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The Fiber Optic Update

Every year is another busy year for fiber optics: new technology, components, applications and usually a few surprises.

On this page we've gathered some of the more important topics, covering new technology and applications that FOA believes every tech needs to know. Many of these articles are from the FOA monthly newsletter, which you can subscribe to here.

We also recommend the FOA "Fiber FAQs" page with tech questions from customers originally printed in the FOA Newsletter. We get lots of interesting questions at FOA.

This page is part of a Fiber U Tech Update Course.

Sections: Technology     Components    Installation      Testing    Safety 

FOA Guide

Got questions? Try the FOA Guide and use the site search.


They've ALL Got It All Wrong - And They Confuse A Lot Of People - YOU CANNOT STRIP THE CLADDING OFF GLASS FIBER!!!

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
- really! - 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?

Bottom Line:
  • Most diagrams of fiber construction are wrong - showing core and cladding as separate - but they are one solid piece of glass.
  • You cannot strip the cladding from glass fibers.

Connector Loss For Splice-On Connectors

FOA received a call from a contractor working on a network. His subcontractor doing termination presented data on terminations using mechanical splice-on connectors where he claimed the TIA standard for these connectors was 0.75dB for the connector PLUS 0.3dB for the splice, for a total of 1.05dB. He wanted to know if this were true.

No, it is not true. These connectors have an internal splice to a stub fiber already glued in the ferrule and factory polished. The loss of the connector used to terminate a fiber must include the splice since it is the termination method and there is no way to test it separately from the connector itself.

Typical mechanical splice-on connector, also called a prepolished/splice connector.

Afusion splice-on connector has a stub fiber that's fusion spliced to the fiber being terminated.

We noted the TIA loss value, 0.75dB was very high compared to adhesive polish connectors which average around 0.3dB loss when tested against a reference connector. In the standards it has remained at 0.75dB to cover this type of connector and array connectors like the MPO.

Another question often asked about these types of connectors as well as spliced-on pigtails for termination is can you differentiate the loss of the connector from the splice loss in an OTDR.The answer is generally no, the resolution of the OTDR is not adequate to distinguish the two losses, unless the pigtail is very long. The splices in a fusion or mechanical splice-on connector are much too close for OTDRs to see both events.

More information about splice-on connectors  

Cable Bend Diameter (Radius)

All fiber optic cables have specifications that must not be exceeded during installation to prevent irreparable damage to the cable. This includes pulling tension, minimum bend diameter or radius and crush loads. Installers must understand these specifications and know how to pull cables without damaging them.

The normal recommendation for fiber optic cable bend radius is the minimum bend radius under tension during pulling is 20 times the diameter of the cable. When not under tension, e.g. cable stored in service loops,  the minimum recommended long term bend radius is 10 times the cable diameter.

Note: Always check the cable specifications for cables you are installing as some cables such as the high fiber count cables have different bend radius specifications from regular cables!

And also note that some manufacturers are now quoting "bend diameter" instead of or in addition to bend radius. Bend diameter is more relevant when dealing with service loops or storage loops, while bend radius is more aimed at bending cable around corners. Remember the diameter is twice the radius of a circle, so the minimum bend diameter of a cable  under pulling tension, e.g. the diameter of a capstan used in pulling cables, would be 40 times the diameter of the cable and the storage loop minimum diameter would be 20 times the diameter of the cable.

Fiber Optic Cable Bend Radius
Under tension (top) and after pulling (bottom)

Bend radius example: A cable 13mm (0.5") diameter would have a minimum bend radius under tension of 20 X 13mm = 260mm (20 x 0.5" = 10") That means if you are pulling this cable over a pulley, that pulley should have a minimum radius of 260mm/10" or a diameter of 520mm/20" - don't get radius and diameter mixed up!

Why is it important? Not following bend radius guidelines can lead to cable damage. If the cable is damaged in installation, the manufacturer's warranty is voided. Here is what one manufacturer's warranty says: "This warranty does not apply to normal wear and tear or damage caused by negligence, lack
of maintenance, accident, abnormal operation, improper installation or service, unauthorized repair, fire, floods, and acts of God.
" And their specifications call our the minimum bend radius as "20 X OD-Installation, 10 X OD-In-Service."

And When An Installer Gets it Wrong

Cable pull

There are two problems here, one visible and one hidden. The visible one is the pulley mounted on the side of the truck used to change the direction of the cable to allow using the capstan mounted on the rear of the truck. The cable is being bent about 120 degrees over a pulley that appears to be about 120mm (5 inches) diameter. That's a radius of 60mm or 2.5 inches. That pulley looks like a stringing block uses for stringing ropes when pulling in power lines.

We believe the cable was a 864 fiber ribbon cable with a diameter of 24mm (0.92") with a minimum bend radius of 360mm or 14".  That means the pulley the cable is being pulled over is ~1/6th the size it should be - shown by the dotted red circle above.

The second problem is the angle of the cable coming out of the manhole. It is exiting a conduit and being pulled almost straight up out of the manhole. If there is no hardware in the manhole, the cable is being pulled over an edge exiting the conduit or the manhole, bending with a very, very small radius.

One can only speculate about the possible damage to a cable when treated like this. What comes to mind first is broken fibers, and that is a possibility. But bending this tightly can also damage the cable structure, including the fiberglass stiffeners, strength members and jacket. Compromising the integrity of the cable reduces its protection for the fibers. Even the fiber ribbons can be delaminated and fibers put under stress. A cable pulled under these circumstances can have damage along the entire length, not just a point where it was kinked.

What should have been done on this pull? The 120mm/5" pulley should have been replaced with one at least 6 times larger. The truck could have been further from the manhole (and maybe turned to be inline with the pull) so the angle of the cable exiting the conduit was less. Hardware should be attached to the conduit to provide a proper bend radius for the cable as it exited the conduit and the cable should have been protected if it contacted the edges of the manhole..

Bottom Line
  • All cables have specifications for minimum bend radius
  • Violating this spec may permanently damage the cable
  • Bend radius is generally 20X cable diameter under tension - 10X after installation
Read more about bend radius.

Optical Loss: Are You Positive It’s Positive?

Update 7/2020: Mystery solved! Investigations into ISO standards showed the international standards committees changed the definition of loss in a way that changes the sign for loss but makes it violate all scientific convention on the use of dB. This is documented below.

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?

The asnwer is no - loss is a negative number, but instruments that only measure loss - OLTS and OTDRs - display loss as a positive number. 

Suppose we set up a test. Let's measure power out of a cable with a power meter and then attenuate the power by stressing the cable. What happens?

FOA created 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. (-22.3dB) - (-20.0dB) = -2.3dB That's basically 4th grade math.

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

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?  Well, it seems they think when we talk about loss, we do not give it a "+ or -" sign, we just say loss, so they just display it as a number without sign,

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!

