinite bandwidth. See the singlemode specification in the link above. Singlemode is also used in passive optical LANs that can be much cheaper to build than conventional networks. See this page  for information on optical LANs (OLANs) including passive OLANs based on FTTH GPON technology.

Fiber Lifetime
I am often ask how long the fiber we are deploying today will last or be useable , I typically say something like it will last at least 20 years and that no one really knows how long it can be used.   What is the oldest fiber optic network or longish segment that is still in production that you know of?
A: Current cables are probably good for 40 years or so. Today there is some fiber being used by telcos from the late 1980s and lots form the late 1990s and early 2000s. Lots of OPGW (optical power ground wire) is in use up to 30 years old. Some of this old fiber is being used at 10Gb//s. But remember that fiber from 20 or 30 years ago may have limitations on bandwidth, since both chromatic and polarization mode dispersion has been reduced in newer fibers for higher speed networks. And spectral attenuation of older fibers may be higher and have the water peak at 1383nm that can affect wavelength-division multiplexing systems. But the weak point may not be the cable or fiber, but the splice and termination points where bare fibers may be exposed to the elements. It's not uncommon to find these fibers have become brittle and are hard to work with. What we always tell people is if it’s working, leave it alone. If you want to upgrade to higher bit rate systems, use fiber characterization to determine if the fibers are capable of use at higher speeds.

Cable Bend Radius
Q: We are working on project where we need to know difference between short term and long term bend radius for fiber optic cable?
A: The bend radius for cables is generally specified under two conditions - under stress, e. g. when being pulled, it is a radius 20 times the cable diameter. Relaxed, after installation, it is a radius 10 times the cable diameter. The relaxed specification, 10X, is considered a long term specification. Some of the new high fiber count cables have different specifications, sometimes 15X or 20X under either condition. Check with the manufacturer for their specific cable. See this article on bend radius in the FOA Guide.

Old Multimode Fiber
We have old multimode fiber and we are still connecting more equipment to it over greater distances and need some way to insure the equipment will work.
A: I do not know of any simple formula for figuring this out. Last time I remember such a formula from around 2000 done for Gigabit Ethernet with VCSELs and the equation reminded me of the quantum mechanics course I took in physics. The problem is you have two bandwidth factors, modal dispersion and chromatic dispersion. Modal dispersion is highly dependent on mode fill, e.g. the metric “encircled flux” was developed to define the mode fill of multimode fiber with VCSEL sources for simulations to estimate bandwidth. Chromatic dispersion is dependent on the fiber spec and the spectral width of the source which is better with lasers and LEDs.
What generally happens is the standards group developing the network standard, especially IEEE 802.3 for Ethernet, runs the numbers and specifies a maximum distance for the particular network and its speed. FOA has a table of these specs here: Specifications for fiber optic links and systems, including FTTx

105 Micron Fiber?

Q: I have a customer asking about 105um fiber. Does it exist? What is it basically used for? After FOA suggested a clarifying question to the customer: The fiber is for Power over Fiber(PoF). The construction is 105um fiber with 125um cladding. The question or assumption would be – The termination would be the same as 50,62.5 or 8.3um with a 125um cladding?
A: The people who do laser surgery and power over fiber use special step index fibers and SMA connectors. The power density can be very high so the heat can build up in the cable. SMA connectors or the metal ferrule swaged-on connectors are often used for their all-metal construction with the ability to withstand heat and sometimes the need to be drilled for special fiber diameters. Because of the high power, the polish needs to be low reflectance, so we’d recommend using a wet polish and end with a very fine polishing film - 0.3 microns or so. Like polishing SM for DWDM.
Cleanliness is very important for these applications. I remember a call from a doctor doing laser surgery who kept ruining cables because they were dirty and the high power literally exploded the dirt and pitted the ends. When that happens, sometimes they can be polished out but often they are ruined. The same thing happened to the 120 inch telescope at Lick Observatory when Joe Wampler tried using it to laser range to the retroreflector Apollo 11 left on the moon. Exploding dust pitted the aluminization on the mirror.

Replacing OM1 MM Fiber
Q: We are an automation system integrator in South Africa. We have a client that has multimode 62.5/125 fibre optic plant wide. None of the runs between components are longer than 2km. We intend to upgrade the technology from a proprietary communication protocol to a standard ethernet protocol at 100 MHZ. The fibre to copper convertors we will be using are using 1300nm light source and have a Fibre Optic Link Budget of 12.8dB for 62.5/125 um and 9.8dB for 50/125 um. The client has been advised to replace the multimode 62.5/125 with multimode 50/125 cabling and we need to know if this is really a requirement.
A: Do you know how old the fiber is? It should be what we called FDDI grade 62.5/125 fiber with a loss of ~1dB/km and a bandwidth of 500MHz-km at 1300nm. A 2km link should have a loss of 2dB for the fiber and ~0.5dB/connection - well under the power budget of the link. 100Mb/s Ethernet variants were designed for 2km or more on this fiber. There is no reason to upgrade at this time, 50/125 fiber would not be needed until Gigabit Ethernet was desired.

Can A Fiber Optic Cable Catch "Fire"
Q: While working on a cut-over of a dwdm circuit something has happened that I am now looking for an explanation. A transmission fiber emitted "fire" on that same fiber! We did not see any optical light we saw fire. About 5 cm of fiber burned and remained smoke. What phenomenon occurred? Are the dwdm amplifiers so powerful enough to generate fire? Unfortunately I could not get into the station to take the model of the equipment.
A: We’ve heard of high power WDM systems exploding dirt of the endface of fiber connectors and damaging them, but this is a new one. We contacted several technical people in fiber companies and found that this can happen if there was a crack in the fiber in the cable near the connector or lots of reflection perhaps caused by a very dirty connector that allowed the very high power to heat the cable enough for combustion. DWDM with many multiplexed signals and a fiber amplifier creates a lot of power confined to a very small core of the singlemode fiber. That power can ignite the acrylate coating on the fiber.

What's The Lifetime Of Fiber?
Q: "The utility I work for has some FO cable installed, some 20 years+ and I am wondering what is industry standard for the useful life of a cable? This is from an asset management point of view. I realize that FO cables can and do last for decades, especially if the work on them is minimized (ie. splicing for repair or relocation), but what would you consider a good book value for useful life?"
A: Cable manufacturers have generally made fiber optic cable for a lifetime of 20+ years and in the last decade or so we’ve been told that 40 years is reasonable for a cable. But that means the cable will retain its specifications for that time frame. Networks, however, do not stand still. In the last 20 years, network speeds have increased by up to 100 times. In the same time period, the fiber has been engineered to accommodate longer and faster links. 20+ year old fiber was installed when speeds were around 1Gb/s, where dispersion was not an issue, nor was dense wavelength division multiplexing (DWDM) being widely deployed. So if you are using fiber at lower speeds, the current cables are probably fine. You might have trouble splicing older fibers in closures because the exposed fibers do tend to get brittle.
If you want to upgrade to faster speeds or DWDM, the older fibers will need testing - we call if fiber characterization - and here is a page in our FOA Guide about it: Fiber Characterization and Testing long haul networks (CD, PMD, Spectral Attenuation)

Differences In Fibers
What is the difference between OM3 and OM4 type fibers and G.654/G.655? They seem to be rated for the same  amount of GBs (10-400) and the only difference seems to be the multi-mode nature of OM3/4 vs. the single mode nature of G.654/655. Can they both be used in long haul communications if laser optimized?
A: OM3 and OM4 fibers are both 50/125 micron fiber but have different bandwidth capability. OM3 is rated at 1500MHz-km while OM4 is rated 3500 MHz-km. OM4 is an evolution of OM3 where design and manufacture allow more bandwidth. More bandwidth translates into slightly longer link lengths in faster networks, ~1-10Gb/s. For example, Ethernet at 10Gb/s will go 300m on OM# and 450m on OM4, which can be important if it is being chosen for a enterprise network backbone.
The differences in G.654 and G.655 are more complicated. G.654 is singlemode fiber optimized for use at 1550nm for long distance use. G.655 is “non-zero dispersion shifted” fiber tweaked for dense wavelength division multiplexing (DWDM) to prevent secondary problems with high power and closely spaced wavelengths of DWDM and fiber amplification. It’s the kind of fiber used in long submarine cables.

Arsenic Coated Cable?
  I was told a contractor installed arsenic coated fiber optic cable because they didn’t want animals to chew through it. Is this true?
A: Some cable has chemicals put in the jacket to make it taste bad to rodents. We have not found any one who claims to use arsenic, in fact, we could find no references to what kinds of chemicals are used.

Why Do Cables "Go Bad"?
It’s been my observation over ~15 years of building and managing fiber channel storage area networks that from time to time cables will fall out of transmission spec.  In terms of communicating with non-storage people, they in essence, “go bad”.  Other than possible damage due to physical disruption of a cable, or contamination at the connectors usually caused by a human being unplugging/replacing, has it been your observation that MM cables can “go bad”?

A: There are some possible causes of problems over time. We know of connectors that fail for several reasons.

  • The biggest cause is with prepolished/splice connectors with mechanical splices. the assumed problem is the index matching get goes bad, but that’s highly unlikely. It’s usually the crimp fails and the fiber pulls out, especially if it has any stress on the fiber.
  • Adhesive connectors can have a bond between the connector and fiber fail, more likely on anaerobic connectors.
  • Any stress on the fiber at the connector is bad. Patchcords should not be left hanging on racks but dressed into horizontal racks below each patch panel.
  • Residual stress in cables can be a problem - tension or tight bends - and they may get worse over time.
  • Moisture is always a worry. It takes years to show up, but indoor cables are not protected from moisture like OSP cables.
  • Of course, transceivers fail too - electronics are generally very reliable but do deteriorate over time and cause failures.
We always say fiber requires no maintenance - set it up right and lock it up. As you pointed out human intervention is often the issue.

Armored Indoor Cable?
Can I get an indoor armored 8 core fiber optic cable?
A: Most cable manufacturers make indoor armored cable using corrugated wrap armor to protect cables from crushing loads from other cables especially in under floor installations.

More Than “Single” Mode?
We're now using SM fibre so it looks like we don't need mandrels in the Ref Lead at the Light Source.  The info I have is that we need to make a couple of air coils 35mm to 50mm in diameter.  Why? 
A: When you launch from a pigtial laser source through a connector into a reference cable, you do have several modes being propogated. It usually takes 100m or so for the second or third order modes to attenuate. So the coil causes them to be attenuated by the stress enough to no longer be significant - it’s a mode filter just like MM. If you do not do this, you will measure higher loss in the fiber and at connections near the source. Since most SM has traditionally been long distance, the effect was small or ignorable, but with short links, it can be significant.
Followup Q: But how do we explain multiple modes in Single Mode fibre?
A: When you get the core of the fiber down to ~5-6 times the wavelength of the light, it no longer acts like geometric optics (like MM fiber). Some of the light can travel outside the core (see the note on “waveguide dispersion”here At launch, significant amounts of power are at higher angles creating short lived modes that are highly attenuated.

Replacing OM1 MM Fiber
We are an automation system integrator in South Africa. We have a client that has multimode 62.5/125 fibre optic plant wide. None of the runs between components are longer than 2km. We intend to upgrade the technology from a proprietary communication protocol to a standard ethernet protocol at 100 MHZ. The fibre to copper convertors we will be using are using 1300nm light source and have a Fibre Optic Link Budget of 12.8dB for 62.5/125 um and 9.8dB for 50/125 um. The client has been advised to replace the multimode 62.5/125 with multimode 50/125 cabling and we need to know if this is really a requirement.
A: Do you know how old the fiber is? It should be what we called FDDI grade 62.5/125 fiber with a loss of ~1dB/km and a bandwidth of  500MHz-km at 1300nm. A 2km link should have a loss of 2dB for the fiber and ~0.5dB/connection - well under the power budget of the link. 100Mb/s Ethernet variants were designed for 2km or more on this fiber. There is no reason to upgrade at this time, 50/125 fiber would not be needed until Gigabit Ethernet was desired.

Old Fiber
We are looking at a company’s fiber network which has been laid at various points in time over numerous years.  In this process, we are trying to identify the changes that were made to either/both the glass fiber and the cladding.  Are there different generations of what was industry standard in creating the fiber?  For example, are you able to identify the difference in a fiber that was laid in 1980 versus one laid today?  Was the cladding the same size/thickness etc. in 1980 as it is today or has this been modified/improved upon over the years?  In all, we are trying to find what modifications have been over the years and how this may improve the life of the network and its capabilities. 
A: This is a common problem today. Many network operators are evaluating their fiber networks for upgrades, hampered by the fact that few are properly documented. Below is a timeline that should answer your questions. What many network owners are doing now is testing their cable plants - a process called Fiber Characterization.  There are contractors who do this service.

