Fiber Optic Update
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 stories for
the year, stories covering topics that FOA believes every tech
needs to know. Many of these articles are from the FOA
monthly newsletter, which you can subscribe
We also recommend the FOA "Fiber
FAQs" page with tech questions from customers originally
printed in the FOA Newsletter. We had lots of interesting
questions in 2018.
This page is part of a Fiber
U Tech Update Course.
questions? Try the FOA
Guide and use the site search.
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
radius and crush loads. Installers must understand these
specifications and know how to pull cables without damaging
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, the minimum
recommended long term bend radius is 10 times the cable
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!
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
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
When An Installer Gets it Wrong
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
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.
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
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..
cables have specifications for minimum bend radius
this spec may permanently damage the cable
radius is generally 20X cable diameter under tension - 10X
Loss: Are You Positive It’s Positive?
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.
recent post on a company’s
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
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?
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:
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
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,
The Detailed Explanation
In order 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
calculated from the ratio of optical powers expressed as power
in Watts, the standard measurement units for optical power, just
like the output of light bulbs and LEDs:
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
less than reference power, the log is negative. If the ratio is
1, the log is 0.
try a graphic explanation of this equation. We measure absolute
power in fiber optics referenced to 1 milliwatt power.
Substitute "1mw" for "reference power in the equation above and
we get power in dBm. Take a look below at this of dBm vs
optical power in the range commonly used for fiber optics
calculated with our equation above. Remember dBm means all power
is referenced to 1 milliwatt optical power.
As the power in milliwatts goes up, the equivalent in dBm become
more positive - 1 milliwatt is 0dBm and 10 milliwatts is +10dBm.
Going the other way, 100microwatts = 0.1 milliwatt = -10dBm,
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:
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.
Maybe the people from fiber optic test equipment companies went
to business school instead of engineering or science!
Lost In Translation
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 - we're supposed to know loss always means the optical
power measurement in dB 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. We 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.
*So the problem is not simply fourth grade math, it also
involves a bit of convention and tradition and marketing.
This 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: https://www.khanacademy.org/math/algebra2/exponential-and-logarithmic-functions/introduction-to-logarithms/v/logarithms
And more basic, here is a tutorial on adding and subtracting
negative numbers https://www.khanacademy.org/math/cc-seventh-grade-math
** 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.
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
in dB is a negative number
that measure loss do not display negative signs with
are displayed with a negative sign
Update - Mystery Solved! 7/2020
Summary: IEC changed the definition of attenuation to
make it a positive number in defiance of mathematical and
metrological standards, centuries of mathematical history and
Change The Negative Sign For Loss, Just Change The
Definition! Who Cares If Everybody Else Does It
recently FOA was reviewing a new proposed update for FOTP-78 IEC
60793-1-40 Optical Fibres - Part 1-40: Measurement Methods and
Test Procedures - Attenuation. This FOTP might be the
most-referenced FOTP since it deals with measuring attenuation,
something that dozens of FOTPs use in their testing of
components. I started reviewing this document by skimming the
Terms And Definitions, where I was stopped by Section 3.1 which
The classic attenuation equation was different.
where (quoting from the standard)
Note 1 to
entry: Attenuation is a measure of the decreasing optical power
in a fibre at a given wavelength. It depends on the nature and
length of the fibre and is also affected by measurement
- A is
the attenuation, in dB
- P1 is
the optical power traversing cross-section 1 (e.g.
before the attenuation you are measuring - what we would
call the "0dB" reference in testing cables)
- P2 is
the optical power traversing cross-section 2. (e.g.
after the attenuation you are measuring - what we would
call the measurement of loss in testing cables)
As we traced this definition in other IEC standards, we find
they are variations of this, and one specifically states that it
expresses attenuation as a positive term.
So there you have it - why attenuation is positive - and
therefore gain - like a gainer on an OTDR - is a negative
number. The IEC standards just turned the measurement upside
down - reversing "Measured Power" and "Reference Power" to get
the term to become a positive number in dB when it's
And I might add, they are unique. See References
below. Undoubtedly some instrument manufacturer wanted the
definition that way and had no broad knowledge of measurement
convention. Nor did they understand fiber optic power meters.
At least now we know where the confusion lies.
are several reasons to object to this from a mathematical and
measurement standpoint. When you measure something against a
reference, it's common to divide the measured value by the
reference. Thus if something is getting smaller, like
attenuation, and the change is the measured value decreases by
50% or half, you expect the ratio of powers to be a number
less than 1 because the value has decreased, in this case the
ration would be 1/2 or 0.5 0r 50%.