Bottom Line: Confusion
  • Loss in dB is a negative number
  • Instruments that measure loss do not display negative signs with loss
  • Gains are displayed with a negative sign

dB or dBm -Still Confusing 4/2020 -


The second most missed question on FOA/Fiber U online tests concerns dB, that strange logarithmic method we use to measure power in fiber optics (and radio and electronics and acoustics and more...). We've covered the topic several times in our Newsletter but there still seems to be confusion. So we're going to give you a clue to the answers and hopefully help you understand dB better.

These are all correct statements with the percentage of test takers who know the answer is correct.

The most answered correctly: dBm is absolute power relative to 1mw of power (78.8% correct. Does "absolute" confuse people? It's just "power" but absolute in contrast to "relative power" which is loss or gain measured in dB.)

This one is answered correctly less than half the time: dBm is absolute power like the output of a transmitter. (41.5% correct, see comment above.)

This one does often get answered correctly: The difference between 2 measurements in dBm is expressed in dB. (23.8% correct)

Here is an example of a power meter measuring in dBm and microwatts (a microwatt is 1/1000th of a milliwatt.)

Watts to dBm

Here is an example of the conversion of watts to dBm. This meter is reading 25microwatts - that's 0.025milliwatts. If we convert to dBm, it becomes -16.0dBm. We can easily figure this out using dB power ratios. -10dBm is 1/10 of a milliwatt or 0.100mW. -6dB below that is a factor of 0.25 so 0.1mW X 0.25 = 0.025mW or 25microwatts. The other way to figure it is -10dB is 1/10 and -6dB is 0.25 or 1/4th (remember 3dB = 1/2, so 6dB = 3dB + 3dB = 1/2  X 1/2 = 1/4) so -16dBm is 1/40milliwatt or 0.025milliwatts or 25microwatts.

Read a more comprehensive explanation of dB here in the FOA Guide.

What's That Fiber?

Regular and BI MMF

A FOA Newsletter reader sent FOA these microscope photos of two MM (multimode) fibers, asking what was the difference with the one on the right. It is a bend-insensitive (BI) fiber and compared to the regular graded-index MM fiber you readily notice the index "trench" around the core that reflects light lost in stress bends right back into the core. You can read more about bend-insensitive fiber in the FOA Guide.

What does Bend-Insensitive Fiber Look Like?

While researching the answers to the question above, we talked to Phil Irwin at Panduit. He mentioned that you could see the structure of BI fiber and sent along this photo:
Bend-insensitive fiber photo    BI fiber structure
At the left, you can see the gray area surrounding the core, shown in the drawing in the right as the yellow depressed cladding region.

If you want to try to see it yourself, it's not easy. Phil tells us that OFS fiber is the easiest to see, Corning a bit more difficult. You need a good video microscope. You may need to vary the lighting and illuminate the core with low level light.

Today most multimode (MM) fibers are bend insensitive fibers. If you buy a MM cable or patchcord, it is probably made with bend-insensitive fibers. That's generally good because thee fibers are less sensitive to bending or stress losses which can cause attenuation in regular fibers.

Many singlemode fibers are bend-insensitive also, especially those used with smaller coatings to pack more fibers into microcables or high fiber count cables.

The Perils Of 2-Cable Referencing

FOA received an inquiry about fluctuations in insertion loss testing. The installer was using a two cable reference method for setting a "0dB" reference where you attach one reference cable to the source, another to the meter and connect them to set the "0dB" reference. The 2-cable reference method is allowed by most insertion loss testing standards, along with the 1- and 3- cable reference methods, although each gives a different loss value.

reference 3 ays
3 different ways to set a 0dB reference for loss testing

When a 1-cable reference is used, one sets a reference value at the output of the launch cable and measures the total loss. With a 2-cable reference, a connection between the launch and receive reference cables is included in making the reference, so the loss value measured will be lower by the amount of that connection loss. The 3-cable reference includes two connection losses so the loss will be lower still.

two cable reference
The problem with the two cable reference is the uncertainty added by including the connection between the two reference cables when setting the
"0dB" reference.

Unless you carefully inspect and clean the two connectors and check the loss of that connection before setting the
"0dB" reference, you add a large amount of uncertainty to measurements of loss. The best way to use a 2-cable reference is to set up the source and reference cable (with inspected and cleaned connectors), measure the output of the launch cable, attach the receive cable (with inspected and cleaned connectors) and measure the loss of the connection before setting the "0dB" reference. If the connection loss is not less than 0.5dB, you have connectors that should not be used for testing other cables. Find better reference cables.

The two cable reference is often used when the connectors on the cables or cable plant being tested are not compatible with the connectors on the test equipment, so you must use hybrid launch and receive cables. Then you can only reference the cable when connected to each other. In that case, you need the 2-cable reference but should expect lower loss and higher measurement uncertainty.

Experiments have shown that the uncertainty with a 1-cable reference is around +/-0.05dB while the 2-cable has an uncertainty of around +/-0.2 to 0.25dB caused by the mating connection between the two reference cables. Those experiments also showed the uncertainty of the 3-cable reference was not significantly larger than the 2-cable reference.

When possible, use a 1-cable reference. When you must use the 2- or 3-cable reference, inspect and clean all connectors carefully before making connections for the reference or test.

Bottom Line:
  • The value of loss you measure depends on how you set your "0dB" reference - more reference cables means less loss.
  • Connections between reference cables when setting a 0dB loss add uncertainty to measurements

Troubleshooting With A VFLFibers Damaged In Splice Trays

Is this a trend? Twice in one week, we have inquiries from readers with problems and both were traced to fibers cracked when inserted in splice trays. The photo below shows one of them illuminated with a VFL. This was the same issue we found in the first field trial of a VFL more than 30 years ago that led to its popularity in field troubleshooting.

Broken fiber found with visual fault locator
Photo courtesy Alan Kojima.

Bottom LIne:
  • VFLs are invaluable troubleshooting tools for finding cable faults
  • But only work close by - 3-4km range max

How "Fast" Is Fiber?

We've probably all heard the comment that fiber optics sends signals at the speed of light. But have you ever thought about what that speed really is? The speed of light most people think about is C = speed of light in a vacuum = 300,000 km/s = 186,000 miles/sec. But in glass, the speed is reduced by about 1/3 caused by the material in the glass. The light is slowed down and the amount is defined as the index of refraction of the glass. V= speed of light in a fiber = c/index of refraction of fiber (~1.46) = 205,000 km/s or 127,000 miles/sec. So in glass, the "speed of light" is about 2/3 C, the speed of light in a vacuum.  And the difference in speed in different materials is what makes fiber work - causing "total internal refraction".

One of the FOA instructors sent us this question:  "I work with at Washington Univ with an engineer who works for an electrical utility. He asked a question about the speed of signal transmission over fiber optics, single mode, at top of towers. They need signal to be sent in 18 millisecs for relays to function properly. Is there a problem over a distance of 150 miles?"