Fiber Tech Timeline
1976 - First field trials, US and UK, using multimode fiber at 850nm
1980 - First long distance networks still using multimode fiber at 850nm, planning to upgrade with wavelength-division multiplexing at 1310nm
1984 - singlemode fiber becomes feasible, telecom drops multimode fiber, all future installations are singlemode - this first SM fiber with a 9 micron core and 125 micron cladding is still available today but with better specifications. Really early fiber may not have good environmental protection and degrades over time. Early speeds were 145-405Mb/s, up to 810Mb/s by the end of the decade.
1990 - around this time, modern fiber begins - better performance and environmental protection. Fibers for wavelength-division multiplexing in the 1500nm range appear allowing multiple signals on a single fiber and fiber amplifiers allow long spans.
1995-2000 - massive build-out of fiber backbone leads to glut of fiber - WSJ ~2001 says 93% of all fiber is dark. Speeds grew from 1.2-10Gb/s over the 90s decade
2000-date - massive Internet growth and mobile device growth eats up glut of fiber and demands many times more. Dense wavelength-division multiplexing becomes the norm. Speeds began at 1.2/2.5Gb/s, upped to 10, 40 and are now at 100Gb/s.
So most fiber installed after 1990 has the possibility of being used at 10Gb/s, after 2000, it’s probably OK for 40Gb, and since 2010, you are probably OK for 100G and maybe more.  To verify performance, you test each fiber for connector condition, loss, spectral attenuation, chromatic dispersion and polarization mode dispersion. There are test sets that will do
Fiber Characterization in basically one step.

How to Clean POF (plastic optical fiber)
Q: I heard that plastic fibres such as PMMA can suffer damage from cleaning from an alcohol solution. Are there alternate cleaning solutions available for these types of fibres."
A: You can use a 10/90 mix of  isopropyl alcohol/water. Typically use with a lint free swab. (from out POF consultants)

Older Fiber?
I have some 62.5 mm and sm inside fiber plant over 20 years old.  When is a good time to upgrade?
A: When you need to or have to. If it's working OK, there is no need to upgrade!

Do You Strip The Cladding For Termination Or Splicing? NO! They've ALL Got It All Wrong

We recently got this email from a student with field experience taking a fiber optic class:""The instructors are telling us that we are stripping the cladding from the core when prepping to cleave MM and SM fiber.  I learned from Lenny Lightwave years ago, this is not correct. I do not want to embarrass them, but I don't want my fellow techs to look foolish when we graduate from this course."

I'll share with you our answer to this student in a moment, but first it seems important to understand where this misinformation comes from. We did an image search on the Internet for drawings of optical fiber. Here is what we found:

bad fiber drawings

EVERY fiber drawing we found on the Internet search with one exception (which we will show in a second) showed the same thing - the core of the fiber separate sticking out of the cladding and the cladding sticking out of the primary buffer coating. Those drawings are not all from websites that you might expect some technical inaccuracies, several were from fiber or other fiber optic component manufacturers and one was from a company specializing in highly technical fiber research equipment.

The only drawing we found that does not show the core separate from the cladding was, no surprise, on the FOA Guide page on optical fiber.

correct fiber drawing

No wonder everyone is confused. Practically every drawing shows the core and cladding being separate elements in an optical fiber.

So how did FOA help this student explain the facts to his instructors? We thought about talking about how fiber is manufactured by drawing fiber from a solid glass preform with the same index profile as the final fiber. But we figured a simpler way to explain the fiber core and cladding is one solid piece of glass was to look at a completed connector or a fusion splice.

We started with a video microscope view of the end of a connector being inspected for cleaning.

fiber view - core/clad

Here you can see the fiber in the ceramic ferrule. The hole of the connector is ~125 microns diameter (usually a micron or two bigger to allow the fiber to fit in the ferrule with some adhesive easily.) The illuminated core shows how the cladding traps light in the core but carries little or no light itself. This does not look like the cladding was stripped, does it?

Here is the same view with a singlemode fiber at higher magnification.

Fiber view - SM

And no connector ferrules have 50, 62.5 or 9 micron holes so that just the core would fit in the ferrule, do they?

What about stripping fiber for fusion splicing. Here is the view of fiber in an EasySplicer ready to splice.

Fusion splice - core/clad

What do you see in the EasySplicer screen? Isn't that the core in the middle and the cladding around it? In fact, isn't this a "cladding alignment" splicer?

We rest our case. If that's not sufficient to convince everyone that you do not strip the cladding when preparing fiber for termination or splicing, we're not sure what is.

Special Request: To everyone in the fiber optic industry who has a website with a drawing on it that shows the core of optical fiber separate from the cladding, can you please change the drawing or at the very least add a few words to tell readers that in glass optical fiber the core and cladding are all part of one strand of glass and when you strip fiber, you strip the primary buffer coating down to the 125 micron OD of the cladding?


MM Splice-on Connector On Singlemode Cable
I encountered a situation where a MM mechanical connector was used on a SM fiber and passed on an OTDR test. The client and I are interested in understanding how these connectors could have passed?
The joint between a multimode and singlemode fiber should have vrey high loss, ~17-20 dB, depending on the mode fill of the MM fiber. However the short length of the MM fiber, ~10mm, might not be enough to cause the modes to fill in the short fiber in the connector, resulting in relatively low loss.
Eric Pearson, one of the most knowledgeable people on connectors expressed this idea then tested it with 100m singlemode connected to a second singlemode cable. The second singlemode cable has an OM3 LC unicam connector, An EXFO ftb-400 OTDR indicates a 2.09 dB drop. That is way too much to pass a test but nowhere near the loss that could be expected from the MM/SM joint. See the OTDR trace below.

MM connector on SM fiber

Connector Mating Adapter Loss
When looking in data sheets on duplex adapters, it’s telling me that it has a Insertion Loss of about 0,2dB. Is that common? Only thought it was the connector that had a loss.
A: Technically, a single connector or mating adapter does not have any loss. It’s not “connector loss” but “connection loss” defined as the insertion loss when two connectors are mated, and for most connectors that requires a 3rd component, a mating adapter to align the ferrules. (Some connectors like MPOs have their own alignment mechanism so the mating adapter merely holds the two connectors - one with pins and one with holes - together.)
The connector manufacturer’s specification for “Connector loss” is the loss of their connector mating to a reference connector with a mating adapter. Connectors are graded in ISO/IEC standards, but not TIA, and the best connectors are ~0.2dB loss when mated to another of the same grade.
If a mating adapter manufacturer is quoting loss, one assumes they mean their adapter with two of the best connectors will have a connection loss of 0.2dB.
Mating adapters for 2.5mm ferrule connectors - FC, SC and ST - have a split sleeve alignment bushing that is the critical element. They have been made with molded glass-filled plastic, phosphor bronze and ceramic. In our opinion/experience, the plastic ones are only good for multimode fiber and wear out in ~10 insertions, discoloring connector ferrules and leaving dust scraped off the plastic on the ferrule ends. The metal ones are good for SM or MM and hundreds of insertions, but they tend to wear and leave marks on the ceramic ferrules. The ceramic ones are recommended for SM and for testing as they work best and last practically forever.

Maximum Fusion Splice Loss
We have set 0.4 dB as our max for all losses per splice and my counterparts argue that customer quality will not suffer with a 1.0db-1.5db loss at a splice. What do you think?
A: We would argue that the issue with high loss splices is more one of reliability. Most fusion splices of singlemode fiber are 0.05 to 0.1 dB A splice that has more than ~0.2dB loss probably has some inclusion (dirt that got on the fiber after cleaving) or an air bubble with means the splice is deficient in strength and may fail over time. If the network is operating at high power with WDM and fiber amplifiers, the inclusions or bubbles may produce heat which can cause failures. At very high speeds or using coherent communications over long lengths, it might affect dispersion.

Loss For APC vs UPC Connectors
Q: I was wondering if there will be a standard connector loss for a UPC connector and a different lower value for an APC connector.. ex. upc has 0.5dB while APC is 0.3dB.
I would like to make all connectors uniform on a new network infra to avoid mismatch and causing any possible damage on the equipment when APC will be plugged into to a flat.
A: There is really no statistical difference between APC and UPC connector loss. The lower reflectance of the APC actually reduces loss since the reflectance represents a factor in connection loss, This issue of connector grades has been discussed at international standards committees for years. ISO/IEC wants to have grades of connectors, rated for connector loss in ranges from 0.1 to 1dB, but I do not think it’s standardized. I recommend using 0.3-0.5dB for loss budgets, where in OSP networks it matters little, since there are only a few connections and fiber and splice loss is a larger factor.
Keeping UPC and APC connectors straight is easy - APCs are Green, UPCs are blue. Everybody just needs to be taught that!

Fusion Splice-On Connectors (SOCs) (From an FOA Instructor)
A question came up from one of our students regarding splice on connectors.  Is there a TIA or other standards body that addresses this issue? We are used to the 0.75 dB loss for a mated pair, however, when this mated pair has two fusion splices that terminate the connector, is there a recommendation? 
One could make the argument that it does not make any difference as the other alternative is splicing a pigtail for termination of a cable.  This pigtail splice is normally included in the link loss budget calculation.   So similarly, with a splice on connector it is the same as splicing on a pigtail.
A: There are no specific TIA or IEC specs that address these splice-on connectors or pigtails. If you used TIA numbers and included the splice and connector it would be 1.05dB - 0.75dB for the connection and 0.3dB for the splice, that’s mated to a factory adhesive/polish connector.   Or if it were two similar connectors, 1.35dB. 
Everybody, including the people in TIA standards groups, know those numbers are too high for most single ferrule connectors. They keep them at 0.75dB for prepolished/splice connectors (w/ mechanical splices) and array connectors (MPOs) which have somewhat unpredictable performance. Internationally, IEC has created grades of connectors from ~0.3 to over 1dB. The newer mechanical splice connector kits now use the Chinese copied cleavers which are super - at least the few we have tested - and the connectors are now much lower loss and consistent.
SOCs (fusion splice-on connectors) are spec’ed as the total termination and are generally just as good as the typical adhesive polish connector - 0.5dB is plenty of margin for a those mated to a factory adhesive/polish connector.
Spliced on pigtails are generally considered a termination and the splice is not broken out - like a long SOC. But I cannot guarantee everybody thinks that way. But a fusion splice is typically <0.1dB anyway.

Can my otdr test upc connectors?  Can I test this distribution without problems?
A: APC and UPC connectors are not compatible. APCs are green, UPCs are blue. See To test UPC connectors you would need a launch cable that is UPC on the OTDR end and APC to to mate to an APC connectors. But if I look behind these green APC mating adapters, I seem to see blue connectors - blue is UPC not APC.  (add photo)

APC Connectors
Q: We've got a discussion going in our department about whether an APC connection is required at both ends of an electronics link or just in one spot when trying to control reflection for broadcast equipment.  Thoughts/advice?
A: It seems you could use APCs on the transmit end to prevent reflectance bothering the laser source, but it would get reflectance from the receiver end, probably not an issue unless you are on short links, like in a data center where the far end reflectance would not be attenuated substantially.  If you put the APC at the receiver end, you would still have problems with reflectance at the transmitter end. Perhaps the biggest problem is managing to keep patchcords straight. We’d vote for the simple solution, all APCs, which is what many people are doing today. Alternatively, use the best UPC connectors which have only about 10dB worse performance as long as they are kept clean.

PON Troubleshooting
I have question about DBM IN GPON system with splitter 1:64. I spliced 2 fiber from splitter to customer going through 4 splicing points and when I measured the loss at the end (customer) the #2 fiber was fine  but the #1 fiber was down 12 dB. I checked the fiber with OTDR without splitter and it looks fine. What you think is the problem?
A: You need to test the splitter itself to make sure all ports are good.

Testing Pigtails
Q: A customer said he said he would test 100 foot pigtails with OTDR. I question that practice and think OLTS Tier 1 and microscope test for defects,
A: Pigtails do not have a connector on one end so that makes OTDR testing more justifiable. Clean connector, mate to connector on long reference cable, check connection and length. OLTS testing would require using bare fiber adapter or temporary splice and might not be very accurate.

Splice Loss
Q: We are installing 216 fiber aerial cable for 12km with 2 splicing points. We use 3 different fusion machines and they report that all splices are 0.00db. But when we check using OTDR we get above 0.04db. The question is how can we get below 0.04db splicing loss?
A: The loss results from both fusion splicers and OTDRs are estimates, with considerable uncertainty. The splicing machines estimate based on the optical images of the fibers. The OTDR estimates loss based on fiber backscatter and may give significant differences depending on the direction of test. The differences you quote are within the uncertainty of the two instruments.