Consider what happens when using the equation above. If P1 is
the reference and P2 the value after it decreases, the ratio
for the example above would be 2. Wouldn't anybody assume that
the measured value had increased instead of decreased it the
ratio was 2?
There are several reasons to object to this from a
mathematical and measurement standpoint. When you measure
something against a reference, it's common to divide the
measured value by the reference - like we do defining dBm where
the reference is 1mw.
We checked and the TIA and IEC standards for measuring power,
FOTP-95, still defines dBm this way. That's good, because we're
used to negative dBm being power smaller than 1mW and positive
dBm being power larger than 1mW.
However if one makes an attenuation measurement using a fiber
optic power meter calibrated in dB and you used the "Zero"
control to set the reference,
the resulting measurement of loss will be a negative number.
Likewise if you measure the two powers in dBm, the
resulting measurement of loss will be a negative number, if you
understand negative numbers.
Remember dBm is defined as Power(measured)/Power(1mw) (see
FOTP-95, Sec. 6.2) and if dBm were defined in this upside
down manner, power levels below 1mW would be positive numbers,
not negative as they are now, and power levels above 1mW would
be negative! How's that for confusing.
The definition assumes you are making measurements in linear
units - Watts, milliwatts or microwatts, then calculating dB.
Does anyone do that anymore? We don't think so. Instruments
measure in dB and dBm. Recognizing that, some standards actually
tell you how to calculate using simple subtraction of dB or dBm
measurements but reverse the values so loss is positive and gain
Maybe it's time to drop the definition from the standards or at
least provide descriptions of how one makes measurements in dB.
References: The method for calculation of attenuation in
dB IEC uses in these fiber optic standards is definitely not how
measurements are normally defined. In fact we looked at several
dozen websites and the result was 100% - attenuation is a
If P is
greater than P0 then LP is positive;
if P is less than P0 then LP is
- definitions of the International Systems of Quantities - If P is
greater than P0 then LP is positive;
if P is less than P0 then LP is
- see Electric Power (telephone)
San Diego Neurophysics - they get it! - (-3dB = half power)
Santa Cruz - with
the measured value less than the reference, we get a negative
Ott Consultants - The unit can be used to express power
gain (P2>P1), or power loss (P2<P1) -- in the latter
case the result will be a negative number.
Notes - Where there is a loss, the deciBel equation will
return a negative value
or dBm -Still Confusing 4/2020 -
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%
Here is an example of a power meter measuring in dBm and
microwatts (a microwatt is 1/1000th of a milliwatt.)
is an example
watts to dBm.
This meter is
If we convert
to dBm, it
out using dB
-10dBm is 1/10
of a milliwatt
that is a
factor of 0.25
so 0.1mW X
0.25 = 0.025mW
The other way
to figure it
is -10dB is
1/10 and -6dB
is 0.25 or
= 1/2, so 6dB
= 3dB + 3dB =
1/2 = 1/4) so
Read a more
comprehensive explanation of dB here in the FOA Guide.
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.
does Bend-Insensitive Fiber Look Like?
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:
At the left, you can see the gray area surrounding the core,
shown in the drawing in the right as the yellow depressed
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.
The Problem Comes When Testing
The problem comes when testing these fibers and using them as
reference cables for testing. BI fibers have regions outside the
core that reflect light lost in bends back into the core,
reducing the effect of bending. However that mechanism causes
these fibers to have more high-order modes than regular fiber
which can affect testing.
problem with testing is as follows:
1. Most standards say do not use BI MMF for reference test
cables. This is because standards call for launch cable
mode conditioning (mandrel wrap or encircled flux) that is
ill-defined for BI MMF. Except one standard. FOTP-171B has one
section saying no BI MMF (Sec. 3.6.1) but another (A.3.2) says
to use the same type of fiber for reference cables as in the
cables you are testing.
2. Manufacturers are mostly making BI MMF today. OFS is
the one manufacturer that says they continue to make non-BI MMF.
3. As far as anyone knows, nobody marks on the jacket of
cables whether the fiber is BI or non-BI. There seems to
be consensus that is should be marked, but no standard exists to
4. Some test equipment vendors provide non-BI MMF for test
cords. Nobody else has been identified as a supplier.