Electrical transmission lines

Let’s do a calculation:

C = speed of light in a vacuum = 300,000 km/s = 186,000 miles/sec
V= speed of light in a fiber = c/index of refraction of fiber (~1.46) = 205,000 km/s or 127,000 miles/sec
150 miles / 127,000 miles/sec = 0.00118 seconds or ~1.2 milliseconds

Another way to look at it is 127,000 miles/sec X 0.018 seconds (18ms) = 2,286 miles

So the fiber transit time is not an issue. The electronics conversion times might be larger than that.

I used to explain to classes that light travels about this fast:

300,000 km / sec
300 km / millisecond
0.3km /microsecond or 300m / microsecond
0.3 m per nanosecond - so in a billionth of a second, light travels about 30cm or 12 inches

Since it travels slower by the ration of the index of refraction, 1.46, that becomes about 20cm or 8 inches per nanosecond.

That is useful to know since an OTDR pulse 10ns wide translates to about 200cm or 2 m pr 80 inches (6 feet and 8 inches), giving you an idea of the pulse width in distance in the fiber or an idea of the best resolution of the OTDR with that pulse width. 

Bottom Line
  • Fiber is "fast" because of its bandwidth capability
  • Light travels in fiber at the speed of light
  • But the speed of light in glass is only 2/3 as fast as the speed of light in air or a vacuum

What Does A FTTH ONT Look Like Today?


That's all there is to the ONT that goes into the home. The arrow points to the 1310 TX/1490 RX transceiver for SC-APC connectors.

Note: That device is now priced at $10-20US. The incredibly high volume of FTTH components has drive costs of these devices down to incredibly low levels. Today media converters for SM fiber are as priced the same as MM. Multimode fiber is beginning to look obsolete.


Here are several technologies that have continued growing in importance in the fiber optic marketplace  -  components that every tech needs to learn about and become familiar with their use.

How Many Fibers? - What's The Optimal Cable Size?

The idea often arises to reduce the number of fibers in a cable and therefore reduce the cable cost, assumed to be important on long cable runs. But is the cost of fiber such a big part of the cost of the cable plant? We decided to analyze cable costs for standard loose tube cable capable of being pulled into conduit for underground or lashed to a messenger for aerial installation.

Gathering data was not easy, but we found several large, reputable US distributors who listed prices for several types of loose tube singlemode OSP cables from top cable makers. All prices are for small quantities (km, not 10s or 100s of km).  Prices are how they were quoted, in $US per foot, so our readers outside the US should feel free to convert into another currency and meters.

This graph shows what we found:

cable cost

The curve looks reasonable above 24 fibers, but unpredictable below that, so we analyzed the data by cost per fiber per foot and got the graph below.


The cost per fiber per foot increases rapidly below 24 fibers, probably because the cost of making cable doesn't change much with fewer fibers; it's the cost of the plastics, strength members and manufacturing process that dominates the cost. However, after 24 fibers, the cost settles down and slowly decreases for higher fiber counts, reflecting then the cost of the added fibers.

Another way to think of this is that below 24 fibers, you are paying for the cable; above 24 fibers you pay for the fibers.

The thing to note of course is the cost of each fiber is less than 2 cents per foot for any cable above 24 fibers. When OSP construction costs are $5-25 or more per foot, the cost of fiber seems to be quite cheap. Certainly installing cable with additional fibers is very cost effective if it means having fibers to expand the network without having to install another cable. And, of course, that applies to urban and suburban networks, not just rural.

Micros: Microcables, Microducts and Microtrenching

Corning MiniXtend cable

144 fiber Corning MiniXtend cable is smaller than a pencil

MIcrocables, microducts and microtrenching - three technologies that have more in common than the prefix "micro" are gaining in acceptance along with blown cable, the obvious method of installation using them. Smaller is always better when it comes to crowded ducts, especially in cities where duct congestion is a problem in practically every city we have contact with.

Bottom Line:

  • Like everything else, cables keep getting smaller
  • Work well with microducts and microtrenching
  • Installers need to become familiar with "blown cable" technology
  • They are already accepted in the marketplace

Fiber Ducts

With the demand for more fiber for smart cities services like small cells and smart traffic signals, not to mention a ton of other smart cities services, installing more cables in current ducts - without digging up streets - is a major interest. Sometimes it's possible to install microducts in current ducts with a cable and blow in a new microcable. Sometimes it's worth it to pull an older cable out and install a new microduct that will accommodate 6 cables, making room for future expansion. The makers of the fabric ducts, Maxcell, can even show you how to remove the ducts in conduit without disturbing the current cables and pull in fabric ducts to install more cable.

Comparison of MaxCell ducts to rigid plastic duct

samples of microducts

Microducts are small ducts for blowing in cable. In the size of a traditional fiber duct, you can get 6 microducts for 6 288 fiber cables.

Microducts And Microtrenching

Nearly invisible microtrenching
Nearly invisible microtrenching

If you have to trench, microtrenching is probably the best choice for cities and suburbs. Rather than digging wide trenches or using directional boring (remember the story about the contractor in Nashville, TN using boring to install fiber who punctured 7 water mains in 6 months?), microtrenching is cheaper, faster and much less disruptive.

All of this implies that contractors are willing to invest in new machinery and training, sometimes an optimistic assumption. Microtrenching machines and cable blowing machines are available for rent, but personnel must be trained in the design of networks using these technologies and operating the actual machinery in the field. That's still a considerable investment.

Bottom Line:
  • Cables and ducts are getting smaller allowing more and more fibers in the same space
  • Microtrenching allows "construction without disruption"

High Fiber Count Cables

More manufacturers are introducing high fiber count cables - 864, 1728, 3456 or even 6,912 fibers. Like this one from Prysmian with 1728 fibers: The applications were first in large-scale data centers but are also seeing use in dense urban centers to support FTTH and cellular small cell systems.

Prysmian 1728 fibers

These cables use bend-insensitive fibers to allow high density of fibers without worrying about crushing loads affecting attenuation. Most also use fibers with 200 micron buffer coatings instead of 250
micron buffer coatings to allow even higher density. Many, or even most, use ribbons of fiber, either the conventional hard ribbons or the newer flexible ribbons, since, as we show below, the time to splice even a 1728 fiber cable is extremely long unless ribbon splicing is used.

High Fiber Count Cables may not be for everyone. Maybe only for a very few. A single cable that has as many fibers as 12-144 fiber cables (1728 fibers) in a cable with a diameter of only twice that of a conventional 144 fiber cable can present challenges.

  • First of all, the cost - it's high. You do not want to waste cable at this price. Engineering the cable length precisely will save lots of money.And it's worse for higher fiber counts.
  • Likewise, making mistakes when preparing the cable for termination can be expensive.
  • The cable may require special preparation procedures to separate fibers for termination. Most use new methods of identifying cables and bundles.
  • Besides skill, the tech working with high fiber count cables needs lots of patience.
  • Splicing multiple cables at a joint can get complicated keeping all fibers straight.
  • These cables will generally use 200 micron buffered fiber and often a flexible ribbon instead of a typical rigid ribbon structure to reduce fiber sizes. This may complicate splicing as the methodology to splice the flexible fibers and splice 200 micron fibers to regular 250 micron fibers is a work in progress.
  • Splicing 200 to 250 micron fibers may be easier with the flexible ribbon designs which make it easier to spread fibers to the same spacing.
  • We've heard the splicing time for flexible ribbons is about 50-100% longer than that of conventional rigid ribbons. So if you use that table below, you may need to increase your ribbon splicing estimates when working with flexible ribbons.