APC Connectors
Why NOT make the use of APC connectors the new standard for all adds, moves and changes to any campus, MDU or similar application using single mode cable?
A: There is absolutely no reason not to use APC connectors other than the cost is slightly higher and one must be careful if they are used in a cable plant that also has PC or UPC connectors because they are incompatible. We recommend them all the time for short links like data centers, passive optical LANs and FTTH where runs of singlemode fiber are short. In fact they are very common in these networks today.

Cleaning Connector Protective Caps
How do you clean LC Fiber Optic end caps (the cap that covers the cleaned fiber cable)? Is there a tool for that?
A: We assume you are talking about the small plastic protective caps on the connector ferrule. There is a joke in the industry that goes “there’s a reason they call them "dust caps’” they’re often full of dust.” The problem is these are plastic molded parts that are made by the billions for various purposes - some just fit fiber optic connectors. They come out of the molding machine and are dumped in barrels. No provision is made to keep them clean, plus they will have some mold release chemicals inside them that can attract or hold dust. Even static electricity is a problem.
We know no way to clean them nor to keep them clean. We recommend using them to protect the connector ferrule - in fact we’re trying to get people to call them “protective caps” - but after they are removed and before use (connecting to another cable or a transceiver or testing them) they need inspection and cleaning.
See these pages in the FOA Guide: Microscope Inspection And Cleaning of Fiber Optic Connectors  Cleaning Fiber Optic Connections

Directional Splice Loss
Q: I have a customer that is splicing a fiber distribution hub to their fiber plant.  The fiber distribution hub utilizes 100FT long fiber stubs of SMF G.657.A1 and the fiber plant uses SMF G.654.D.  The project has a contract fusion splice passing spec of 0.2dB loss, averaged bi-directional and also a one-way <0.3dB loss (either direction) specification; using an OTDR for measurements.
From my research, if the splices OTDR’s test results for the 2 directions are -0.2dB / +0.6 (average of +0.2), the network is not actually seeing a +0.6dB loss; but this is how the OTDR interprets the backscatter information… the OTDR being somewhat confused due to the bend insensitive fiber characteristics.
A: Correct - the directional differences are due to the mode field diameter variations in the two fibers.  G.654 is a large MFD fiber, ~12.5microns, compared to ~9 microns for G.657.A fiber. The OTDR measures based on backscatter which will be very different for the two fibers.

Connector Mating Adapters
: I am looking for a standard that describes the value of parameters (IL, RL) and class (if we can talk about class) for FO mating adapters.
A: Mating adapters are part of the connection but like each connector they only contribute to the total loss of the connection and cannot be separated from the other two when talking about loss. They can be specified by mechanical dimensions and materials. For example on 2.5mm ferrule connectors (SC, ST, FC and the obsolete FDDI and ESCON duplex connectors) the mating adapters have had alignment sleeves made of molded glass-filled thermoplastic, phosphor-bronze and ceramic. The plastic ones are cheap but wear out quickly - 10 insertions will leave plastic dust all over the mating connectors. Phosphor-bronze mating adapters last longer - maybe 500 cycles. The ceramic sleeve ones last almost indefinitely. We know this because we were in the test equipment business for 20 years (we started FOTEC in 1980 and sold it to Fluke in 2000) and we tested these mating adapters for longevity with reference test cables used in insertion loss testing. We had many calls from techs with problems caused by the adapters with plastic sleeves. So the way we know the mating adapters are graded is by alignment sleeve materials.

APC Connectors
With a fiber optic pathway that has multiple patching points...if the end user requires APC connections, isn't it only important to have those angled connectors at the end/equipment connections with UPC being acceptable throughout the middle part of the link?
A: Reflectance at the connection is the issue, of course.
Reflectance near a transmitter can affect the laser transmitter causing nonlinearities or noise in the device. That’s always been a major concern.
The second issue is reflectance causing background noise in the link. If you have ever seen a ghost on an OTDR, you have seen a reflectance at a connection that is bouncing back and forth in the fiber and is of high enough amplitude that you can see it at the source. Of course if it reflects back and forth in the fiber link, it will also show up at the receiver end, becoming noise and/or distorting the receiver pulses. In a bidirectional single-fiber network like a PON, it affects receivers at both ends.
Some refer to this as multipath interference. It is being studied by international standards groups but nothing has been published on it as far as I know.
We are familiar with a link that was ~1km of SM fiber with hand-polished ST connectors at several connections. The link had acceptable loss for all the fibers in the cable but none would work with electronics. Replacing the connectors with fusion spliced pigtails cleared the problem up immediately. Was the problem
With that background, I would answer you question this way. APC connections at each end of the link will effectively stop any reflectance issues going back and forth in the whole link. Using UPC or PC connectors in the link with reflectance better than -40dB are unlikely to cause problems. (Keep them clean of course since dirty connectors show high reflectance.) If the links are very short (<1km), the fiber will not attenuate any reflectance substantially, so short SM links (FTTH and passive OLANS for example) often use APC connectors everywhere.
And a final practical issue - mixing APCs and PC connectors is very bad, perhaps damaging the surfaces. If you do mix them in a link, you must train personnel how to handle them. If you have patch panels with PCs and equipment with APCs, for example, you have to ensure the patch cords are color coded properly (blue = PC, green = APC) and everybody knows not to mix them

Connector Loss At Patch Panel
If I have two SC connections in a cabinet eg one incoming cable jumperd to an out going cable. Should I be looking for a loss of no more than .75db across the two of them as per TIA-568
A: TIA 568 has included a connector loss of 0.75dB for decades. Even the committee is aware that this is a bogus number for most connectors but they leave it in because the manufacturers of MPO connectors need it to comply with the standard.
SC connections should be ~0.2-0.3dB if the connectors are good and properly cleaned. Now in the patch panel you describe, each of the two connections should be in that range for a total loss of 0.2-0.6dB. TIA would allow 0.75dB for each connection or 1.5dB total.

Differences Between Singlemode or Multimode Connector Mating Adapters
What is the difference between singlemode and multimode bulkhead/adapters (mating adapters). My understanding is you cannot use the singlemode with the multimode and visa versa.
A: There are 3 types of adapters - rated for SM or MM - based on the alignment sleeve material.
-Plastic (glass filled thermoplastic) alignment sleeves are cheap, not very precise and wear quickly (you can see ceramic ferrules get dirty using them) - only good for multimode and one or two insertions - not recommended
-Metal (phosphor bronze) alignment sleeves are better with good alignment but still wear some - OK for MM, some are rated for singlemode (check before you buy), and are OK for most uses but will wear out if used for repetitive testing
-Ceramic alignment sleeves are the best and most expensive. They are very precise in alignment and last for a long time. Recommended for all singlemode and all testing purposes.
Don’t use MM adapters for SM but SM adapters are OK for MM.

Polishing Films
  Are there different grades (micron) polishing films/papers for multimode and single mode fiber cables in ODF termination ? If yes, What are the grades polishing papers for multimode 50/125 um and 62.5/125 um fibers.
A: The polishing of MM and SM fiber is indeed different. Both start with an “air polish” with 12micron alumina polishing film to remove the protruding fiber. Then the polishing continues on a soft polishing pad (3mm 80 durometer rubber).
MM uses a 3micron alumina polishing film polished dry then a final 0.3micron alumina film polish. See
SM is usually done with a wet polish using as special polishing slurry and diamond polishing film. The diamond film will polish both the ferrule and the fiber to get the best end finish. See
There are even more pages of information on the FOA Guide at

MPO Connector Loss
: Is there a current standard, for maximum allowable loss, for MPO fiber connectors? If so… what is the standard # from EIA/TIA? (Was it amended in 568B, since they were introduced?) Would it be similar to standard connectors @ 0.75dB Max allowable loss?
A: The MPO is covered under the TIA 568 standard. All fiber optic connectors are the same - 0.75dB.
There are discussions being held at TIA and ISO/IEC on using a different method of specification, statistical in nature, that says X% would be less than YdB in several stages from 0.1-0.2 to over 1dB, but it’s led to some headed discussions.
MPOs for MM are probably no less than 0.5dB and SM are near the 0.75dB mark. At least the SM ones are APC (usual 8 degrees, but still a flat polish).
I’ve recently learned that MPOs are polished for fiber protrusion to try to get fiber contact, but the evenness along the line of fibers is harder to control.

image from SUMIX showing protruding fibers in MPO connector

Fusion Splicing Regular And Bend-Insensitive Singlemode Fiber
Would fusion splicing single mode bend insensitive to standard fiber with same core cause a numerical aperture mismatch? We are seeing loss but it’s hard to tell from what. Going from a drop to BI inside cabling. Any direction is appreciated!

There is a lot of controversy in this area and has been for some time. The issue is mode field diameter(MFD)  differences between regular and bend-insensitive (BI) fiber caused by the low index trenches around the core that are used to limit bend insensitivity.

Some (maybe most) manufacturers make BI SMF to match MFD of their regular SMF, since a common use is splicing BI SMF pigtails onto regular SMF. With so much BI SMF fiber being used in microcables and high fiber count cables, the opposite situation could be an issue also.

Another factor at play here is the fusion splicing program. The different structures of the fiber may need special programming in the fusion splicer to get heat and feed right for the two different fibers.

FOA hopes to have some independent data on this topic soon. FOA Master Instructor Joe Botha has done tests before on splicing dissimilar fibers when BI fiber first became available (read the report here)  and has planned a more extensive set of tests to update that data for more recent fibers soon.

Fusion Splicing Live Fibers
Is it safe to fusion splice a live fiber, or is there a chance that
the light from the arc will damage the detectors in the modules at the end (20km-rated SM for us).
A: I have never heard of this being a problem. The amount of light coupled into the fiber from the splicing would be very small compared to a properly coupled laser. When a cable is broken you might be splicing the fibers that are live without knowing which are live and not caring. On your newer splicers this is not a problem. On the older splicers with the LID system you would have to reduce the power to get a good splice which they would do by putting a bend in the Fiber.

Maximum Fusion Splice Loss
Q: We have set 0.4 dB as our max for all losses per splice and my counterparts argue that customer quality will not suffer with a 1.0db-1.5db loss at a splice. What do you think?
A: We would argue that the issue with high loss splices is more one of reliability. Most fusion splices of singlemode fiber are 0.05 to 0.1 dB A splice that has more than ~0.2dB loss probably has some inclusion (dirt that got on the fiber after cleaving) or an air bubble with means the splice is deficient in strength and may fail over time. If the network is operating at high power with WDM and fiber amplifiers, the inclusions or bubbles may produce heat which can cause failures. At very high speeds or using coherent communications over long lengths, it might affect dispersion.

How Long Does Termination Take?

FOA received a request from a consultant recently wondering if we had information on the termination times for fiber optic cables. After some looking in our archives, we realized we had a document online that compared times for various fiber optic termination processes. The paper was written after several FOA instructors did a comprehensive time and motion study on termination processes. The document is about 15 years old but still relevant.

You can read it here in the FOA Online Guide.

Testing Connectors (From A Patchcord Maker)
What are the chief defining standard(s) that specifies connector and assembly IL (insertion loss) and RL (return loss or reflectance) for both SM and MM fiber?
A: The description on our Guide is here:  
FOTP-34 covers connector testing as a qualification test for the type of connector - basically a "destructive" test for connector manufacturers.
Reflectance is described on that page and here also:
Testing an assembly like a patchcord is covered under FOTP-107

"Connector Loss" or "Connection Loss"
Q: I have always counted the loss of a connector as .75 dB (568B-3) and 1.5 for a mated pair. Is that correct?
A: While the industry always says "connector" loss, it is actually "connection" loss. As we explain in the page on termination and splicing ( When we say "connector" loss, we really mean "connection" loss - the loss of a mated pair of connectors, expressed in "dB." Thus, testing connectors requires mating them to reference connectors which must be high quality connectors themselves to not adversely affect the measured loss when mated to an unknown connector. This is an important point often not fully explained.  In order to measure the loss of the connectors you must mate them to a similar, known good, connector. When a connector being tested is mated to several different connectors, it may have different losses, because those losses are dependent on the reference connector it is mated to."
The TIA spec of 0.75dB is for a mated pair of connectors. If you have been passing connectors tested @ 1.5dB may have some very bad connectors in your cabling!

Test & Measurement

OTDR questions are the most common questions on testing so they have their own section below.

Basic Tests For Fiber Optic Cable Plants
Q: I
did some research and I noticed that there is a bunch of tests that can be done to fiber optics and I was wondering if there is a list of primary tests that can be done as a basic test.
A: Fiber optic testing does have a hierarchy of tests.
  • At the top of the list is "insertion loss" testing which uses a light source and power  meter to test the fibers in the same way that a communications system transmits over the fiber. It is a simple test and the equipment needed is inexpensive.
  • Techs will also use a microscope to inspect the fiber optic connectors for dirt and damage, a big issue for fiber.
  • The instrument called an "OTDR" takes a snapshot of the fiber using a technique like radar. Most outside plant cables are tested with an OTDR and the data ( the snapshots are called "traces") stored for future reference. OTDRs are more expensive and require more training to use properly.
Here is a link to a page on the FOA Guide site that explains the technical,details:
FOA also has information just for users of fiber optic networks, see

Is A "Flashlight Test" Adequate?