Update 7/2020: Standards committees are concluding that
test results with non-BI or BI fiber are both acceptable. It
may take years for standards to be updated, however.
Multimode Fiber With BI Fibers: Real Test Data
received a technical inquiry regarding insertion loss
differences in test results when using OM2 and OM4 fiber for
launch cables. This table shows losses measured with OM4
bend-insensitive (BI) fiber launch cables and OM2 non-bend-insensitive
fiber launch cables.
Note the same cable under test (DUT) showed significantly lower
loss when tested with an OM2 (non-BI) launch cable than with an
OM4 (BI) launch cable (-the same test source was used.) The OM4
cable, based on the photo below, is BI fiber, had higher modal
fill and gave much higher loss.
Bottom Line: Confusion!
MM fibers are BI fibers
say don't use BI fibers for reference cables
are not marked so the user doesn't know if its BI or not
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?
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”
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.
Guide pages on OSP Construction created by Joe Botha for
his course in South Africa talks about manholes and handholes on
page near then end.
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.
Broadband General Network Specifications: see page
need to be big enough for the cables they must contain
Near Fiber Optic Cables
we ran a Q&A question about blasting near fiber optic
cables. Bill Graham, FOA Board Member and long-time instructor
in Canada, tells us that he suggests a more conservative
approach. This is what he has taught in his classes:
and Fiber Optic Cables
1.) Aerial on poles or towers
2.) Buried in ground
Blasting close to poles with fiber optic cables can depend
on the soil between the blasting area and the pole. For
example: In parts of Ontario there are large areas of
solid granite. The pole is generally bracketed to the rock.
(Some areas have started drilling the rock) there is a solid
connection from the blast hole to the pole, up the pole onto the
bracket and to the Fiber Optic cable, which they seem to forget
We recommend the following:
1.) Cover the aerial cable with Big O (4”
perforated drain pipe slit along the length) as shown in the
photo above. This protects only from flying rock.
2.) Removing the cable from the pole clamp and
hanging it from the bracket with a Bungie strap.
Blasting locations are carefully engineered… however, if the
crew wants to get home early on Friday and they double up on the
blasting the damage risk increases substantially. ( The red cups
in the photo are blast holes.) Most companies are wary
enough not to guarantee “no glass damage”. This area is almost
all solid granite.
Cables Buried Underground
Regardless of Kinder Morgan’s recommendation, We suggest
5-6 meters separation is not adequate
should be at least 12 to 15 meters away
Perils Of 2-Cable Referencing
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.
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.
The problem with the two cable reference is the
uncertainty added by including the connection between the
two reference cables when setting the "0dB"
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
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.
value of loss you measure depends on how you set
your "0dB" reference - more reference cables means
between reference cables when setting a 0dB loss add
uncertainty to measurements
a OTDR trace from EXFO that FOA Master Instructor Eric Pearson
uses in one of his books to illustrate bend sensitivity of
fiber and different wavelengths. It shows the loss of a bend
in a singlemode fiber at 3 wavelengths, 1310, 1550 and 1625nm.
You can see the loss is greater at longer wavelengths, the
reason that OTDR testing at longer wavelengths can be used on
most fibers to find bending or stress losses. However, this
will not be as useful in the future as bend-insensitive fibers
are more widely used.
Footnote 2: We found another interesting Corning
ap note, AN3060,
March 2014, on OTDR testing of SM fiber fusion splices.
What interested us was this graph, showing OTDR loss measurement
differences depending on direction and mode field diameter MFD
This shows the direct relationship between MFD and OTDR errors -
if you test a joint or splice going from smaller MFD to larger,
you get a gain. Going from larger MFD to smaller, you get an
Note the magnitude of the loss difference for such small
differences in MFD - up to 0.25dB!
are caused by differences in mode field diameter (MFD)
in fiber splices or joints.
can have big errors when tests are done in only one
Loss With A Mandrel Wrap, Testing The Effect In Class
The fiber optic industry has always known about the effects of
modal distribution and has created metrics to measure and
standardize it for testing multimode fiber. The methods
included MPD (mode power distribution), CPR (coupled power
ratio) and the latest, EF
In class, we decided to test connectors with and without a
mandrel wrap mode conditioner to see if it made a difference.
adding the mandrel wrap to the launch cable, we tested the LED
test source using a HOML (higher order mode loss) test as
described in the page on Encircled
Flux. With the mandrel wrap, the power
was reduced by ~0.6dB, so we left the mandrel on for our
Adding the mandrel wrap certainly did make a difference.