We've been looking for directions on how to deal with high fiber count cables. Several contractors tell us ribbon splicing is the way to go, and most of these cables now use a version of the new ribbon types that are flexible. We've  put together this table from some articles on splicing ribbons:

Corning generously sent FOA some samples of 1728 and 3456 "RocketRibbonTM" cable. We took some photos and must admit that these cables are fascinating updates on the traditional fiber optic cables.

high fiber count cable

Here are Corning RocketRibbon 1728 fiber (bottom) and 3456 fiber (top) cables. To get an idea of these cables size, look at this photo:


The 3456 fiber cable is 32mm diameter, 1.3 inches. The 1728 fiber cable is 25mm, 1 inch diameter.

These are cables made from conventional "hard" ribbons, not the "flexible" ribbons used on some cable designs. As a result of using hard ribbons, the fibers are arranged in regular patterns to get high density.


These are the tubes of ribbons from these cables. Each of those tubes of ribbons has the equivalent of 24 ribbons of 12 fibers each (actually 8 X 12 fibers and 8 by 24 fibers stacked up) for 288 fibers total. The 1728 fiber cable has 6 tubes and a center foam spacer, with 144 ribbons total. The 3456 fiber version has 12 tubes and no spacers, 288 fiber ribbons total.

What amazes us is the density of fibers.


We calculated the "fiber density" of this 3456 fiber cable based on 200 micron buffered fibers and determined that 54% of the cable is fiber. Compare that to a typical 144 fiber loose tube cable, which is about 14% fiber or a 144 fiber microcable which is about 36% fiber.

We're hearing rumors that the new high fiber cables are getting fibers broken during installation with the possible cause(s) being exceeding bend radius or pulling tension, using improper installation equipment or maybe even the cable designs. We're investigating this and will report back in the near future. But please ensure installers follow manufacturer's recommendations carefully.

From a reliable source within the industry: Within a couple of years, the old inflexible  hard ribbon cables will be extinct. Everything will be flexible ribbons and mostly 200 micron fibers and BI (bend insensitive) fibers (G.657). Besides changing how these cables are handled, one thing will be lost - the ability to print ID info on the ribbons so matching fibers to splice will be more difficult.

Looking at the end of this cable reminded us of nothing so much as this PR photo from AT&T from their introduction of fiber optics in 1976:


Not the fiber, the dense cable of copper pairs!

Of course the cable is much lighter than copper but much heaver than you are used to with fiber - it weighs 752 kg/km or about 1/2 pound per foot. And it's stiff. Very stiff. The minimum bend radius is 15 times the cable diameter or 480mm (~19 inches), about a meter or yard in diameter.

As we noted in the photo above, Ian Gordon Fudge of FIBERDK taught some data center techs how to handle a 1728 fiber Sumitomo cable with a slotted core. Ian sent FOA this photo to illustrate the number of fibers in the cable he was using for training. Impressive!

Fiber DK

Here is the slotted core that separates the flexible fiber ribbons
in the Sumitomo cable:

slotted core

More on high fiber count cables and our continuing coverage

ribbon splicing

High fiber count cables are all ribbon cables, some with hard ribbons and some with flexible ribbons, All require ribbon splicing because of the construction and the time it would take to terminate them. This is a table of estimated termination times. Is that realistic? We've heard the flexible ribbons may take
50-100% longer than conventional ribbons due to the need to carefully arrange and handle fibers.

High Fiber Count Cables - Continued Updates - Installation

Continuing our ongoing research on high fiber count cables, last month we were invited to visit Corning's OSP test and training facility to experience the processes of installing these cables for ourselves. We had the opportunity to handle some of these cables ourselves and see how experienced techs worked with this cable.

Once you get a chance to handle this cable and see how big, stiff and heavy it really is, you understand that it's quite different from any fiber optic cable you have worked with, with the possible exception of some hefty 144/288 fiber loose tube cable that's armored and double jacketed. With a bend radius of 15X the diameter of the cable, the minimum bend radius of a 1728 fiber cable is 15" (375mm) and that's a 30" (750mm - 3/4 of a meter) diameter. Just the reel it's shipped on is outsized - it should have a ~750mm (30 inch) core and will be probably ~1.8m (6 feet ) in overall diameter. 3300 feet (1km) of this cable will weigh 550-750kg (1200-1700 pounds.) and the reel will weigh another ~300-400kg (700-900 pounds). Will that fit on your loading dock? Can you handle a ton of cable? (Metric or English)

I tried bending one of the 1728 fiber cables and (with the manufacturer’s OK) tried to break it. The 1728 fiber cable I was bending took an enormous amount of muscle to bend, and when I got down to about an 8 inch radius, it broke, with a sound like a tree limb of similar diameter cracking. In the field, that would have been an expensive incident.

The stiffness of these cables affects the choice of other components and hardware. You will not fit service loops into a typical handhole, you need a large vault like the one shown in the photos taken at Corning. You will also need close to 100 feet (30m) of cable for a service loop. You may need to figure 8 the cable on an intermediate pull and that will require lots of space and a crew to lift the cable to flip it over.

This 1728 fiber cable is stiff, does not easily twist and only bends in one direction because there are stiff strength members on opposite sides of the cable. Placing it into a manhole or vault and fitting service loops into it is not easy. In this case, it helped to have two people and one was the trainer. You need to have a "feel" for the cable - how it bends and twists - to make it fit. The limits of bend radius, stiffness and unidirectional bending makes it necessary to work carefully with the cable to fit loops into the vault. Sometimes it's necessary to pull a loop out and try in a different way to get it to fit. But it can be done as you see at the right.

cable handling
Pulling the cable out of conduit in the vault without damaging it also requires care. You can see in the back the orange duct coming into this vault. When pulling the cable, it's important to not kink the cable while pulling it out of a duct. A length of stiff duct can be attached to the incoming duct to limit bend radius. Capstans, sheeves and radius cable sheaves need to be chosen to fit the required cable bend radius. A a radius cable sheave with small rollers can damage the cable under tension and are bot a good choice unless the rollers are used with a piece of conduit to just set the bend radius.