Q: I contracted a firm to install an OM3 of 200 meters. On one  end I have an SFP 1000SX ,on the other a 1000SX converter from optical to UTP. We made pings but they never reached, and I didn’t see the laser at the extreme of the fiber. They promised me to send me the certification they supposely made ,though they assured me the fiber is ok, because  WITH A FLASHLIGHT THEY SENT WHITE LIGHT FROM ONE SIDE TO THE OTHER AND IT WAS VISIBLE. I saw the light too, and I thought the culprit was my switch or my SFP. I want to know: is this a good demonstration that the fiber is ok?
A: A visual continuity test is not adequate - your eye is not calibrated! The power of the lamp is unimportant as each eye’s sensitivity is different. And your eye probably cannot see the light from a 850nm VCSEL source - most people’s eyes are not sensitive at that infrared wavelength. The installer should have tested the link with a light source and power meter ( and given you the loss in dB. The connectors should also be inspected with a microscope to ensure proper polishing and cleanliness ( If the SFP output is -6dBm, what is the power at the receiver? 1000base-SX is supposed to work with 4.5dB loss (see The fiber loss should be ~0.6 dB, so you must have >4dB connector losses! That says bad installation! The 1000SX link should work over 200m if the fiber has been properly installed.

Using A Visual fault lOcator
What are the best practices for using a VFL to locate fiber faults?
See in the FOA Online Guide


Microscope Magnification (11/13)
I am doing a lot of fiber optic jumpers for control systems,  either single mode or multimode. I want to get a scope to inspect the ends after I clean them would you recommend a 200X,  400X handheld or one similar to a Noyes OFS 300 200C?
A: We prefer to use lower magnification and have a wider view so I can see more of the ferrule to determine its condition. You can see the fiber effectively at 100X but 200X may be better. 400X may be too much for most tasks like inspecting for cleanliness, but may be good if you are polishing SM for good reflectance. We've used the Westover units for years because they offer two different methods of illumination - direct and at an angle. If you are doing a lot of patchcords, I recommend a video microscope. I've used the Noyes unit that interfaces to a PC to create the FOA Microscope Inspection YouTube video here: and it works well.

Calibration of Fiber Optic Power Meters
Why does this tester sold so cheap show the same value for 1310 and 1550?
Power meter calibration
We’ve purchased and tested several of these inexpensive meters based on questions from our instructors and readers. They have a similar “feature” - they are not calibrated to international standards of optical power, so when you measure dBm, the result at any wavelength may vary considerably from a meter by a major manufacturer who does calibrate properly.

However, these meters have a menu item that allows you to calibrate them yourself. We've calibrated all the ones we tested to match our calibrated instruments and after that they work OK.

Interestingly, they show loss as negative dB, which is how we think it should be. Some large, famous manufacturers manipulated the standards to make loss a positive number, so if you measure gain, it is considered a fault.   

Differences in OTDR Traces
What causes the differences in otdr traces for fibre cores that are in the same cable?
A: Several things can cause the fibers  in a cable to have differences in their OTDR traces:
Differences in fiber from different production batches including fiber that may come from different preforms.
Differences in stress on the fiber caused by inconsistent cable design and manufacture
Of course differences in splices including stress on fibers in a splice closure and terminations including stress on fibers in racks and panels.

Test Source Variations
Q: When we plug in the patch cable to use as a reference to get ready for our insertion loss test, our power meter gives us different readings each time we plug in and unplug the cable.  Is this normal?
A: If you plug a reference cable into a source, it is likely to have some variation in coupling each time so the power out of the cable is different. Once you plug the cable into the source and a reference set with the meter, you should not remove the cable until you finish testing.

Test Source Modulation Options
Q: Our light source user’s manual listed the following: “In the actual project, it is necessary to load the audio carrier in the optical signal to identify the optical fiber. The equipment contains three carrier frequencies, which are 270Hz, 1KHZ, and 2KHz.” Can you tell me if I need to be concerned about setting this to a certain value?
A: Sources often have an option to modulate the source for use with another instrument called a “fiber identifier.” Then it offers the output as DC or CW (a steady unvarying signal) or modulated at 270HZ,1 or 2 kHz. When testing loss the DC or CW setting should be used.

850 LED Test Source
I need to test multimode fiber at 850 nm but sources are hard to find and expensive. Can I use a laser?
A: Multimode fibers should be tested with an LED. Lasers have several problems in multimode fiber that may cause untrustworthy readings. The problem is that LEDs are no longer used for transmission systems; every MM system now uses VCSEL transmitters, a surface emitting laser. As a result most LEDs at 850nm for MM have gone out of production. Two engineers I know who have been looking for them say there is now only a few sources and the price is much higher that of a few years ago. We've been buying used test sources on eBay for training.

Optical Power Of FTTH Signals
I wanted to know on a fiber to home what is the optimal signal strength  I should receve at? I have a leg that is 21 km long I receive at 1490 at -22 dBm is that to low, everything looks good on my traces
A: The specifications for GPON are here.
The standard for GPON calls for receiver power at -13dBm max to -28dBm minimum, so -22 dBm is OK.

Documenting Fiber Optic Cable Plants
I am looking for information or training materials on documentation standards for OSP cable.  We currently have a number of large backbone cables along with mid span cables.  I know companies like the phone company labels each cable with a number and terminals off of the mid span drop with the specific pair numbers.  Is there a standard way to label each cable, mid span cable, splice box, terminal, etc.?
A: The usual way is to document every fiber in every cable with a fiber designation, color code and connections on each end. There are some standards on numbering schemes but most companies I know use their own designations created as they got started. There are software packages that will do this work, simplifying the process,

Documenting Test Results
We’re currently working on a bid that includes presenting some test sheet documentation for OTDR & Light loss testing. What should I do?
A: High end LSPM or OLTS should store data and have some software to report test results. Simpler units should simply require logging data into a spreadsheet showing Cable ID, Fiber ID, wavelength and loss. Details like launch & receive cables and test results can be kept separately on the spreadsheet. Today’s OTDRs will show you a trace and an event table that lists each even in the fiber tested as well as overall loss. Whatever OTDR you use should have software for reporting test results. Here is an example of a report from an EXFO and a trace from a Yokogawa.

Maintaining Dark Fiber
Do you have any standards that speak to how often dark fiber should be tested with OLTS and OTDR? Such as just at installation and when troubleshooting, or should they be done on a regular basis?
A: We at FOA know of no standards calling for periodic testing of fiber optic cable plants.
Fiber optic networks generally do not require maintenance and it is often detrimental to the network. It is the opinion of FOA and most people in the industry that testing should be done upon completion of the installation and data submitted to confirm proper installation of the cable plant. Data should then be stored for reference in case of problems requiring troubleshooting or when new dark fibers are turned up. Before lighting a dark fiber, it should be tested and the results compared to earlier data. Since both tests have some uncertainty, test results can vary as much as 0.5dB on short cables, higher on longer runs.
If older fiber is being upgraded to higher speeds, now cities like Santa Monica where we live are upgrading to 100G networks, fiber characterization including chromatic dispersion, polarization mode dispersion and spectral attenuation (for DWDM) are advised. Of course, every time a connection is opened, it should be inspected and cleaned. And patchcords should be tested; even new ones in sealed packages are often dirty. There is a reason people call the plastic protective caps on connectors “dust caps!”
Otherwise, with fiber, we suggest the patch panels be locket to keep unauthorized personnel from accessing them and causing problems. Even disconnecting a connector can add dirt to the connections and cause problems.

Identifying Users On A PON Network
How or what testing tool or technique can I use to verify whether there is a live customer w/ONT working on any fiber i may select @ a splice enclosure prior to getting further down the cable and  to the MST service terminal. All our fibers have light on them leaving the CO so when we go into a splice enclosure to pick a fiber to connect a drop to, to service a home, they are usually all lit up in that enclosure.
A: The simple answer for a tool or technique that can tell you if a customer is connected on an output of a PON splitter is “documentation.” If you know where each fiber is connected going downstream. Then the IT person who programs users into the system can tell you if that fiber is connected to a customer. There is a possibility that there is a test solution. Have you ever heard of a “fiber identifier”? It’s a gadget that can tell if there is signal in a fiber and some can identify the direction it comes from. What I don’t know if the unit can somehow indicate bi-directional traffic. Nobody we contacted seems to know either.

Microscope Power For Connector Inspection
What power microscope do you recommend to inspect singlemode/multimode in 1.25/2.5 format (ST, SC, LC)?
A: Microscopes in the range of 100-400 power are available. Many people assume higher power is best - and it is for examining polishing results in the center of the ferrule - but lower power helps inspect more of the ferrule for dirt when used in the field before connecting or testing cables. We prefer the lower power.
So for patchcord manufacturers, 400, field techs 100. Patchcord manufacturers will undoubtedly use video microscopes, most field tech the optical ones.

Power Budget For PON
Do you have any information on guidelines for avoiding over saturation in a PON network? Our ONTs have a power window of between -8dBm and -27dBm.  OLT transceivers transmit at around 4dBm.  So our designers budget for no more than 28dB of loss. However, some ignore the -8dBm maximum power spec.  With a short run from OLT to ONT and a small splitter, installers are sometimes seeing light levels at the ONT at around -6 to -7dBm. What would you recommend as a minimum loss budget in this case?  Do we need margin?
A: The GPON spec does have a max power at the ONT generally expressed as a minimum loss in the cable plant - 13dB for GPON. There is a graph about halfway down this page ( that shows a graph of BER vs Receiver power. To have a link work properly, it must have sufficient power to be above the minimum S/N - signal to noise - ratio for the link but not so much power that it saturates the receiver.
This is a very common situation in telco networks where links are designed for relatively long distances but may be used on short ones - e.g. a 40km link being used over 10km in a city. Their solution is simple - add an attenuator ( Lots of these links use attenuators.
In a PON, there are several ways to go. 1) Brute force - test each ONT and add attenuators as needed. Techs could carry a selection of 5dB or 10dB attenuators to get at least to the 13dB minimum needed. 2) Rather than require testing at each ONT, have the designer do a loss budget based on the link length and specify a minimum splitter in the link (8:1 would probably work well) which would probably be cheaper than testing and adding lots of attenuators.

VFL Works On Multimode And Singlemode
Will a VFL for single mode work with a multi mode as well?
A: Yes, a VFL works for either SM or MM fiber. Tight buffer MM is no problem. On patchcords, the ability to see light through the jacket of the cable depends on the color and transparency of the plastic. On buffered fiber, it works well on most 900 micron buffer fibers.

Troubleshooting Links
I have a run of 12 Strand single mode fiber from a remote closet back to my main equipment room.  I unpluged it this morning to check another cable and when i plugged it back i have lost communication with the remote closet.  Can you help me with some trouble shooting ideas.  I have already tried a new cable from the termination point back to the switch but still no communication.
A: The first guess is that the fibers are not connected correctly - transmitter to receiver. That’s the first thing to check. The second possibility is dirt on the connector or in the connector housing on the transmitter or receiver. Cleaning might be the solution. Third and worst possibility is the connector was damaged when being disconnected.
Q2: I ended up powering down the switch in the remote closet and plugging the fiber in then powering it back up and it picked up communication again.  Strange ??
A: Nope, makes sense. Many new networks shut down unless they have full duplex communications. If the link is broken, it shuts down until reset.

Testing Samples Not Everything
: Instead of testing everything, how does one determine how many fibers or components to test for a reasonable statistical sampling?
A: The relevant term is AQL - acceptance quality limit - a term that is used for statistical sampling for testing. Here is a web page that explains it: Let testing and inspection evolve. At first test thoroughly, but drop testing anything that never fails, it’s a waste of time. Qualify vendors and test trusted vendors less. 

GPON Power Test
What is normal Range for good power in an FTTH fiber?
A: The GPON specification for downstream power from the OLT is OLT transmitter power should be 0 to +6dBm and link attenuation in the range of 13 to 28dB, which says receiver power the ONT must be a maximum of 13 dB less than +6dBm or -7dBm and a minimum of 28 dB less than 0dBm or -28dBm, so -7 to -28dBm at the receiver.
Upstream, the similar calculation is ONT transmitter -4 to +2dBm  and the receive power at  theOLT is -11 to -32dBm.
See for the full specifications for GPON.

esting Cable Before Installation
Q: Does the FOA publish a standard for assessing single-mode fiber optic cables, prior to use on a specific project?
A: The ANSI/NECA/FOA-301 fiber optic installation standard covers this in Section 4.1. It recommends visual inspection and testing if there is any suspicion of damage to the cable. Many contractors will test a couple of fibers with an OTDR before installing any cable, just for assurance. It requires an OTDR with a pigtail launch cable and a mechanical splice.