Connections tested single-ended withowithut the mandrel
wrap ~0.6dB loss were measured at ~0.2dB loss with the
mandrel wrap - that's 0.4dB less. That's how much
difference modal conditioning can make on a single connection.
Think about that the next time you are testing multimode fiber!
you are testing multimode fiber, use a mandrel wrap mode
With A VFL: Fibers
Damaged In Splice Trays
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.
are invaluable troubleshooting tools for finding cable
only work close by - 3-4km range max
ALL Got It All Wrong - And They Confuse A Lot Of People - YOU
CANNOT STRIP THE CLADDING OFF GLASS FIBER!!!
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."
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:
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.
No wonder everyone is confused. Practically every drawing shows
the core and cladding being separate elements in an optical
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
We started with a video microscope view of the end of a
connector being inspected for cleaning.
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
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.
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?
diagrams of fiber construction are wrong - showing core
and cladding as separate - but they are one solid peice of
cannot strip the cladding from glass fibers.
Manufacturers Beginning To Realize That It's Time To Go
years ago when optical fiber technology was being developed by
the scientists at Bell Labs in Murray Hill, NJ they were focused
on future technologies. At the time, the installed fiber links
were based on 850nm Fabry-Perot lasers (VCSEL technology was 20
years in the future) and multimode fiber (62.5/125 micron was
their standard then, later replaced with higher bandwidth 50/125
fiber for the early long distance links.)
But they knew the future would be dominated by singlemode fiber.
These scientists, many of whom had worked on the Bell Labs
projects associated with millimeter wave transmission in the
1960s, knew that multimode fiber had the same problem as mm RF
waveguides - noise and bandwidth limits caused by multimoding.
To realize the potential of optical fiber, it was necessary to
move transmission to singlemode fiber.
And that's what happened. Telcos went multimode in the
mid-1980s. Multimode fiber was still used for premises cabling
and short links because the transceivers were cheaper.
But that may be changing. Links speeds are getting higher so
multimode is running our of bandwidth - but singlemode is still
cruising along. The millions of links in data centers and
millions of subscribers connected on FTTH networks has caused
the price of laser transceivers to drop to prices comparable to
multimode versions with cheap VCSELs and now singlemode links
are often cheaper since singlemode fiber is much cheaper than MM
Fiber techs who have worked with multimode in premises cabling
often claim singlemode is much harder to install. Maybe that was
true a decade ago, but the technology developed for outside
plant and FTTH, especially multi-dwelling units and data centers
has changed all that. Bend-insensitive fiber allows the cables
to be made much smaller (microcables) and the same for ducts.
Microducts allow fiber to the "blown in" more quickly. Splice on
connectors (SOCs) and low cost fusion splicers solve the
is needed for today's high speed links
is becoming cost effective for all applications
technology makes singlemode easy to install
new networks are based on singlemode fiber (see below)
Maybe it's time. But don't hold your breath. Like Cat 5,
multimode will not "go gentle into that good night..." Dylan
See also the article below on
fiber types in data centers.
New High Speed Networks
In the article below on faster Ethernet standards, we've highlighted the fiber types to emphasize the dominance of SM fiber. Note that 6 of the 7 use singlemode fiber and 4 of the 7 use WDM over 2 fiber links.