Corning also showed us a new feature of their RocketRibbon Cables. A high fiber count cable has a lot of fibers, even a lot of ribbons, so identifying ribbons can be a problem. In addition to printing data on each ribbon, Corning now tints the ribbons with color codes to simplify identification. Great idea.

tinted ribbons

Tight Fit: 6912 Fiber Cable Pulled in 1.25 inch Conduit

Furukawa Electric Co., Ltd. (FEC) conducted an experiment in its Mie, Japan facility to demonstrate the installation of a 6912-fiber optic cable with an outer diameter of 1.14 inches (29 mm) in a 696 foot (200m) long conduit with three 90 degree curves and an inner diameter of 32mm. The conduit used was a standard product installed in conventional data center campuses. Engineers confirmed a maximum pulling tension of 84 pounds (372N), well below the maximum pulling tension of 600 pounds (2700N) specified for the cable.

FEC Cable  FEC Cable

The cable was installed in a 1.25 inch (32mm) conduit with a maximum length of 1,411 feet (430m) in a North American data center campus in 2020 to support live traffic. The high fill ratio in this application is not typically recommended for Outside Plant (OSP) cable installation. However, in this application, the end-user was willing to accept the installation risk in return for maximum fiber density. The installation demonstrated that FEC’s 6912 fiber optic cable can be successfully installed into 1.25 inch (32mm) conduit using appropriate tools, work procedures, and optimum installation conditions.

“The FEC 6912 fiber optic cable at least doubled the fiber count possible in a 1.25 inch conduit, compared to competing available designs,” said Ichiro Kobayashi, General Manager of optical fiber & cable engineering department, FEC.

Furukawa PR also on OFS Website. OFS is a FEC company.

Bottom Line
  • High fiber count cables allow extremely high fiber counts in small cable sizes, perfect for dense applications in data centers and metro areas
  • With so many fibers, ribbon splicing is the only sensible way to splice them
  • Ensure you splicing machines can handle 200micron buffer fibers
  • Because bend radius limits are so high, they require special consideration for installation and storage - BIG manholes for example

Cable Marking Mystery

You are all familiar with the information printed on a typical fiber optic cable which includes the manufacturer, how many fibers in the cable and distance markings, plus sometimes other information like the manufacturer's part number.

But recently two people made reference to the small symbol that looks like an old-style telephone handset. One thought the manufacturer of the cable used that symbol to show where the helical winding of the buffer tubes reversed, a reference point for preparing the cable for midspan access. Another thought it was to indicate this was a telecom cable not a power cable.

telephone symbol on cable

FOA has been reaching out to people at cable companies to see if anyone has a definitive answer as to what this symbol means, and the answer comes from Rodney Casteel and his engineers at Commscope.

"The handset symbol is mandatory for cables “suitable for direct burial applications” per ICEA 640 and Telcordia GR-20.  I think this handset symbol started a long time ago so data cables could be identified if they were dug up. My guess is the first standard to mandate this was Telcordia GR-20 Issue 1 back in the 80s."

From ICEA-640:

telephone symbol

And Bellcore/Telcordia GR-20:

telephone symbol

More information on interpreting cable jacket markings   

Cable Markings

New Connectors

We're seeing some interesting new connectors being introduced. 3M announced a multifiber array connector using expanded beam technology and several new ideas of making a duplex connector smaller.

3M Expanded Beam Connector
3M Expanded Beam connector 3M

Details are sketchy but from the video on the 3M website, the connection is made by a small plastic fixture that is shown by the arrow in the top photo. The plastic seems to turn the beam 90 degrees so the connection is made when two pieces overlap., in the direction of the arrow in the lower photo. The connectors are hermaphroditic - that is two identical connectors can mate. There are models for singlemode and multimode fibers and you can stack the connection modules to handle up to 144 fibers. We understand this was not part of the 3M fiber optic product line recently acquired by Corning. 3M Expanded Beam Connector.  

For more information on expanded beam connectors, see the FOA Newsletter for October 2018 that discusses the R&M QXB, another multifiber expanded beam connector announced last Fall. 

In the FOA Newsletter for January 2018, we featured the SENKO CS connector, a miniature duplex connector using two 1.25mm ferrules, but much smaller than a duplex LC. The CS is sell on its way to becoming standardized with a FOCIS (fiber optic connector intermateabliity standard), but on the SENKO web page, there is another new connector, the SN, that makes the SC look huge! The big difference is the vertical format that allows stacking connectors very close. That can allow transceivers to have more channels, a big plus for data centers. Here is more information on the SENKO CS and SN connectors.


Comparison of SENKO CS (L) and SN (R) connectors with duplex LC.

US Conec MXC and MDC Connectors
The R&M and 3M expanded beam multifiber connectors reminded us that US Conec introduced the
MXC connector over 5 years ago, using similar technology for up to 64 fibers per connector. The MXC is on the US Conec website, but seems to be aimed at board level connections, not far off its original purpose as a connector for silicon photonic circuits. But when we checked the US Conec website, there was a connector name we dis not recognize, the MDC. The MDC (below) is a vertical format duplex connector using 1.25mm ferrules that looks similar to the SENKO SN above. Here is information on the US Conec MDC duplex connector.

US Conec MDC connectorUS Conec

Its All About The Data Center
Just like the high fiber count cables discussed above, the CS, SN and MDC connectors are aimed at high density cabling and transceivers for data centers. All three are specified for the new QSFP-DD pluggable transceiver multi-source agreement.

Bottom Line:
  • Like everything else, connectors keep getting smaller
  • Too early to determine if they will be accepted in the marketplace and can compete with LCs

Splice-On Connectors

Terminating with SC SOC in EasySplicer

Termination has been seeing greater acceptance of the SOC - splice-on connector - using fusion splicers. It's popularity started in data centers for singlemode fiber where the number of connections is very large so the cost of a fusion splicer is readily amortized and the speed of making connections is the real cost advantage. The performance of SOCs is much better than prepolished/splice (mechanical splice) connectors simply because of the superiority of a fusion splice and the cost of the SOCs are much less since they do not have the complex mechanical splice in the connector.

We have used SOCs in training and the techs take to them readily. In classes you can combine splicing and termination in one session. The cost of fusion splicers has been dropping to near the cost of a prepolished/splice (mechanical splice) connector kit so the financial decision to use SOCs is easier to make.

Bottom Line:
  • Splice-On Connectors (SOCs) are easy to install, low loss and low cost
  • Less hardware than pigtail splicing
  • Premises or OSP

Splice-On Connector Manufacturers and Tradenames   7/2020

FOA Master Instructor Eric Pearson of Pearson Technologies shared a list he has researched of prepolished splice connectors with mechanical splices and SOC - splice-on connectors for fusion splicing. This list shows how widepread the availability of these connectors has become, especially the SOCs and low cost fusion splicers.