Testing Cable Before Installation
Q: Does the FOA publish a standard for assessing single-mode fiber optic cables, prior to use on a specific project?
A: The ANSI/NECA/FOA-301 fiber optic installation standard covers this in Section 4.1. It recommends visual inspection and testing if there is any suspicion of damage to the cable. Many contractors will test a couple of fibers with an OTDR before installing any cable, just for assurance. It requires an OTDR with a pigtail launch cable and a mechanical splice.

Test Multimode At 1310nm?
Can I test multimode over 1310 wavelength?
A: Certainly you can, but why? In fact, multimode fiber has been tested at both 850 nm and 1300 nm for most of its history. Some standards still call for testing at both wavelengths.
When network speeds were 100Mb/s or less, sources were LEDs at 850nm used for shorter links - a few hundred meters - and LEDs at 1300nm were used for longer wavelengths, up to 2km for networks like Ethernet, FDDI and ESCON.
When Ethernet jumped to 1 gigabit/s, LEDs could not be used; they were limited to ~200 Mb/s. The new fast VCSEL sources (vertical cavity surface-emitting lasers) were adopted for most links. Some networks still offered a 1300 nm option using 1310 nm Fabry-Perot lasers since VCSELs are limited to about 950nm wavelength max. The 1310 lasers available were generally pigtailed with singlemode fiber and required some special launch cables to prevent modal problems, and even though they offered longer range, they were not really cheaper than using singlemode fiber and never gained much popularity. (See Specifications for fiber optic links and systems, including FTTx)
So multimode fiber became almost exclusively used at 850nm. In standards committees we discussed dropping the requirement for testing at 1300nm, but some argued that since the fiber is more sensitive to stress/bending losses at 1300nm, testing at 1300nm provided information on the stress in the fiber. Once multimode fiber became almost exclusively bend-insensitive fiber, that argument lost validity.
While some standards still call for 1300nm testing and many test sets offer 850nm and 1300nm LED sources, it’s probably not worth the time.

Testing OM2/3/4 Multimode Fiber
Q: Is there any loss concerns when mating an OM2 fiber to an OM4 fiber (vs using the same type only).
The question applies to both use of an LED source and an VCSEL laser source. We can assume it is only for short fiber lengths (< 25meters)
A: OM2, OM3 and OM4 fibers have the same basic specifications. Based on this specification, the two fibers are geometrically identical and there should be no difference between them.
However, about 10 years ago, multimode fiber was introduced with bend-insensitive structure.  Today almost all multimode fiber is bend-insensitive. Earlier MM fibers, including the OM2 fibers, are not bend-insensitive. Unlike SM fiber, where bend-insensitive singlemode fiber is given an different designation - G.652 becomes G.657 - multimode fiber is not differentiated between regular and bend-insensitive, cables are not marked, and it takes a knowledgeable tech with a microscope to tell the difference.
Until recently, testing standards called for test reference cables to be non-BI fiber because the early BI fibers simply added a lower index trench around the core to capture the lost modes which gave them a larger core diameter and higher effective NA and these fibers did not respond the same way to traditional mandrel wrap mode filters - in fact the specified mandrel wrap had virtually no effect at all. When non-BI fibers were tested with BI fibers, the effective core diameters produced directional loss effects. Over a decade, MMF manufacturers learned more about BI fiber structure and modified the index profile of the fiber to essentially make BI fiber about the same as non-BI fiber. In addition, mode control was changed to encircled flux controlled sources instead of mandrel wrap mode filters, which was more effective  with both fibers.
So recently, MM testing standards have changed. You can use whatever 50/125 fiber you have to test OM2, OM3 and OM4 fiber.

Mismatched Fiber Losses
If I have 50 micron test leads for my OLTS and I used them to test a 62.5 micron fiber link what can I expect in terms of results?  Will the 50 micron leads give me (generally) higher or lower loss values?
A: Yes, you will see higher and or lower loss depending on which way you test. See this page in the FOA Guide on Mismatched Fibers.

Reflectance And Return Loss
Help me understand measuring reflection little better. Why do we consider -55dB to be a better reading than, say, -25dB? If reflection and return loss are inverse readings and we had a 55dB return loss, would that positive reading for return loss be considered good?
A: Reflectance is measured as the ratio of reflected to incoming signal at a connection. The confusion comes because reflectance and return loss are inverse readings. Consider this:
If we have 1/1000 of the light reflected, the reflectance would be -30 dB (1/1000 = -30 dB) but the return loss would be 30dB since it is defined as 1000/1, the inverse, and is described as +30 dB.
Likewise, an APC connector would have a reflectance of -50 dB or a return loss of 50 dB.
However, return loss as tested by all OTDRs is not be the reflection from a single event but the total of all reflectance events plus total backscatter from the length of fiber being tested in the trace.
This is where most people are confused and misuse the terms.

Q: What is the importance of reflectance and all the other numbers in installing and trouble shooting a fiber circuit?
A: Reflectance has always been a secondary issue to connection loss but has some important issues that need consideration. There are two basic issues with reflectance, affecting with the output of laser transmitters and creating background “noise” in a fiber link.
Reflectance can interact with the laser chip itself, causing laser transmitters nonlinearities or random fluctuations in the output. The background noise is a secondary issue, but can be seen in ghosts in an OTDR trace. The light bouncing back and forth in the fiber that causes ghosts will be added to the signal at the receiver end, adding noise to the actual signal. Both these effects are more significant on shorter links, for example FTTH or LANs using PONs (passive optical networks). We always recommend using APC (angled physical contact) connectors on short SM links. And most short SM networks do use APC connectors.
FOA tries to stick to the definition that reflectance is the light reflected from a connection but some others call it “return loss.” Return loss has been defined generally as the combination of reflectance and backscatter from the fiber, and that’s how OTDRs measure return loss. Standards vary in the definition sometimes.
Here is a FOA Guide page on reflectance that gives the basics and explains how it is tested.

Reflectance Testing Expanded Beam Connectors
We have a contract where we are required to test assemblies that vary from 0.5m to 800m in length with connectors of either PC to Expanded beam or expanded beam to expanded beam type.

      LC/SC/ST -----------to------------- MIL-DTL-83526
     MIL-DTL-83526 -------to---------- MIL-DTL-83526

We currently have a back reflection ‘power meter’ on loan but have been told that the results for assemblies above 5m lengths may not be reliable to measure the connector termination due to the nature of the expanded beam termination and cumulative scatter reflection in the cable.
Therefore we are considering OTDR equipment for our back reflection testing but trials we have done so far show that the ‘standard’ OTDR testers are not able to separate events that occur within a 1 – 2m length so we are not able to separate connections on short lengths.
We are coming to the conclusion that we will now need to purchase both a back reflection meter and OTDR to cover the 0.5m to 800m range but would appreciate any industry knowledge we can find.
We took some time to try to research measuring reflectance on expanded beam connectors and was surprised how little I could find. FOA has a web page summarizing reflectance testing ( and a full chapter in our book on testing ( but I don’t remember ever being asked about reflectance on expanded beam connectors.
The best information I have found indicates that reflectance is fairly high because of all of the optical surfaces. It’s possible to test all the cables - short to long - by using a mandrel wrap attenuator after the connectors under test. But if these are multi-pin connectors with multi-fiber cables, that won’t work.
Generally OTDRs are not good for short length testing but fortunately there are specialized instruments designed for your application. Here is the Luciol OTDR from Switzerland which can do the job (
I believe there are others like this also.
Having both types of instruments is problematic, as the readings will not agree between the two measurement methods. I suggest using the Luciol instrument with the concurrence of your customer so the tests can be comparable. Otherwise, I would not be surprised at 3-5 dB differences or more.

Reflectance Testing
Q: Do you know whether anyone has compared the reflectance measurement determine by the OTDR calculation to that make with a reflectance test set?
A: Measuring reflectance/return loss is a complex task requiring measurement over a high dynamic range with limited accuracy. We devoted a 15 page chapter to it in the testing book. The problem is establishing a reference - like most other tests - but the uncertainty is probably no better than +/-3dB.
Here is a place where the OTDR measurement is probably the preferred method because the test is made with mated connectors compared to the backscatter background and the meter/source/spitter measurement has to compensate for cable lengths and the reflectance from the far end of the mated cable which needs termination (dip in index matching fluid) to prevent that reflectance from affecting the measurement.

What Do Testing Results Indicate?
I was told the other day by a network technician that it is possible that a fiber optic strand that is tested to standard, 850, 1300, mm and 1310-1500 SM bi-directional can pass a test but when connected to an optic it doesn’t work. I told him that the optic is the variable but if a strand passes the testing its qualified to “work”  or pass light.
A: There are several reasons it can be true.
Either MM or SM
-The installed cable plant is OK but the patchcords are bad. Or mixed up - we know instances where systems did not work because MM systems were connected with SM patchcords and vice versa - instant 17-20dB loss.
-The polarity is wrong so the transmitter does not go to the proper receiver. For MPO networks, this is a major problem since there are so many different polarities used. (See MPO array/parallel connectors and how to test them). This is often a documentation problem.
-Post testing, the connectors get contaminated and not cleaned or are damaged.
-The link meets the loss budget but the length is too long for the fiber type to support the transmission bit rate - e.g. OM2 fiber  on a 10G system that is near 300m long (see Specifications for fiber optic LANs and Links for the list of lengths supported)
-Mixing PC and APC connectors. Bad for loss - may cause serious damage too.
-Reflectance problems. Interestingly this question was asked this morning by the tech boss at a giant university. Here is the question and my answer:

Testing At 820 or 850nm
I am working on calibration of an optical power meter. It is an old Photodyne 2285XQ and I need to test it at 820nm, I believe this wavelength is (or used to be) fairly common in military applications. The problem is the only equipment I have available is  all based around 850nm/1310nm/1550nm. Our optical power meters and optical spectrum analyser can certainly operate at 820nm, but I’m having real difficulty finding a commercially available light source at 820nm.
A: That Photodyne meter was probably built before the US National Bureau of Standards agreed around 1983 to create the first calibration standards for fiber optic power meters. NBS created standards for the three primary wavelengths of fiber optics - 850, 1300 and 1550nm - based on the available laser wavelengths for calibration with their standard ECPR - electrically calibrated pyroelectric radiometer.  The 850nm range was never considered a problem because in the early days LEDs were sometimes called 820nm and sometimes called 850nm, but the spectral width is quite wide and the variation in actual peak wavelength, even as called out in standards, is 850+/-30nm or 850+/-20nm, making it a very “broad” standard.
I know the rules for MIL standards and calibration so here are several solutions. Your solution will require a transfer from some transfer standard power meter.
1. Use the 820nm laser and calibrate the output with the H-P set at 820 if that is possible. Possible error due to calibrating only a small spot on the Photodyne detector.
2. You can also make the cal above with a high-intensity source with a filter around 820nm.
3. Use an 850 LED and calibrate the output with the H-P set at 820 if that is possible. This assumes the photodetector sensitivity curves are similar.

APC Connectors On Power Meters
We need to test a fibre link terminated with APC SC pigtails. I am using SC-APC Ref Leads to interface the LSPM to the fibre link. I am using the 1 Test Cord Method. Step 1 means connecting the LS to the PM via one Ref Lead. That means I have an APC Green SC connector plugged into the PM.  Is that OK? 
A:The SC APC connector should only be mated to another SC APC connector to prevent potential damage to the fiber/ferrule end. But most power meters have adapters for the connector that have an air gap above the detector to prevent contact to the detector window. Plus, the detector should be large enough to capture the light from the SM fiber exiting at a small angle. Thus you can plug the connector into the power meter directly.
Some meter manufacturers make SC APC adapters for their power meters that angle the connector toward the detector but that is generally not necessary unless the meter has a very small detector.
However if the power meter has a pigtailed detector - that is the meter has a fiber>fiber interface, you will need to add an adapter patchcord to mate the APC connector to it. When you set a 0dB loss reference all those connections will be zeroed out.

OTDR AutoTest
Would we say that OTDR 'Smart' test capabilities are commonplace on newer models or only on some manufacturers meters ?
Maybe it's additional software that can be thought of as an upgrade ?
Some form of “auto test” has been available on most OTDRs for 20 years or more. Early versions were not very good; they usually just made a test under some average test conditions and reported the results. Modern OTDRs use more powerful computing power to make several tests and determine which conditions are best for the fiber being tested. By optimizing the range, pulse width, number of averages, etc. it can usually produce fairly good results. We don’t think the cost of the OTDR is an issue for new ones because users expect all of them to have a good auto test function. As to whether an older unit could be upgraded, that would depend on the manufacturer and if they still support that product. An OTDR less than 5 years old should probably be able to be upgraded.