December, the Ethernet committee approved a new standard, IEEE
Std 802.3bs-2017: 200 Gb/s and 400 Gb/s Ethernet with seven
200GBASE-DR4: 200 Gb/s transmission over four lanes (8 fibers
total) of singlemode optical fiber cabling with reach up
to at least 500 m
200GBASE-FR4: 200 Gb/s transmission over a 4 wavelength division
multiplexed (WDM) lane (i.e. 2 fibers total) of singlemode
optical fiber cabling with reach up to at least 2 km
200GBASE-LR4: 200 Gb/s transmission over a 4 wavelength division
multiplexed (WDM) lane (i.e. 2 fibers total) of singlemode
optical fiber cabling with reach up to at least 10 km
400GBASE-SR16: 400 Gb/s transmission over sixteen lanes
(i.e. 32 fibers total) of multimode optical fiber cabling
with reach up to at least 100 m
400GBASE-DR4: 400 Gb/s transmission over four lanes (i.e. 8
fibers total) of singlemode optical fiber cabling with
reach up to at least 500 m
400GBASE-FR8: 400 Gb/s transmission over an 8 wavelength
division multiplexed (WDM) lane (i.e. 2 fibers total) of
singlemode optical fiber cabling with reach up to at least
400GBASE-LR8: 400 Gb/s transmission over an 8 wavelength
division multiplexed (WDM) lane (i.e. 2 fibers total) of
singlemode optical fiber cabling with reach up to at least
An MPO-16 plug and receptacle is required to support the
32-fiber 400GBASE-SR16 multimode application. The MPO-16
plug is designed with an offset key to prevent accidental mating
with a standard MPO/MTP receptacle. All 2-fiber applications may
be supported with a 2‑fiber LC singlemode interface and all
8-fiber applications may be supported with standard MPO/MTP
graphic shows how many "outlaw" Ethernet versions there are:
100G SWDM4 is for VCSEL WDM on OM5 fiber. 100GBase-ZR is a
tech marvel using special modulation and coherent technology
like long haul telecom. As you can see from the list above, the
400G standards are now approved along with some 200G not listed
here. Remember our quote (above) from Bob Metcalfe, co inventor
"The wonderful thing about standards is we have so many to
"Fast" Is Fiber?
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
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
Let’s do a calculation:
C = speed of light in a vacuum = 300,000 km/s = 186,000
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
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
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
is "fast" because of its bandwidth capability
travels in fiber at the speed of light
the speed of light in glass is only 2/3 as fast as the
speed of light in air or a vacuum
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
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
Care of Your Fiber Optic Tools
Nothing is more frustrating that trying to accomplish a task
and having problems with your tools. It doesn’t matter whether
it is not being able to find a tool or finding a damaged tool
put back in the toolbox without being repaired or replaced, it
is a problem.
ever noticed how careful automobile mechanics are with their
tools? The right tools are absolutely necessary for their work
and they know they must keep them in good condition and stored
in the tool’s proper location when not being used. Tools are
expensive - not just for mechanics but for fiber technicians
too - so learn how to use them correctly and take care of them
so they will work properly when you need them.
ever, go out on a job unless you have inspected your tools and
test equipment back in the office and verified your tool kit
is complete, your test equipment is working properly and you
have all the supplies and consumables you need.
corollary of this is never take new gear into the field until
you have tested it in the office and are familiar enough with
its use that you will not have problems in the field caused by
unfamiliarity with it. When I was in the fiber optic test
equipment business, it amazed us how many help calls we got
from customers who were at the job site and wanted to know how
to use the equipment!
more specific. Start with your tools. Clean off a table and
open your fiber optic tool kit. Are all the tools there? Grab
a notepad and list what you are missing. Even better create a
list of tools you need and use it as a checklist so you don’t
forget anything. In fact, I’ll include on the online version
of this article at http://www.ecmag.com a list of recommended
tools you can use as a checklist. And keep a copy of that list
in your toolkit for reference.
your tools are bulletproof, but some are delicate and/or wear
out. Check the condition of your fiber optic strippers and
scribes in particular. They both should be carefully cleaned
and inspected. Use a magnifying glass or loupe to check the
working areas. Then get some fiber and test them to make sure
they work properly. I recommend you have spares of these two
tools in your kit since they do wear out or can be damaged, so
spares are warranted.
equipment is battery powered, so having spare batteries and/or
keeping the batteries charged is important. Check the
condition of the batteries in each piece of gear, turn it on
and make sure it works properly. If your gear has adapters for
various fiber optic connectors, make certain that all those
adapters are there and kept in marked plastic bags to identify
and protect them. Find all your reference test cables and
of your reference test cables and use it to check the
operation of your connector inspection microscope. At the same
time, you will be checking the condition of the reference
cable connector. Does it look nice and clean and free from
scratches? Reference test cables wear out after hundreds of
tests, even when you clean them regularly, so use the
microscope to check the condition of every connector on every
reference cable and set aside those which look questionable.
your light source and power meter to test all those reference
cables. Use a single-ended insertion loss test to determine if
they are still in good condition, with a loss of well under
0.5 dB, and discard the bad ones or set them aside for
re-termination. If you have a checklist, keep track of the
loss and watch how the loss will increase as they are used
more and more. If you have an OTDR and associated launch
cables for it also, use the light source and power meter to
check them too.
Finally, check all your cleaning supplies. Make sure you have
enough for the next job. If not, add that to your notepad list
of things to order ASAP. Don’t wait until the next job comes
up; order all the replacement tools and supplies you need now
and be ready.
work depends on your tools - take good care of them!