Mechanical Splice
1.    Corning Unicam® (50, 62.5, SM)
1.    FIS Cheetah (???)
2.    Panduit OptiCam® (50, 62.5, SM)
3.    Commscope Quik II  (50, 62.5, SM)
4.    Cleerline SSF™ (50, SM)
5.    LeGrand/Ortronics Infinium® (50, 62.5, SM)
6.    3M/Corning CrimpLok (50, 62.5, SM)
7.    Leviton FastCam© (50, 62.5, SM)

Fusion Splice
2.    Inno (50, 62.5, SM)
3.    Corning FuseLite® (50, SM)
4.    FORC (50, 62.5, SM)
5.    Siemon OptiFuse ™ (SM, MM)
6.    Belden OptiMax?? FiberExpress (SM, MM)
7.    AFL FuseConnect® (SM, MM)
8.    OFS optics EZ!Fuse ™ (50, 62.5, SM)
9.    Sumitomo Lynx2 Custom Fit® (50, 62.5, SM)
10.    Commscope Quik-Fuse (50, SM)
11.    Ilsintech Pro, Swift® (50, 62.5, SM)
12.    LeGrand/Ortronics Infinium® (50, 62.5, SM)
13.    Greenlee (50, 62.5, SM)
14.    Hubbell Pro  (50, SM)
15.    Easysplicer (SM)

Note: There are additional manufacturers from the Peoples Republic of China, which advertise on Amazon and eBay.


Midspan Access - Simplifying Installation Of Drops

Many installations involve dropping a small fiber count cable from a large backbone cable. Backbone cables of 144-288 fibers are common and larger ones are becoming more common too. Drop cables are often only 2-14 fibers, meaning most fibers are continuing straight through the drop point. Midspan access involves opening the cable by removing the jacket and strength members, opening the buffer tube and splicing only the fibers being dropped at that point. The untouched buffer tubes from the opened cable are carefully rolled up and stored in the same splice closure as the fibers that will be separated and spliced to a drop cable.

Midspan access

If there is a method of splicing only the 4 drop fibers instead of the 144 fibers, we will only have 4 splices instead of 144 or 146 depending on the architecture of our system. The difference is according to how the drop is configured.

midspan access
If you are building a star network where every drop links back to the origin of the network, you will splice 4 fibers in the cable to the drop cable, leaving 4 splices on 4 fibers (instead of 144 splices if the backbone cable is cut and respliced.

midspan access
If you are building a ring network, you may only be splicing two fibers going to the drop and two others that are continuing along the ring network.

All this may seem obvious but in actual practice requires some knowledge, skills and careful workmanship. To do a proper job. Fortunately, manufacturers of cables and tools have good information available online on how to do it, and FOA Master Instructor Joe Botha has provided FOA with a application note on how midspan access is done in his classes also.

The basic process is simple. We will look at a loose tube cable but processes exist for ribbon cables also. You remove the jacket of the cable for a specified length according to the cable type and splice closures used. After removing the cable jacket, you remove unnecessary strength members, leaving enough of the stiff central member on both ends to attach to the splice closure. Identify the tube with the fibers to be spliced to the drop cable and set aside while carefully coiling the other tubes for storage in the closure.

To open the buffer tube, you need a midspan access tube that shaves off a section of the tube to allow removal of the fibers without damaging them. Here two types of Miller tools that shave the tube:

midspan access tool  midspan access

After shaving the tube and removing the fibers - count carefully to ensure you remove all the fibers! - you can cut the tube off to have bare fibers only for the length you need to splice on the drop cable. All these fibers will be placed in a splice tray for safe storage but only the fibers being dropped will be cut and spliced to the drop cable. This is what the closure will look like, ready for splicing the drop cable.

midspan access

In the case of the particular user who contacted us, not every drop would use midspan access. His cable plant was 15miles (25km) long with roughly 17 locations where cable drops were needed. The cable he was using could only be made in 5km lengths, so there would have to be several locations where the cable would be spliced in the 25km run.

The design would need to carefully determine how much cable was needed along each section of the route, including lengths for service loops and midspan access or splicing, to determine which drop points would be using midspan access ans which would be used as splice points for the entire cable.

That's why fiber optic network design is important but sometimes complicated.

Search online for "midspan access" to find lots of application notes and videos on the subject. Or talk to your fiber optic cable vendors.

FOA Guide Page on Midspan Access

Nanotrenching Failure In Louisville, KY

Google FIber tried a new way to install cable in Louisville, KY, that turned out to be a very expensive failure. Nanotrenching is what some call very shallow trenching for installing fiber optic cable - see the photo below - and filling with rubber cement. It did not work.

Google Fiber ending service in Louisville

Chris Otts, WDRB Louisville,
Feb 7, 2019

LOUISVILLE, Ky. (WDRB) – Google Fiber is leaving Louisville only about a year after it began offering its superfast Internet service to a few neighborhoods, citing problems with the method it used to build the network through shallow trenches in city streets.

The shut off will happen April 15, said Google Fiber, a unit of Silicon Valley tech giant Alphabet, in a blog post Thursday.

Google Fiber has served about a dozen cities, and Louisville is the first it has abandoned."

Shortly after Google announced Louisville as a possible location in 2015, the Metro Council passed a utility pole ordinance at Google’s behest, then spent $382,328 on outside lawyers to defend the ordinance in lawsuits from AT&T and the cable company now called Spectrum.

Mayor Greg Fischer said in early 2016 that Louisville’s landing Google Fiber was “huge signal to the world.”

Louisville’s public works department allowed Google Fiber to try a new approach to running fiber – cutting shallow trenches into the pavement of city streets to bury cables.

It led to a lot of problems, including sealant that popped out of the trenches and snaked over the roadways.


Louisville street,
Copyright 2019 WDRB Media. Reproduced with permission.

It feels like you are using us for a science-fair experiment,” Greg Winn, an architect who lives on Boulevard Napolean, told Google Fiber representatives during a Belknap Neighborhood Association meeting last year. “…Our streets look awful.”

Google Fiber would go on to fill in the trenches with asphalt, what company executives said was like filling a 60-mile long pothole.

Google Fiber never ended up using the utility pole law -- a policy called One Touch Make Ready -- that Louisville passed at its behest, as the company only buried its wires instead of attaching them to poles.

A public relations representative for Google Fiber said no one was available for an interview.

In written responses, the spokesman said Google Fiber initially chose not to use the utility pole access because of "uncertainty" about whether the ordinance would hold up. Now that it has cut trenches in the streets, the company has no desire to start over.

Even using (One Touch Make Ready), we’d need to start from scratch, and that’s just not feasible as a business decision," the spokesman said.

FOA: Be sure to watch the video from WDRB.

Copyright 2019 WDRB Media. Reproduced with permission.

Bottom Line:
  • Before you try some new idea, ask some experienced installers what they think

Manhole/Handhole Size

Q: What you recommend when it comes to manjole/handhole sizing.  If they are being used for splicing, do you have a general formula of length of splice closure plus X factor more for cables in/out of closure and slack storage? 


A: FOA has been doing some research on underground construction to expand our section in the FOA Guide. We are looking at what people are specifying on some projects since we do not know of any industry standards.

There are links of some interesting/useful information below.