Directional Testing
I have taught for several institutions and throughout all my years of doing this I was always taught that when testing for insertion loss and back reflection for singlemode cable links that testing bi-directional is an imperative.  Recently when I was attending a meeting involving members of the military along with folks involved in the development of the military manuals, it was mentioned that with singlemode testing that bi-directional testing is not necessary.
A; First of all, there is a directional difference in splice or connector loss - and maybe reflectance (that term is now almost universally used in place of “back reflectance” which is a poor term since a reflection is always back) - as long as two different fibers are being spliced. Fiber geometry is the main difference - mode field diameter in SM and core diameter in MM - but it can also be a matter of the fiber composition.This happens if two different manufacturers’ fibers are joined or bend-insensitive (BI) fiber with differences in depresssed cladding geometry are joined.
If you test bidirectionally with sufficient accuracy, you can see the difference. It’s an OTDR trace that most people are familiar with  -  if you see a gainer, you shoot the other direction and average to get the “actual” splice loss. When splicing or connecting different types or manufacturers SM fiber you may see directional differences of up to 0.3dB or more. Same for MM, not only for differences in core diameter but also for connecting BI to non-BI fibers. While those OTDR measurements are actually differences in backscatter levels, they are indicative of real differences in connector loss or splice loss in opposite directions.
Are those differences enough to be of concern? That’s a judgment call.
We’ve found most people do their bidirectional testing all wrong. With an OTDR, you should disconnect the instrument, not your launch and receive cables, and take just the instrument to the other end. Disconnecting the launch and receive cables changes the fibers at the connections to the cable under test and you lose the connection you want to test from the opposite direction.
Bidirectional testing with a test source and power meter is more confusing. You have to do your “0 dB” reference, check the launch and receive reference cables, measure the cable under test, then move just the source and meter to the other end, test the cable under test again, then disconnect and then measure the output of the source to get the “0 dB” reference used in that direction. That’s confusing!
Even if you do test bidirectionally, you do not get the “actual” connection or splice loss like everybody says - you get the average of the two directions. Unless you are willing to do a lab setup and some careful testing, that’s the best you can do.
Now we are into measurement uncertainty. If the measurement uncertainty is around the same as the typical bidirectional difference, does bidirectional testing gain you that much?
That’s a judgement call.

The FOA book on testing goes into this - a complete chapter is devoted to measurement uncertainty.

FOA textbook on fiber optic testing

When To Test Fiber
Should testing of the fiber plant be done before the Optical Network Terminal is installed?
A: Fiber optic testing is generally done when the cable plant is installed to confirm proper installation and check that the performance is adquate for the electronics planned for use on it.
There are several processes - first an overview of testing:
Check cable before installing - continuity if it looks OK, OTDR testing if the reel is damages
Test installed cable when splicing - check the fusion splicer estimate of loss and do OTDR testing if questionable and inspect every connector as termination is done to confirm the connector is good.
Test cable plant after splicing and termination - end-to-end insertion loss and OTDR testing for longer OSP (outside plant) links.
New singlemode cable plants for high speeds may need "fiber characterization” - adding in CD (chromatic dispersion), PMD (polarization mode dispersion) and SA (spectral attenuation for DWDM wavelengths) testing. If one is considering upgrading a cable plant that is already installed or has been used, these same tests should be done - inspection/cleaning, insertion loss, OTDR.
When the system is installed, one should know it should work because of the testing done during installation. One should inspect and clean patchcords before installation and test them if suspect. In fact any connector needs inspection/cleaning before hooking up equipment. Dirt is the biggest problem with fiber optic systems.

VFL for 10km?

Q: I have 10 kilometers of singlemode cable installed that was not labeled. It has been suggested that we shoot a VFL down the fiber and label it. I am having trouble finding a VFL that will shoot this far. Any ideas?
A: Occasionally we see some imported VFL that claims to go 10km or more. That tells us the company is clueless about fiber optics. VFLs work at ~650nm in the visible red spectrum while SM is optimized for 1300-1600nm in the infrared where it has a loss of ~0.3dB/km. At 10km it has a total loss of ~3dB or half the input signal. At 650nm, singlemode fiber has a loss of ~10dB/km which means it loses 90% of its power per km. At 10km, you have 100dB of loss - leaving you with 0.00000001% of the input power - not much!
VFLs have enough power for 2-3km max. To identify fibers at 10km, you need a 1310nm laser source and a power meter to do continuity. Or a gadget called a fiber identifier. For more info, see and

Testing PON Meters And Sources
We're evaluating PON power meters and test sources. How should we test them? Do we need a PON network?
A: There is no requirement for having a PON to test the meters. I would check it against a meter you trust to test
1) if the reading of absolute power (dBm) agrees - should be within +/-0.2dBm. Compare at several power levels, as high as possible (~0dBm with a laser), medium, (~ -20dBm) and very low (~ -40dBm)
2) make some loss tests of cables and attenuators over the range of 1-5-10-15-20dB and compare to a meter you trust.
3)the extra calibration at 1490 is not an issue - the difference between 1490 and 1550 is very small and providing that calibration can be more a confusion factor since there are no transfer standards for that wavelength.
3) The big issue with sources is stability. Connect the source with a short cable to a trusted power meter, connect it to its power supply, turn it on and monitor the output over time. There should be a short warm-up period and then it should be stable within a few 0.01s dB. Let it run on batteries until the batteries run down to ensure that the source has a proper power supply that keeps the light output stable over time as the batteries discharge.

PON Testing with a 2Xn Splitter Instead of 1Xn
When testing upstream back to the CO on a PON, how can we get a good OTDR trace if the primary splitter (nearest one to ODF) has a 2:8 split ratio rather than the usual 1:8?
It's just like looking at a 1X2 downstream - you will see the combined traces of each fiber. If the second port is for testing, it may be short and connector, so it will not affect the longer trace very much. If it's the same length and used as a spare, you need to test each fiber downstream to the splitter. Nothing is easy with OTDRs and splitters!

High Loss At 1383nm?
We tested one link of 90.8 km at 1310/1383/1550nm and we got high loss at 1383 while other wave lengths have good results. What's up?
A: That wavelength is the center wavelength of the OH+ water peak, so you are seeing the extra attenuation there. Older fibers will have attenuation of 2-3 dB/km at that wavelength but new “low water peak” fibers will be <1dB/km. See “Low Water Peak Fibers” here

Bi-Directional OTDR Testing
Should the testing be done with the same piece of equipment from both ends then merge the results or does that not matter - can you use traces from two OTDRs as long as the test equipment is compatible and settings are adjusted properly.
A: Yes, you should use the same test set from each end but this way - take a trace, disconnect the OTDR from the launch cable and go to the far end of the receive cable and connect it there to take the second trace. The usual way people do bi-directional tests is to disconnect the launch cable and take it to the far end and shoot back up, often not using a receive cable at all, figuring they get the far end connector on the second test. But when you disconnect the launch cable (and/or the receive cable) you lose the connection you want to test in the other direction! As for using the same OTDR, every OTDR is different and the results you get may be significantly different, esp. if they are not calibrated recently - and few OTDRs are ever calibrated.

Calibrating An OLTS
I have a question about the OLTS - do you have to recalibrate it every day ?
A: Any optical loss test set needs to be calibrated for “0dB” whenever anything changes - the launch cable - source output - or even every few tests to ensure the connector is clean and undamaged - plus they wear out. See 5 different Ways To Test Fiber Optic Cables.

Insertion Loss
I have not been able to find a good definition of “optical insertion loss” or “insertion loss” or “optical loss.”
A: Insertion loss was the term originally used for the loss of a connector tested by a manufacturer. They would set up a source and length of fiber connected to a meter, measure power, insert a pair of connectors and measure the loss. Since it was an inserted connection, it became known as insertion loss.
Over time, the term insertion loss became more widely used to contrast with the loss measured by the OTDR, an indirect measurement using backscatter that may not agree with the loss with a light source and power meter.
Insertion loss, therefore migrated to meaning a loss measured of a cable or cable plant inserted between the launch and receive cables attached to a light source and power meter for double ended testing used with installed cable plants. For patch cord testing, you do not use a receive cable attached to the power meter but connect it directly to the cable under test, making the test just include the one connection to the launch cable.
Two other terms often mixed up are attenuation and loss, which are essentially the same, except when discussing a fiber. In fibers, attenuation is often used instead of attenuation coefficient. Attenuation is the absolute loss i dB while attenuation coefficient is the characteristic attenuation of a fiber expressed in dB/km.
Here is probably the best explanations:

Using Hybrid 2.5-1.25mm Connector Mating Adapters
Can I use the hybrid 2.5-1.25mm adapters for connecting SC connectors to LCs or MU connectors. It would make testing much more convenient.
A: We do not recommend them for most uses, especially testing, as they can be highly unreliable. Reserve them for emergencies and use hybrid patch cords instead.

Test MM Fiber @ 1300nm?
Q: What is your opinion about the need for testing at 1300 nm on OM3 and OM4 fiber especially now that bend insensitive multimode fiber is taking over?
It’s unnecessary and costly. It’s rooted in the FDDI/100M days 25 years ago when 1300 LEDs were used and is now obsolete. The only actual uses at 1300nm I know are the extremely rare systems using 1310 lasers which may be standards but simply don’t seem to ever be used. As you say, BI fiber makes the issue of finding stresses moot.

Another Way Of Expressing dB?
Just wondering what to think about presenting dBm as a percentage of power, using either a linear measurement or quadratic equation ?
I recently came across this article : []  I realise it's Wi-Fi signals here, but can you compare this to anything concerning optical loss or gain , given we're still using dB and dBm ?
A: I had to read this 3 or 4 times to get the idea. Basically he suggests converting dB, a nonlinear log scale, to a linear scale expressed in %. Following his steps (assuming I understand his system) , 100% = 0 dBm (1mw), 90% = -10dBm = 1/10mw  = 10% of the original signal, 80% = -20 dBm = 1/100 mW = 1% of the original signal.. So 80% = 1% of the original signal. And that’s where it seems a bit nonsensical. 70% would be 0.1% of the original signal,,,
We fail to see what this “new math” accomplishes.

OTDR Questions

Testing Bare Fibers With OTDR
We are starting to test some OPGW cables. We have an OTDR but we don’t find some reusable connectors. If we have to test an OPGW with 48 fibres, we can’t set up 48 SC connectors!
Are there some reusable connectors in the commerce?
A: I assume you mean you need to test with a bare fiber on the OPGW. For testing bare fiber, use a splice, not a connector. Have a long pigtail on the OTDR as a launch cable, long enough for the test pulse to settle, say 100-500m, then use a splice for a temporary connection. You can fusion splice the fibers then cut the splice out or use a removable splice like the Corning Camsplice (
If you use a mechanical splice, you need a high quality cleaver just like with fusion splicing and after several uses, you need to add more index matching gel or liquid - mineral oil works OK.
See the FOA page on Testing Bare Fiber.

OTDR Dead Zone
Q: What is dead zone when using OTDR?
A: The "dead zone" is the length of fiber near the OTDR that is blanked out by the overload from the test pules. See this page on OTDRs in the FOA Online Guide:

OTDRs - Launch Cables And Range
I have a question about OTDR launch cables.  In all readings about OTDR testing, it states that the launch cable "needs to be of sufficient length ...".  What length is sufficient?  How long should a launch cable be?  What is the maximum length of cable plant that can be tested at one time?
A: OTDR launch cables need to be long enough to allow the OTDR to settle down after the test pulse leaves the instrument and reflectance at the output connector overloads the receiver. The dead zone is a function of the OTDR test pulse and the condition of the output connector. If you are testing short cables (<1km) with very short test pulses, a launch cable can be 20-50m long. If you are testing a very long cable with very wide pulses (some OTDRs have pulses ~4microseconds long, equivalent to ~1km) you would need a 2-5km launch cable. So the answer to that question is it depends on how long the fibers are you are testing.
As to how far a OTDR can reach, the answer is generally not specified in km but in dB. The best OTDRs have a reach of ~40dB at 1550nm which corresponds to ~150-200km, spending on how good the splices are. That length of fiber would have ~30 splices for say 3dB splice loss.
Here’s the FOA Guide page on OTDR testing and the FAQs page Frequently Asked Questions about OTDRs.