Fiber Count Cables
this first in the March 2019 FOA Newsletter with updates in
May and June. We received some feedback and have been talking
to people in the industry also. We even have samples to
photograph and have visited a manufacturer for demos and
training. We thought we'd share some of what we've been told
and see if others agree. Feel free to comment!
Fiber Count Cables
FOA has recently gotten several inquires about the new high
fiber count cables - 864, 1728, 3456 or even 6,912 fibers.
Like this one from Prysmian with 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
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.
making mistakes when preparing the cable for termination can
cable may require special preparation procedures to separate
fibers for termination. Most use new methods of identifying
cables and bundles.
skill, the tech working with high fiber count cables needs
lots of patience.
multiple cables at a joint can get complicated keeping all
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.
200 to 250 micron fibers may be easier with the flexible
ribbon designs which make it easier to spread fibers to the
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
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:
Fiber Count Cables
We've had a
continuing feature on high fiber count cables in the FOA
Newsletter and we now have some interesting photos to show you.
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.
Here are Corning RocketRibbon 1728 fiber (bottom) and 3456 fiber
(top) cables. To get an idea of these cables size, look at this
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.
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.
Looking at the end of this cable reminded us of nothing so much
as this PR photo from AT&T from their intro of fiber in
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
Here is the slotted core that separates the flexible fiber
in the Sumitomo cable:
More on high
fiber count cables and our continuing coverage.
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
longer than conventional ribbons due to the need to
carefully arrange and handle fibers.
Fiber Count Cables - Continued Updates - Installation
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.
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
Here's links to some of the information we've been reading
and watching online:
sticks with solid ribbons in high density cables.
ribbon splice closure for 1728 fibers.
Directions from Corning
on ultra high-density cabinets
a high fiber count cable with flexible ribbons - SEI.
(Japan) Highest density Optical Fiber Cable.
Presentation on 200micron buffer, bend insensitive, high fiber
250 micron loose tube fibers for splicing, AFL Fujikura.
AFL "SpiderWeb ribbon cable.
250 micron loose tube fibers for splicing, Sumitomo.
for ribbonizing and de-ribbonizing fibers. (Telonix).
things you need to know about splicing 200micron buffered
things you need to know about the new ribbon cables
These current links to news may disappear over time.
fiber count cables allow extremely high fiber counts in
small cable sizes, perfect for dense applications in data
centers and metro areas
so many fibers, ribbon splicing is the only sensible way
to splice them
you splicing machines can handle 200micron buffer fibers
bend radius limits are so high, they require special
consideration for installation and storage - BIG manholes
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
Expanded Beam Connector
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
CS and SN
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
The R&M and 3M expanded beam multifiber connectors
reminded us that US Conec introduced the
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.
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
pluggable transceiver multi-source agreement.
everything else, connectors keep getting smaller
early to determine if they will be accepted in the
marketplace and can compete with LCs
Microcables, Microducts and Microtrenching
MiniXtend cable is smaller than a pencil
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
everything else, cables keep getting smaller
well with microducts and microtrenching
need to become familiar with "blown cable" technology
are already accepted in the marketplace
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
Microducts are small ducts for blowing in cable. In the size of
a traditional fiber duct, you can get 6 microducts for 6 288
Microducts And Microtrenching
Nearly invisible microtrenching
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.
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
and ducts are getting smaller allowing more and more
fibers in the same space
allows "construction without disruption"
with SC SOC in EasySplicer
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
(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.
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.
Connectors (SOCs) are easy to install, low loss and low
hardware than pigtail splicing
DAS or Small Cell?
provides perspective better than looking at something as an
outsider. Especially an outsider who's just trying to understand
something instead of an insider trying to perform successfully
as an insider. That's how we feel about wireless communications.
If you say "wireless" to an IT or LAN person, they think WiFi.
But to a telecom person they think cellular. FOA's involvement
is based on trying to understand the infrastructure to support
wireless, OSP or premises, WiFi or cellular, tower site or small
We're basically outsiders on the technology looking at the
infrastructure to support them. Recently we've been trying to
understand the technologies, markets and applications for both
to better include the two technologies in our training and
The initial question we had dealt with distinguishing DAS
(distributed antenna systems for cellular) and small cells (also
cellular). In most ways they seem to be very similar, except
perhaps DAS is indoors and small cells outdoors.