From our standpoint, the minimum size would be determined by the bend radius of the fiber optic cable (see article above), how much slack (service loops) would be stored (slack from how many cables - see photo below) size of the splice closures, and how many ducts and cables would be served. Generally you will have 20-30 feet in service loops to allow for splicing, Typical cable up to 1/2” needs a loop >20” but I don’t know how you would ever get a loop that small for that much cable, so you probably have minimum 2’ loops, at least 5 coils. Add a closure and you probably need a 2’X4’ handhole, at least 2’ deep, as a minimum - see the “good” photo below.

We were at Corning training last Spring on high fiber count cables and those cables require ~6’ X 4” min manholes just to fit the loops of cable. Handholes can be smaller, depending on the type of splice or drop, midspan access, etc.

The FOA Guide pages on OSP Construction created by Joe Botha for his course in South Africa talks about manholes and handholes on this page near then end.

This Jensen web page shows the number of different designs and sizes.

Detail from Central FL Expressway Design Standards offers several sizes: 4' X 4' X 4',  4' X 6.5' X 6.5', 4' X 6.5' X 6.5' and specifies a duct organizer.

Here is a Wisconsin DOT spec for a 4’ diameter manhole.

NYC Broadband General Network Specifications:  see page 24ff

Bottom Line:
  • Manholes need to be big enough for the cables they must contain
  • They usually aren't!

Installation - Cleaning

Bad Advice

Our inbox recently had a message with this thought:

"It is time for spring cleaning, and we don't mean just at home. Contaminated fiber end faces remain the number one cause of fiber related problems and test failures. With more stringent loss budgets, higher data speeds and new multifiber connectors, proactively inspecting and cleaning will help you ensure network uptime, performance, and reliability. Despite "everyone" knowing this, fiber contamination and cleaning generates a lot of failed test results."

Well, experience tells us that "proactively inspecting and cleaning" can generate a lot of damage to operating fiber optic networks.

Inspection and cleaning should be done whenever a fiber optic connection is opened or made, of course. But the act of opening the connection exposes it to airborne dirt and the possibility of damage if the tech is not experienced in proper cleaning. Fiber optic connections are well sealed and if they are clean when connected, they will not get dirty sitting there. Fiber optic connections do not accumulate unseen dirt like under your bed or sofa, requiring periodic cleaning, as implied in this email.

Clean 'em, inspect 'em to ensure proper cleaning, connect 'em and LEAVE THEM ALONE!!!

And, duh, remember to put dust caps on connectors AND receptacles on patch panels when no connections are made

Was this perhaps another early April Fools' this one we ran several years ago about the wrong way to clean connectors:

Connector cleaning - NOT!

Why You Clean Connectors Before You Make Connections

Brian Teague of Microcare/Sticklers send us this series of photos showing what happens when you make connections with dirty connectors. It speak for itself!

Dirt on fiber optic connectors

How To Backfill A Trench For Underground Construction

backfill a trench

Here's the answer to a question we've gotten. Where did we find the answer? In the new FOA Guide section on OSP Construction developed using Joe Botha's OSP Construction Guide which is published by the FOA. Joe's book covers underground and aerial installation from a construction point of view, covering material after the FOA's design material and before you get into the FOA's information on splicing, termination and testing.

DO NOT FORGET THE MARKER TAPE! It makes the cable easy to locate and hopefully prevent a dig-up.

The 2019 update of the FOA Reference Guide To Outside Plant Fiber Optics contains this and lots of other new material on OSP construction.

FCC Adopts One Touch Make Ready (OTMR) Rules For Utility Poles

On August 3, The US Federal Communications Communications Commission adopted a new rule that allows "one-touch make-ready" (OTMR) for the attachment of new aerial cables to utility poles. From the FCC explanation of the rule, "the new attacher (sic) may opt to perform all work to prepare a pole for a new attachment. OTMR should accelerate broadband deployment and reduce costs by allowing the party with the strongest incentive to prepare the pole to efficiently perform the work itself."

You may remember that FOA has reported on the "Pole Wars" for several years. Battles over making poles available and/or ready for additional cable installation has been slowing broadband installations for years and now threatens upgrading cellular service to small cells and 5G in many areas.

Is OTMR A Good Idea?

OTMR has the potential to speed deployment of new communications networks if handled properly. However, one hopes the installers doing OTMR know what they are doing. We've heard so many horror stories about botched installations, cut fiber and power cables, punctured water mains and gas lines done by inept contractors that we just hope this doesn't cause even more trouble.

For example, here are 2 poles in the LA area where small cells are being installed. Can just any contractor handle OTMR on these poles?

Pole wars  figure 8 on a pole

Bottom Line
  • OTMR may be problematic if contractors doing installation are not competent

Fiber Optic Testing

Test Sources For Multimode Fiber Testing

One of our schools recently asked up for recommendations on test sources for multimode fiber, wondering if the sources should be a LED or laser. Multimode test sources are always LEDs and these sources should be always used with a mode conditioner, usually a mandrel wrap. See here. This is how all standards for testing multimode fiber require test sources.

Years ago, as systems got faster and LEDs were too slow at speeds above a few hundred Mb/s. Fortunately 850nm VCSELs were invented to provide the solution for faster transmitters. But VCSELs were not good for test sources. They had variable mode fill and modal noise, so testers continued using LEDs for test sources, but with mode conditioners like the mandrel wrap that filtered out higher order modes to simulate the mode fill of an ideal VCSEL

The bigger issue with MM fiber is whether to test at both 850 and 1300nm. In the past, we did both because there were systems that used 1300nm LEDs or Fabry-Perot lasers for sources because the fiber attenuation was lower at 1300nm than 850nm. As network speeds increased to 1Gb/s and above, bandwidth became the limiting factor for distance, not attenuation.  VCSELs only worked at 850nm and all systems in MM basically have been switched to 850nm VCSELs.

We also used to test at both wavelengths because if a fiber was stressed, the bending losses were higher at 1300nm, so you could determine if a fiber had problems with stress. But since MM fiber has all gone to bend-insensitive fiber, that no longer works and the need or reason to test at 1300nm went away. It has not been purged from all standards yet however.

To complicate things, standards say that you should not use bend-insensitive fiber for test cables (launch or receiver reference cables) because they modify modal distribution, but it’s a moot point - practically all MM fiber is bend-insensitive so you have no choice but to use it. And most links will have BI to BI connections that should be tested. But we checked with some technical contacts and there are no specifications for BI fiber mandrels as mode conditioners.

Best solution - 850 LED with a mode conditioner on non-BI fiber (if you can find it - see above).

Bottom Line

  • Multimode fiber needs testing with a 850nm LED source

Power Budgets and Loss Budgets

Not only was this topic a long discussion with our new instructors but it's a common question asked of the FOA - we received two inquiries on loss budgets in the last month alone. The confusion starts with the difference between a power budget and a loss budget, so we'll start there. and we'll include the points where we were stopped to explain things.

What's The Difference Between Power Budget And Loss Budget?
  • A power budget is the amount of loss the link electronics can tolerate - transmitter to receiver. You use this to compare to the cable plant link loss budget when designing a cable plant to ensure the link will work on the cable plant design.
  • The link loss budget is the estimated loss of the fiber optic cable plant including the loss of the fiber, splices and connections. You compare that to the power budget to ensure the link will work on the cable plant being designed, then again after installation to compare to test results.