Seeing Splices On OTDR Traces
Q: The reason why I am reaching out is because the CEI is having trouble understanding that not all trace files will show splices. As we both know that means that there is low loss and the network will work more efficiently. I was wondering if you could possibly help give a more in depth explanation so everyone can understand why they are not seeing splices.
A: Nothing in fiber optics is more confusing that an OTDR trace!
First it is necessary to understand how the OTDR measures loss, so start on this page in the FOA Guide: When you get about 3/4 down the page, there is a section called “OTDR Measwurement Uncertainty”  that explains the way a splice loss is measured and the uncertainty of the measurement caused by the difference in backscatter coefficient in the two fibers being spliced.
Next consider how a splice is made - fusing or welding two fibers together. The typical loss of the splice is under 0.1 dB. The difference in fiber backscatter can cause directional loss variations higher that the loss of the splice. If the difference in backscatter is 0.1 dB and the splice loss is 0.1 dB, in one direction it will show 0 dB loss and in the other directin it will show 0.2 dB loss, so the average is about right, 0.1 dB. This is of course how we get “gainers” when the backscatter difference is much higher than the splice loss.
But also consider this. The OTDR digitizes the signals in both axes. If the dB range shown is 40dB and the digitization is 10 bits, each bit represents 0.04 dB. A good fusion splice can be so small, the OTDR cannot detect it because it is less than 1 bit of the measurement resolution.
This is the reason we tell people that documentation is so important. If you know where the splice is, you can look for it and pat yourself on the back if you can’t find it because you are so good at splicing!

OTDR Resolution
If testing a 40KM link with 1KM launch and receive cords should I be able to see the connector and cassette splice on each side? My OTDR setup is at 64KM, 300ns pulse and 10 second test at 1310/1550/1625. It shows as a single event so far but with the pulse width at 300ns won’t that combine the events into one event during analysis?
A: You will not be able to resolve a connector and splice close together, especially on a long link like that. 300ns is almost 60m pulse width! You will see an even of the splice and connector combined.

OTDR AutoTest
Q: Would we say that OTDR 'Smart' test capabilities are commonplace on newer models or only on some manufacturers meters ?
Maybe it's additional software that can be thought of as an upgrade ?
A: Some form of “auto test” has been available on most OTDRs for 20 years or more. Early versions were not very good; they usually just made a test under some average test conditions and reported the results. Modern OTDRs use more powerful computing power to make several tests and determine which conditions are best for the fiber being tested. By optimizing the range, pulse width, number of averages, etc. it can usually produce fairly good results. We don’t think the cost of the OTDR is an issue for new ones because users expect all of them to have a good auto test function. As to whether an older unit could be upgraded, that would depend on the manufacturer and if they still support that product. An OTDR less than 5 years old should probably be able to be upgraded.

Fiber Backscatter Coefficient
I was wondering if you have an idea of how accurate the numbers from a fiber datasheet are for the stated backscatter coefficient of the fiber.  When looking at these numbers reported on datasheets, they are nice round numbers like -77dB, -82dB etc., which tells me that those aren’t actual measurements.  They are more ballpark values.  Would you happen to know roughly what sort of accuracy those would have for a given length of fiber?
A: We believe is the number is an average from some tests done whenever the fiber was designed and first manufactured and is +/- several dB. Remember it’s not a guaranteed spec like attenuation coefficient which they do measure.
Remember it is dependent on the pulse width. Corning in the SMF-28 data sheet quotes the numbers you have for a 1ns pulse width. ( You can probably surmise that 1 ns pulse is hardly a square wave and there will be lots of variations in the integrated power in a 1ns pulse created by different instruments. And who knows how you relate that power to a 5, 10, 20 ns or whatever pulse.
This paper by Corning talks about reflectance and backscatter and has some interesting points: The key is backscatter is inversely proportional to mode field diameter, so small changes in fiber diameter can cause changes in backscatter - and MFD variations of several percent are possible.

OTDR Calibration
I know user who want to send their OTDR’s in for calibration, especially those that do government work.  Are you aware of a check list of the required tests to comply to what would amount to an “OTDR Calibration”?
ORDR calibration is covered on FOA Guide page on OTDR FAQs -   OTDR calibration is a unsolved problem. I was first involved with it at NBS in Boulder (now NIST) in the mid to late 1980s. They could not justify having a set of “golden fibers” to send around because they could not figure out how to make a “better" OTDR to use as a standard as we/they did on fiber optic power meter calibration.
Later some tried building electronic calibrators that could calibrate the timebase and linearity of the receiver, but that ignored the laser transmitter - you would need to calibrate it’s wavelength also. Wavelength variation could cause up to 3% difference in backscatter and loss measurements alone. Then the OTDR allows for changing the index of refraction to different fibers which affects length measures.
At one time some manufacturers looked at OTDR cal systems but the only electronic cal units for OTDRs I know that made production came from Belarus. There is a single system in the Navy at Corona, CA however.
Bottom line, test it’s operation according to manufacturer's spec.

Variations In OTDR Length Measurements
I am performing OTDR testing using 3 different wavelengths (1310, 1550, and 1625) on the same fiber but I get 3 different results for fiber length.  The test set and testing parameters do not change.
A: The OTDR calculates length by measuring the transit time to an event (half the total time since it measures the pulse time both down and back) and multiplies it by the speed of light in the fiber. Speed = C(speed of. Light in a vacuum) divided by N(the index of refraction of the glass). Most fibers have an N~1.46.
In an OTDR that speed of light is set using index of refraction in the setup parameters. In the fiber, the light actually travels at different speeds according to the wavelength of light - that is what causes chromatic dispersion. For Corning SMF -28 SM fiber,  effective refractive index at 1310nm is 1.4674, and at 1550nm is 1.4679, not much difference but adds up at 10km or more.
So your OTDR probably has the wrong setup.

Negative Loss?
We were sent this OTDR trace and asked why some traces showed negative loss - gains.
The issue with this OTDR test of a factory-manufactured patchcord was not straightforward. To summarize, the patchcord was plugged directly into the OTDR port, with no launch cable. The OTDR has no reference for measuring the loss of the first connector on the cable nor the second connector since neither launch or receive cables were used. Without a launch cable, the OTDR is trying to get information from the connection to the instrument itself which is basically impossible since it’s suffering from overload caused by the test pulse - even with APC connectors. The OTDR (Yokogawa) is one of the cleanest OTDRs at the interface (we have one on loan at FOA right now) but it’s still not designed to measure that loss. Furthermore, using the instrument interface to plug in every connector will wear that connector in the unit out quickly and require servicing by the factory. The second issue is the difficulty of measuring on short cables like this. Note the vertical digital resolution of the display and think about the location of the second marker - it’s measured dB value will jump around as it goes from digital segment to the next digital segment. When you use a launch cable and measure the loss of the connectors using the “4-point” measurement - also called “least squares approximation” in the FOA Guide to OTDRs. That will overcome the digitization errors as well as the settling times of the pulses.

OTDR Setup

Q: What are good OTDR settings for a 300-500m fibers? We’re using a 1.5 m launch cable and sometimes got (-) loss,
A 1.5m cable is not a launch cable. A launch cable must be long enough to allow the OTDR to settle down after the test pulse. THe negative loss is because the OTDR has not settled down sufficiently.
Generally the minimum launch cable for testing short cables would be 10-20m for MM, 100m for SM. Then use the shortest test pulse, ~1km range, average enough to reduce the noise.

The Value Of Knowing What Your Are Testing And Proper OTDR Setup

FOA consistently makes the point that the most important part of a network design or installation is the documentation. It tell the installers what to build and records their test results. It helps troubleshoot faults if a cable plant problem arises and restoration becomes necessary.

Documentation is also a big help when testing with an OTDR, as two technical inquiries this month showed. One concerned the testing of a cable still on the reel before installation. In this case, just knowing the exact length of the cable or fiber would be helpful.

Here is the first trace sent to FOA:

fiber trace gainer

The sender wanted to know why there was a reflective event in the fiber - had it been broken or spliced? That reflective event is a ghost, easily recognized by the fact that the connection between the launch cable (500 feet long) was high loss and highly reflective (it was a bare fiber adapter) and the second reflection was another 500 feet away. The decay from the connection to the launch cable was due to a bare fiber adapter with ~4 dB loss and a lot of reflectance (-25.7 dB) overloading the OTDR receiver.

There was nothing wrong with the fiber.

The second trace was for a longer fiber, still a cable on a reel before installation:

noisy fiber trace

The question was why was there a "gainer" in the middle of the fiber. The trace sent to FOA showed a noisy trace with a "gainer" at around 5500 feet. Again there seems to be no problem with the fiber, but the OTDR setup caused the noisy trace and that "gainer" was because the noise exceeded the threshold for events set in the OTDR. Redoing the trace with a longer test pulse or longer integration time should reduce to noise. In this case, just knowing the fiber was continuous with no splices tells you a gainer (caused by the joint between two different fibers) was not the cause of the event. And looking at the noise on the fiber says the OTDR is either not properly set up or defective.

I have 4 questions about OTDRs:
What is dynamic range I read many time but can’t understand yet, whether it is a range of losses can be measured by OTDR for example if an OTDR has 45 dB dynamic range, it can read the losses of point up to 45 dB or what it means.
do not believe there is a standard definition of dynamic range, but it is generally accepted to be the highest loss of the longest cable where you can see the end of the cable. That usually means using the longest test pulse and most averaging  and assuming the end of the cable has a significant reflection.

What is dead zone is it fixed in meters mean an OTDR cannot measure up to initial 5, 10 or 20 meter
A: The dead zone is a function of the pulse width and speed of the OTDR amplifier. For most OTDRs it’s about 2-3 times the test pulse width.

What Type of OTDR settings are needed before launching a test
A: See FOA Lecture 18: OTDR Setup or the section "Modifying OTDR Setup Parameters For Best Test Results” in OTDR testing. A: Basically you set up wavelength(s), test pulse width (long enough to reach end of cable but short enough for best resolution), index of refraction or group velocity (a function of the fiber type and wavelength) and the number of averages (enough to mitigate noise but not take too long)

Q: Reading a test with 1310nm and 1550nm - why values different for a same length of fiber.
A: The attenuation of the fiber will be different at each wavelength and the index of refraction which is different at each wavelength causes a difference in length. The OTDR measures length by measuring time and then multiplying that by the speed of light in the fiber (which is the inverse of the index of refraction.)

The FOA page "Frequently Asked Questions About OTDRs" answers these questions and more.

"Connector Loss" or "Connection Loss"
Q: I have always counted the loss of a connector as .75 dB (568B-3) and 1.5 for a mated pair. Is that correct?
A: While the industry always says "connector" loss, it is actually "connection" loss. As we explain in the page on termination and splicing ( When we say "connector" loss, we really mean "connection" loss - the loss of a mated pair of connectors, expressed in "dB." Thus, testing connectors requires mating them to reference connectors which must be high quality connectors themselves to not adversely affect the measured loss when mated to an unknown connector. This is an important point often not fully explained.  In order to measure the loss of the connectors you must mate them to a similar, known good, connector. When a connector being tested is mated to several different connectors, it may have different losses, because those losses are dependent on the reference connector it is mated to."
The TIA spec of 0.75dB is for a mated pair of connectors. If you have been passing connectors tested @ 1.5dB may have some very bad connectors in your cabling!

Optical Loss: Are You Positive It’s Positive? A question we get often.

A recent post on a company’s blog and article on the CI&M website discussed the topic of the polarity (meaning “+” or “-“ numbers) of measurements of optical loss, claiming loss was a positive number. The implication was that some people failed fourth grade math and did not understand positive and negative numbers. The claim is that insertion loss is always a positive number.

Is that right? Well, it depends if you argue from the standpoint of scientific convention or from the standpoint of one isolated standard!

In IEC (and TIA documents adopted from IEC documents, the definition of attenuation in Sec. 3.1 is written to have attenuation calculated based on  Power(reference)/Power (after attenuation). This definition leads to attenuation being a positive number as it is normally displayed by an OLTS or OTDR. However if one uses  a fiber optic power meter calibrated in dBm, the result will be a negative number, since dBm is defined as Power(measured)/Power(1mw) (see FOTP-95, Sec. 6.2). If dBm were defined in this manner, power levels below 1mW would be positive numbers, not negative as they are now, and power levels above 1mW would be negative!

Well the real problem is that to understand this you need to understand logarithms and that’s Algebra II*, way beyond fourth grade addition and subtraction. You see dB is defined as a logarithmic function:

dB equation

With logarithms, if the ratio of measured power to reference power is greater than 1, e.g. measured power is more than reference power, the log is positive. If the ratio of measured power to reference power is less than 1, e.g. measured power is more than reference power, the log is negative. If the ratio is 1, the log is 0.

Since the logarithm for optical power ratio is base 10 and then multiplied by 10, each change of 10 in the ratio of the measured and reference power becomes a change of 10dB. E.g. +10 dB is a factor of 10 (10 times log10 10 which is 1), +20dB is a factor of 100  (10 times log10 100 which is 2), +30dB a factor of 1000  (10 times log10 1000 which is 3)and so on. Negative dB means division, so -10 dB means a factor of 1/10th  (10 times log10 0.1 which is -1), -20dB a factor of 1/100th  (10 times log10 0.01 which is -2) and so on. 0 dB means the measured power to reference power ratio is 1 – they are equal.