We've started to interview insiders in both technologies to try
to understand how they work and why we should have both. Right
off, we found that there appears to be a general lack of
technical understanding about the other from almost everybody we
talk to who works with one of them. And we're talking real
basics - what frequencies are used, protocols, coverage,
bandwidth, etc. etc. etc. Even the jargon is different, but
that's not unexpected. So we've tried to consolidate information
on the three different premises wireless technologies
appropriate for general usage. Over time we expect to refine
this comparison with more data and user feedback. (got any? send
it to us)
Based on the current evaluation, WiFi is essential to premises
spaces and because of the ubiquity of WiFi, it is inexpensive.
However, WiFi connections for cellular mobile devices appears to
have not yet been refined sufficiently to provide reliable
coverage for cellular voice, but data is good and video, maybe.
Given the cost structure of data plans, using cellular for video
can be very expensive but WiFi is preferable since it is only
limited by bandwidth.
The choice between small cell and DAS in premises spaces is
simple - small cells are generally single carrier connections
and that is too limiting for most users. DAS is similar
technology but has the advantage of offering multiple service
providers. If better cellular service is desired indoors and
WiFi connections for cellular calls is unreliable, a DAS is the
Small cells appear to be a good solution for better cellular
service outdoors in metropolitan areas but the capital costs for
building systems is quite high - Deloitte, you might remember
from an earlier FOA Newsletter, forecast a cost of over $200
billion. It makes one wonder if the carriers can make that
investment while simultaneously investing in 5G.
tablets, phones, many other devices
tablets, some other devices
tablets, some other devices
2.5GHz (802.11n, 14 - 40MHz channels, 3 max
5GHz (802.11ac or 802.11n, 24 - 80 MHz channels,
23 max non-overlapping)(more bandwidth, less range)
3G: 850, 1700, 1900, 2100 MHz
4G/LTE: 600, 700, 850, 1700, 1900, 2100, 2300,
CBRS (Citizens band Radio Service, shared, unlicensed):
3600 MHz, 20MHz channels,
5G: Eur: 24-27GHz, US: 37-48GHz, 71-74GHz
3G: 850, 1700, 1900, 2100 MHz
4G/LTE: 600, 700, 850, 1700, 1900, 2100, 2300,
CBRS (Citizens band Radio Service, shared, unlicensed):
3600 MHz, 20MHz
5G: Eur: 24-27GHz, US: 37-48GHz, 71-74GHz
in to each new private system required, limited handoffs
between WiFi systems or WiFi and cellular
handoffs subject to coverage
(bring your own device)
on service provider device connects to
Max data rate:
802.11ac: ~400Mb/s - 7 Gb/s (MIMO)
5G: ~Gb/s (proposed)
5G: ~Gb/s (proposed)
Cellular on WiFi: not optimal, depends on
with proper coverage
with proper coverage
5G: Good (proposed), cost?
5G: Good (proposed), cost?
backbone to Cat 5, POE
sometimes Cat 5
sometimes Cat 5
for data on PCs, tablets, smartphones, good for VoIP
systems, marginal on cellular devices
for cellular devices since can cover all service
providers, not optimal for high throughput data (today,
future 5G ?)
for cellular devices but can cover only one service
provider, not optimal for high throughput data (today,
future 5G ?)
Learned From Visiting A Wireless Conference
attended the WIA's Connect(X) conference in Charlotte, NC.
This was the first wireless show we'd attended in over a year
and the topics of conversation were similar to last year - 5G
topped the list. We attended several tech sessions and our
takeaway from one was the answer to an attendees question to a
speaker: "When can we expect a standard for 5G." The answer
was revealing: "5G is not a standard, 5G is a goal."
search the web for cellular standards, you will probably end
up at a Wikipedia
page called "Comparison of Mobile Phone Standards." It's
an interesting history of the development of cellular systems.
Nothing on that page refers to 5G, but there is a page
on 5G that starts off saying "This article is about
proposed next generation telecommunication standard.
5th-Generation Wireless Systems (abbreviated 5G) is a marketing
we don't recommend using Wikipedia for technical information
because it is too often edited for commercial bias, (that's
why we created the FOA
Guide,) but in this case the candor is refreshing.
toured the trade show exhibits, we did see something new, this
"Standalone Small Cell" from Zinwave. What's notable, is that
like a similar device we saw last year from Ericsson that saw
at the IWCE wireless meeting and we reported on in the June
2017 FOA newsletter, it looks similar to a WiFi wireless
access point including Gigabit Ethernet interfaces to standard
Category-rated copper cabling. DAS, it seems is migrating to
operating off Cat 6/Cat 6A in a structured cabling system.