Consider this diagram:
Fiber optic loss budgets
At the top of the diagram above is a fiber optic link with a transmitter connected to a cable plant with a patchcord. The cable plant has 1 intermediate connection and 1 splice plus, of course, "connectors" on each end which become "connections" when the transmitter and receiver patchcords are connected. At the receiver end, a patchcord connects the cable plant to the receiver.

Definition: Connection: A connector is the hardware attached to the end of a fiber which allows it to be connected to another fiber or a transmitter or receiver. When two connectors are mated to join two fibers, usually requiring a mating adapter, it is called a connection. Connections have losses - connectors do not.

Below the drawing of the fiber optic link is a graph of the power in the link over the length of the link.  The vertical scale (Y) is optical power at the distance from the transmitter shown in the horizontal (X) scale. As optical signal from the transmitter travels down the fiber, the fiber attenuation and losses in connections and splice reduces the power as shown in the green graph of the power.

Comment: That looks like an OTDR trace. Of course. The OTDR sends a test pulse down the fiber and backscatter allows the OTDR to convert that into a snapshot of what happens to a pulse going down the fiber. The power in the test pulse is diminished by the attenuation of the fiber and the loss in connectors and splices. In our drawing, we don't see reflectance peaks but that additional loss is included in the loss of the connector.

Power Budget: On the left side of the graph, we show the power coupled from the transmitter into its patchcord, measured at point #1 and the attenuated signal at the end of the patchcord connected to the receiver shown at point #2. We also show the receiver sensitivity, the minimum power required for the transmitter and receiver to send error-free data. The difference between the transmitter output and the receiver sensitivity is the Power Budget. Expressed in dB, the power budget is the amount of loss the link can tolerate and still work properly -
to send error-free data.

Link Loss: The difference between the
transmitter output (point #1) and the receiver power at its input (point #2) is the actual loss of the cable plant experienced by the fiber optic data link.

Comment: That sounds like what was called "insertion loss" with a test source and power meter. Exactly! Replace "transmitter" with test source, "receiver" with power meter and "patchcords" with reference test cables and you have the diagram for insertion loss testing which we do on every cable.

The loss of the cable plant is what we estimate when we calculate a "Link Loss Budget" for the cable plant, adding up losses due to fiber attenuation, splice losses and connector losses. And sometimes we add splitters or other passive devices.

Margin: The margin of a link is the difference between the Power Budget and the Loss of the cable plant.

Determining The Power Budget  For A Link

Question: How is the power budget determined? Well, you test the link under operating conditions and insert loss while watching the data transmission quality. The test setup is like this:

Measuring fiber optic link power budget
Connect the transmitter and receiver with patchcords to a variable attenuator. Increase attenuation until you see the link has a high bit-error rate (BER  for digital links) or poor signal-to-noise ratio (SNR for analog links). By measuring the output of the transmitter patchcord (point #1) and the output of the receiver patchcord (point #2), you can determine the maximum loss of the link  and the maximum power the receiver can

From this test you can generate a graph that looks like this:
fiber optic BER
A receiver must have enough power to have a low BER (or high SNR, the inverse of BER) but not so much it overloads and signal distortion affects transmission. We show it as a function of receiver power here but knowing transmitter output, this curve can be translated to loss - you need low enough loss in the cable plant to have good transmission but with low loss the receiver may overload, so you add an attenuator at the receiver to get the loss up to an acceptable level.

You must realize that not all transmitters have the same power output nor do receivers have the same sensitivity, so you test several (often many) to get an idea of the variability of the devices. Depending on the point of view of the manufacturer, you generally error on the conservative side so that your likelihood of providing a customer with a pair of devices that do not work is low. It's easier that way.

The Perils Of 2-Cable Referencing

FOA received an inquiry about fluctuations in insertion loss testing. The installer was using a two cable reference method for setting a "0dB" reference where you attach one reference cable to the source, another to the meter and connect them to set the "0dB" reference. The 2-cable reference method is allowed by most insertion loss testing standards, along with the 1- and 3- cable reference methods, although each gives a different loss value.

reference 3 ays
3 different ways to set a 0dB reference for loss testing

When a 1-cable reference is used, one sets a reference value at the output of the launch cable and measures the total loss. With a 2-cable reference, a connection between the launch and receive reference cables is included in making the reference, so the loss value measured will be lower by the amount of that connection loss. The 3-cable reference includes two connection losses so the loss will be lower still.

two cable reference
The problem with the two cable reference is the uncertainty added by including the connection between the two reference cables when setting the
"0dB" reference.

Unless you carefully inspect and clean the two connectors and check the loss of that connection before setting the
"0dB" reference, you add a large amount of uncertainty to measurements of loss. The best way to use a 2-cable reference is to set up the source and reference cable (with inspected and cleaned connectors), measure the output of the launch cable, attach the receive cable (with inspected and cleaned connectors) and measure the loss of the connection before setting the "0dB" reference. If the connection loss is not less than 0.5dB, you have connectors that should not be used for testing other cables. Find better reference cables.

The two cable reference is often used when the connectors on the cables or cable plant being tested are not compatible with the connectors on the test equipment, so you must use hybrid launch and receive cables. Then you can only reference the cable when connected to each other. In that case, you need the 2-cable reference but should expect lower loss and higher measurement uncertainty.

Experiments have shown that the uncertainty with a 1-cable reference is around +/-0.05dB while the 2-cable has an uncertainty of around +/-0.2 to 0.25dB caused by the mating connection between the two reference cables. Those experiments also showed the uncertainty of the 3-cable reference was not significantly larger than the 2-cable reference.

When possible, use a 1-cable reference. When you must use the 2- or 3-cable reference, inspect and clean all connectors carefully before making connections for the reference or test.

Bottom Line:
  • The value of loss you measure depends on how you set your "0dB" reference - more reference cables means less loss.
  • Connections between reference cables when setting a 0dB loss add uncertainty to measurements

More information: 5 ways to test a fiber optic cable plant  

Safety On The Job

bucket truck job  

Safety is the most important part of any job. Installers need to understand the safety issues to be safe. An excellent guide to analyzing job hazards is from OSHA, the US Occupational Safety and Health Administration. Here is a link to their guide for job hazard analysis.

Warning For Techs Doing OSP Restoration


FOA received an inquiry about whether techs working on restoring OSP links should be concerned about eye safety if the link used fiber amplifiers. To answer this question, we had to do some research on fiber amplifiers. The short answer is YES, you should be concerned. The long answer is more technical and includes details that every OSP tech needs to know.

See "Fiber Amps And Restoration" in the FOA Newsletter Archives..

Why We Warn You To Be Careful About Fiber Shards

Fiber in Finger

Photo courtesy  Brian Brandstetter,  Mississauga Training Consultants

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