Let’s try a graphic explanation of this equation. Take a look at this “semi-log” graph (logarithmic on the x axis and linear on the y axis) of dBm vs optical power in the range commonly used for fiber optics and calculated with our equation above. Remember 0 dBm means all power is referenced to 1 milliwatt optical power.

dB to watts

As you move to the right, power increases and the value in dBm gets more positive – that would be gain. So from 1mw to 10mw, we see a gain from 0dBm to +10dBm or 10dB, a positive change.

As you move to the left, to lower optical power, as would be loss, the dBm value gets more negative. From 1mw to 100microwatts (that’s 1/10mw), we go from 0dBm to -10dBm, or -10dB; that negative change indicating a loss of 10dB.

That shows gain is positive dB and loss is negative dB. Now we’re getting to the fourth grade math.*

How about an example? Let’s say we decide to test a singlemode cable plant. We start with a laser source and launch cable which we measure our reference level for loss with a power meter to have an output of 0dBm. That’s 1 milliwatt of power, about the normal output of a fiber optic laser. After we attach the cable plant to test and a receive cable to our power meter, we measure 3dB loss.

What power did we measure? The power must be lower, of course, since we have loss, and 3dB is approximately a factor of 2, so the power the meter measured is 1mw divided by 2 = 1/2milliwatt or 0.5mw. Since our power meter is measuring in dBm, it will read minus 3dBm (-3 dBm), since lower optical power is always more negative. If it read +3dBm, the power measured would be 2mw and that would be a gain from our reference (0dBm) which we know is incorrect – passive cable plants are not fiber amplifiers.

Here is the graphical version of this loss test:

 dB loss or gain

And then there is this short movie on the FOA Guide page explaining dB showing how a power meter shows loss when a cable is stressed to induce loss:

dB on a power meter

As the fiber is stressed, inducing loss, the power level goes from -20.0 dBm to --22.3 dBm.That's a more negative number.

No question – loss means a more negative power reading in dB and a negative number in dB indicates loss.**

But if you are a manufacturer of fiber optic test instruments that offers optical power meters and sources to test loss, why would this confuse you? 

Perhaps we should blame accounting.

Suppose you have a company that has $1million in sales and $900,000 in expenses. What’s the profit? It’s $1,000,000 - $900,000 = $100,000. That’s a profit, right?

But suppose your company has $1million in sales and $1,100,000 in expenses. What’s the bottom line? It’s $1,000,000 - $1,100,000 =  - $100,000. Wait a minute, that is a negative number – that’s not a profit, it’s a loss.

So in accounting, profits are positive numbers and losses are negative numbers when we do the math, but when we talk about loss, we don’t say we have a loss of “-$100,000,” we just have we have a loss of $100,000. Then we’ll put that number in parentheses when we publish our P&L like this ($100,000) and hope it doesn’t get noticed by investors, but you know it will.

Loss and gain in fiber optic measurements are similar. If you are using a separate source and power meter, loss will be a negative number and gain will be a positive number. But because of convention, we sometimes drop the signs when we report the values because loss always means the optical power measurement was negative and gain means the optical power measurement was positive. But maybe that’s not what the convention has evolved to.

Optical loss test sets (OLTS) aren’t designed to measure and display optical power, just loss. The actual power measured is lost in the algorithms used for calculating loss based on the “0dB” reference power and the measured loss. Long ago, most OLTS measured loss and displayed it as a negative number, but some companies who got into the fiber optic test equipment business from other test businesses arbitrarily decided to display loss as a positive number, and today most OLTS do show loss as a positive number. But when the instrument sees a gain, which it can do if improperly used, it therefore displays a negative number, which can be very confusing to a trained fiber tech who understands fiber optic power and loss measurements.

OTDRs do the same thing. I looked at traces from a half-dozen OTDRs and all showed loss as a positive number and gain as a negative number. And yes, when you have a gainer in one direction, they show it as a negative number. Telling them that is wrong will fall on deaf ears, I’m afraid.

The same article/blog post goes on to discuss optical return loss and reflectance, which has similar issues but they get it more or less right, which is confusing. Why can they understand that more negative numbers for reflectance means lower power in the reflectance but claim the opposite for insertion loss?

The “less right” is that with most OTDRs reflectance of an event and optical return loss (ORL) are not the same thing. ORL is the summation of all reflectance events and fiber backscatter from the entire length of fiber.

And, please, please stop saying “back reflection;” a reflection always goes back toward the source so the term is redundant and was dropped from fiber optics years ago.

*So the problem is not simply fourth grade math, it also involves a bit of convention and tradition and marketing. And it requires understanding logarithms that create the negative number of loss. That’s more like Algebra II or 7th grade math, and here is a good tutorial from Kahn Academy on that:

And more basic, here is a tutorial on adding and subtracting negative numbers

** If you want to calculate this yourself, FOA has a XLS spreadsheet you can download that will calculate the equations for optical power for you.

The FOA has an explanation of dB on our online Guide and a couple of graphics that illustrate what happens with loss.

Note: In IEC (and TIA documents adopted from IEC documents, the definition of attenuation in Sec. 3.1 is written to have attenuation calculated based on  Power(reference)/Power (after attenuation). This definition leads to attenuation being a positive number as it is normally displayed by an OLTS or OTDR. However if one uses  a fiber optic power meter calibrated in dBm, the result will be a negative number, since dBm is defined as Power(measured)/Power(1mw) (see FOTP-95, Sec. 6.2). If dBm were defined in this manner, power levels below 1mW would be positive numbers, not negative as they are now, and power levels above 1mW would be negative!


Using FOA Resources To Learn
I've been working in this industry since October 2018. Started as a Field Service Engineer fielding aerial and underground. I have since become a Project Engineer working with aerial OSP and ISP. What is the catch? This is just an amazing platform to continue with for me. It's been all about the OJT, but this is just a great resource for me now and the YouTube videos allow me to watch your videos from today as well as 10 plus years ago.
How can I use the website to benefit me? Where should I start? I just want to watch every YouTube video before I focus on your website, but maybe I want to do both, Suggestions?
A: FOA has three options to get information:
FOA Guide is the FOA knowledgebase, ~1000 pages of technical material generated by the FOA technical advisors around the world. The link is to the Table of Contents where you can find pages on just about any topic in fiber optics from fiber to coherent communications. Every year about 1/2 million users download about 4 million pages!
Linked from the Guide above is the FOA YouTube channel which you have found. It’s over 100 videos, about 60 lectures on tech topics, where you can get very familiar with my voice - I’m the lecturer. The FOA videos are listed here:
You can also go to the FOA channel on YouTube: go to the FOA Channel “thefoainc"
Fiber U  is our free online learning site. We started online learning at Fiber U in 1997. Today it has over two dozen free online self-study courses that lead to a Fiber U Certificate of Completion. Courses include Basics of Fiber Optics with an accompanying Basic Skills Lab, where many people start, especially if they are aiming at FOA CFOT certification, the primary certification from FOA.
We also have basic courses on premises cabling, OSP construction and installation, splicing, termination, testing, network design and about a  dozen that cover specific applications. The FTTH course was developed when Verizon approached FOA in 2005/6 to help with the rollout of FiOS - training and recruiting installers.
FOA”s problem is we have too much “stuff”! It’s so much it can be confusing on where to start. We generally recommend going to Fiber U and picking courses that are important to your work. Those courses will lead you to the appropriate pages in the FOA Guide and videos on YouTube.
But we are always here to help. Tell us what you are interested in and we can point you to the right places (often including websites of manufacturers of products who also have immense amounts of applications information.)

We’re now working on a “Roadmap” to help people find their way, but that will take time, there are a lot of paths to connect!

Are FOA Videos and Web Up To Date?
Are the videos on YouTube still relevant by today's standards are are they out of date?
A: Excellent question. We’ve discussed this within the FOA many times.
For example the live action videos on cable preparation, termination, mechanical and fusion splicing and testing are quite old by tech standards but the processes have not changed in two decades. Preparing loose tube, armored or tight buffer cables has not changed in over 20 years, nor has adhesive/polish connector termination. Prepolished/splice connector and SOC process are different and those processes have been updated. Testing processes are the same with the main difference being the automating of OTDR testing. Manufacturers have dumbed-down OTDRs so well that it seems few techs know how they work or how to read a trace, evidenced by the results of the FOA CFOT Certification exam where questions on OTDRs are the most often missed.
We just did a review of the copper installation for the Premises cabling (CPCT Certification) and that has not really changed in three decades - since the introduction of Cat 5 cable!
We review and update the technical pages in the FOA Guide all the time. Look at the Table of Contents (FOA Guide-  and see how many pages have the NEW symbol, indicating updates in the last couple of months.
Also FOA is adding YouTube videos ( ) and Fiber U MiniCourses ( on many topics regularly - monthly this year, covering new tech and the topics we know are lesser-known or new to most techs.
And let us know if there are topics you think we should focus on in the future.

Math of Fiber Optics
Do you by any chance recommend any books as an introduction to understand the mathematics of fiber optics? Fiber optics confuses me,  particularly how so much data can travel over light without interfering with each other, 
A: If you are interested in information theory, that’s really covered by Claude Shannon at Bell Labs 70 years ago. That’s actually pretty simple. Shannon proved that digital data was best and how much data could be transmitted with a given amount of bandwidth (Read more). There are several levels of math associated with fiber optics. The real theoretical basis is probably in dozens of books but I like these:

Top Level: Gerd Keiser  
Middle Level: Jeff Hecht.

If you are interested in non-math explanations, the FOA website has that:
Fiber Optic Datalinks
Wavelength-Division Multiplexing

For testing math (dB loss, metrology, etc.) these links or the FOA book on Testing
Metrology and Fiber Optics
The Math of Insertion Loss Testing - Reference Methods

Have An Old Cell Phonet? Use it to test for infrared light in fiber optics. See the video on Corning on YouTube
Yes! The camera in your old cell phone is sensitive to infrared light - lots more than your eye - and can detect light in an optical fiber or from a transmitter.  Chris Hillyer,CFOT/CFOS/I, Master Instructor, Northern California Sound & Communication JATC brought this to our attention.
IR Viewer 850 nm  IR Viewer 1300 nm

If you have an old cell phone, try it. Our experience is that older cell phone cameras have better sensitivity at IR wavelengths than newer phones, so you may want to toss that old flip phone into the toolbox.


Where Are The Jobs In Fiber Optics?

Fiber Optic Installation Banner

The FOA was chartered to "promote professionalism in fiber optics through education, certification and standards." Our focus on creating a professional workforce to properly design, install, maintain and repair communications network infrastructure has led us to work with groups in many different areas of technology that use fiber optics, way beyond the basic telecom applications that most of us think of first. FOA has probably worked with most of the potential applications of fiber optics, but we're always learning about new ones!
In addition, we get lots of calls and emails from our members looking for information about where the jobs are and how to train for them. FOA has created three ways to help you find jobs, train for them and apply for them.

Where Are The Jobs In Fiber Optics?
FOA has created a 20 minute YouTube video that talks about all the applications for fiber optics, what jobs are involved and the qualifications for the workers in the field. Besides telecom and the Internet, we cover wireless, cable TV, energy, LANs, security, etc. etc. etc. It's a quick way to get an overview of the fiber optic marketplace and we give you an idea of where the opportunities are today.

Watch the new FOA YouTube Video: Where Are The Jobs In Fiber Optics?

What Training Is Needed For The Jobs In Fiber Optics?
As you will learn from the video described above, the jobs in fiber optics are quite diverse. FOA has investigated these jobs to understand the needs of workers for those jobs and, when necessary, create curriculum and certifications to properly train workers. For example, the FOA FTTx certification was developed at the request of Verizon who needed specialized installers for their FiOS program. Now we are working with the industry on the OLAN (Optical LAN) program (see below).
We have summarized the jobs and required training in a new web page that has two uses - 1) If you have FOA certifications, what jobs are you specifically qualified for? - 2) If you are working in a specialized field or want to get a job in that area, what training and certifications will qualify you for those jobs?
What Training And Certifications Are Needed For Jobs In Fiber Optics? 

How To Find And Apply For Jobs In Fiber Optics
We get many questions from CFOTs, students at FOA-Approved schools and others contemplating getting into the fiber optic business regarding jobs in fiber optics - and how to find them - so we’ve created a new web page to share some information we've gathered about jobs in our industry. The information is designed to help you understand what jobs are available in fiber optics, how to find them and apply for them.
If you are looking for a job in fiber optics, here is the FOA's guide to jobs. 

We hope you find this useful. FOA tries to find new to increase the professionalism in our industry and helping qualified people find jobs is our highest priority - read the article below to see why! If you have feedback on how we can help you and our industry, contact us at

Join FOA on 
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Updated 11/21

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