Since most offices need both cellular (small cell or DAS) and
WiFi, this makes sense.
tried to find a link to this Zinwave device on the company
website and could not find it, we found something even more
interesting on a page called "Cellular
As A Service": Unfortunately, carriers are no longer
spending on in-building commercial cellular coverage in the
way they used to. That means building owners—whether they
are in commercial real estate, healthcare, hospitality, or
the enterprise—are now having to find and fund the solution
themselves, and it’s not easy. It’s difficult to budget for
the kind of capital outlay needed to deploy an in-building
indicate a movement to make indoor cellular more accessible
using small cells replacing DAS. We've been told that DAS is a
declining market because most of the large public areas like
sports arenas and convention centers have been done.
Enterprise DAS has not been as big but if small cells on LANs,
similar to WiFi, becomes cost effective - and at least one
person told us it would be - then we are looking at a change
in enterprise networks.
At FOA: This addition of cellular wireless to WiFi and
of course the usual fiber or copper Ethernet connectivity
expected of a corporate network is something we've seen
before, and it's the reason FOA expanded it's course offerings
and certifications to include a general "Fiber For Wireless"
programs. We now offer a free
For Wireless" program on Fiber U, a curriculum for our
schools to teach, and of course a
page on the FOA Guide.
Underlying Wi-Fi Problems with Ultrafast Broadband"
by Adtran, a provider of equipment for networks.
Watching: A YouTube
video on how the Mexican city San Miguel Allende installed a
small cell/DAS system in a historic city. La ciudad de
San MIguel de Allende se pone a la vanguardia en
telecomunicaciones, al contar con un sistema de antenas
distribuido conectado por fibra óptica. (In Spanish with
and WiFi will probably coexist indoors since they have
cells are outdoors, being installed for 4G/LTE with
expected upgrades in the future to 5G
Failure In Louisville, KY
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.
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
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.
Be sure to watch the video from WDRB.
Copyright 2019 WDRB Media. Reproduced with permission.
you try some new idea, ask some experienced installers
what they think
Adopts One Touch Make Ready (OTMR) Rules For Utility Poles
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.
A Good Idea?
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
may be problematic if contractors doing installation are
Center Connections - 40G Is Obsolete Already
centers have moved on - 100G is now standard, 200/400G +
is starting to become accepted
Fiber For Data Centers?
The Leviton Blog:
Market Has Spoken: OM4 (MMF), OS2 (SMF) Leave No Place for
Unproven OM5 (MMF)
industry standards and associations set the stage for the
next-generation of cabling and infrastructure that support
network communications. But there are instances when the market
decides to take a different route. This is currently the case
with the recently standardized OM5 fiber. Even though TIA
developed a standard for OM5 (TIA-492AAAE), this new fiber type
very likely won’t see wide industry adoption because there is no
current or planned application that requires it.
Due to new transceiver launches, coupled with customer
perception of their needs and network requirements, the market
is ignoring the new, unproven OM5 cable and sticking with proven
solutions like OM4 and single-mode fiber.
This trend is supported by a recent Leviton poll that found a
significant jump in OS2 single-mode, compared to surveys from
Some of the follow-up comments from the Leviton survey included
responses about OM5:
“I do not believe that OM5 offers a real advantage, it’s mainly
a marketing ploy by manufacturers.” — IT manager at a global
“OM5 isn’t needed. There is no real place for it between OM4 and
OS2.” — communications consultant
to CI&M for bringing this to our attention.
may not become accepted because of lack of equipment
manufacturer support and the almost total move to SM fiber
in data centers
Sources For Multimode Fiber Testing
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
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.
solution - 850 LED with a mode conditioner on non-BI fiber (if
you can find it - see above).
fiber needs testing with a 850nm LED source
Budgets and Loss Budgets
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?
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
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:
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
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
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
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
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
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 tolerate.
test you can generate a graph that looks like this:
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.
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
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
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
this perhaps another early April Fools' joke...like this one
we ran several years ago about the wrong way to clean
Clean Connectors Before You Make Connections
Teague of Microcare/Sticklers
send us this series of photos showing what happens when you make
connections with dirty connectors. It speak for itself!
Backfill A Trench For Underground Construction
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.
On The Job
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.