questions? Try the FOA
Guide and use the site search.
Loss: Are You Positive It’s Positive?
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?
Well the real problem is that to understand this you need to
understand logarithms and that’s Algebra II*, way beyond fourth
grade addition and subtraction. You see dB is defined as a
With logarithms, if the ratio of measured power to reference
power is greater than 1, e.g. measured power is more than
reference power, the log is positive. If the ratio of measured
power to reference power is less than 1, e.g. measured power is
more than reference power, the log is negative. If the ratio is
1, the log is 0.
Since the logarithm for optical power ratio is base 10 and then
multiplied by 10, each change of 10 in the ratio of the measured
and reference power becomes a change of 10dB. E.g. +10 dB is a
factor of 10 (10 times log10 10 which is 1), +20dB is a factor
of 100 (10 times log10 100 which is 2), +30dB a factor of
1000 (10 times log10 1000 which is 3)and so on. Negative
dB means division, so -10 dB means a factor of 1/10th (10
times log10 0.1 which is -1), -20dB a factor of 1/100th
(10 times log10 0.01 which is -2) and so on. 0 dB means the
measured power to reference power ratio is 1 – they are equal.
Let’s try a graphic explanation of this equation. Take a look at
this “semi-log” graph (logarithmic on the x axis and linear on
the y axis) of dBm vs optical power in the range commonly used
for fiber optics and calculated with our equation above.
Remember 0 dBm means all power is referenced to 1 milliwatt
As you move to the right, power increases and the value in dBm
gets more positive – that would be gain. So from 1mw to 10mw, we
see a gain from 0dBm to +10dBm or 10dB, a positive change.
As you move to the left, to lower optical power, as would be
loss, the dBm value gets more negative. From 1mw to
100microwatts (that’s 1/10mw), we go from 0dBm to -10dBm, or
-10dB; that negative change indicating a loss of 10dB.
That shows gain is positive dB and loss is negative dB. Now
we’re getting to the fourth grade math.*
How about an example? Let’s say we decide to test a singlemode
cable plant. We start with a laser source and launch cable which
we measure our reference level for loss with a power meter to
have an output of 0dBm. That’s 1 milliwatt of power, about the
normal output of a fiber optic laser. After we attach the cable
plant to test and a receive cable to our power meter, we measure
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:
And then there is this short movie on the FOA
Guide page explaining dB showing how a power meter shows
loss when a cable is stressed to induce loss:
As the fiber is stressed, inducing loss, the power level goes
from -20.0 dBm to --22.3 dBm.That's a more negative number.
No question – loss means a more negative power reading in dB and
a negative number in dB indicates loss.**
But if you are a manufacturer of fiber optic test instruments
that offers optical power meters and sources to test loss, why
would this confuse you?
Perhaps we should blame accounting.
Suppose you have a company that has $1million in sales and
$900,000 in expenses. What’s the profit? It’s $1,000,000 -
$900,000 = $100,000. That’s a profit, right?
But suppose your company has $1million in sales and $1,100,000
in expenses. What’s the bottom line? It’s $1,000,000 -
$1,100,000 = - $100,000. Wait a minute, that is a negative
number – that’s not a profit, it’s a loss.
So in accounting, profits are positive numbers and losses are
negative numbers when we do the math, but when we talk about
loss, we don’t say we have a loss of “-$100,000,” we just have
we have a loss of $100,000. Then we’ll put that number in
parentheses when we publish our P&L like this ($100,000) and
hope it doesn’t get noticed by investors, but you know it will.
Loss and gain in fiber optic measurements are similar. If you
are using a separate source and power meter, loss will be a
negative number and gain will be a positive number. But because
of convention, we sometimes drop the signs when we report the
values because loss always means the optical power measurement
was negative and gain means the optical power measurement was
positive. But maybe that’s not what the convention has evolved
Optical loss test sets (OLTS) aren’t designed to measure and
display optical power, just loss. The actual power measured is
lost in the algorithms used for calculating loss based on the
“0dB” reference power and the measured loss. Long ago, most OLTS
measured loss and displayed it as a negative number, but some
companies who got into the fiber optic test equipment business
from other test businesses arbitrarily decided to display loss
as a positive number, and today most OLTS do show loss as a
positive number. But when the instrument sees a gain, which it
can do if improperly used, it therefore displays a negative
number, which can be very confusing to a trained fiber tech who
understands fiber optic power and loss measurements.
OTDRs do the same thing. I looked at traces from a half-dozen
OTDRs and all showed loss as a positive number and gain as a
negative number. And yes, when you have a gainer in one
direction, they show it as a negative number. Telling them that
is wrong will fall on deaf ears, I’m afraid.
The same article/blog post goes on to discuss optical return
loss and reflectance, which has similar issues but they get it
more or less right, which is confusing. Why can they understand
that more negative numbers for reflectance means lower power in
the reflectance but claim the opposite for insertion loss?
The “less right” is that with most OTDRs reflectance of an event
and optical return loss (ORL) are not the same thing. ORL is the
summation of all reflectance events and fiber backscatter from
the entire length of fiber.
And, please, please stop saying “back reflection;” a reflection
always goes back toward the source so the term is redundant and
was dropped from fiber optics years ago.
*So the problem is not simply fourth grade math, it also
involves a bit of convention and tradition and marketing. And it
requires understanding logarithms that create the negative
number of loss. That’s more like Algebra II or 7th grade math,
and here is a good tutorial from Kahn Academy on that: 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.
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, I suggest 5-6
meters separation is not adequate and should be at least
12 to 15 meters.
Strip The Cladding For Termination Or Splicing? NO! They've
ALL Got It All Wrong
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, no surprise, 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?
We Warn You To Be Careful About Fiber Shards
Photo courtesy Brian Brandstetter, Mississauga
Question! Tech Questions/Comments Worth Repeating
Questions From FOA Newsletter Readers
Communications Cables on Utiity Poles
Is there a standard that service providers such as ISP, FTTH or
cable TV should follow when installing their cables on existing
electric poles. For necessary clearances etc. ?
A: The location of comms cables is in the
“Communications Space.” At the top of the pole is the “Supply
Space” for power conductors and between the two is a “Safety
Zone Space.” It is There are guidelines of various types, mostly
referring to NESC Rule 235. One of the best documents on this is
Electrical Service. This presentation
from Finley Engineering offers a good summary.
"Snowshoes" On Aerial Cable
overhead installation, can snow shoes, or other service loop
devices, hold two separate cables?
A: Snowshoes are sized for different
cable sizes and types. Some snowshoes are big enough for
several cables, that’s no problems.
Fiber and Power Sharing Conduit
Q: We are working on a project that has miles of
underground 7 cell innerduct conduit with existing fiber already
running through one of the cells. Is it possible to run
electrical conductors through the 2 of the other cells? The
conductors would be no larger than 1/0 AWG at 480V or 600V. Both
the fiber and the electrical are being installed for the same
A:We questioned several people in the electrical side
that also do fiber work. The opinion is that the electrical may
use the other ducts. If the fiber cable has conductive members,
e.g. armor, it must be properly grounded. And any cables spliced
in manholes need separation and marking. The concern is over
what happens with a dig up, but as long as the electrical is
turned off before restoration begins, there should be no
Test Multimode At 1310nm
working on some MM fiber and am unsure if I need a fan out
kit. Is there a way I can tell if I do?
Is it 250 micron or 900 micron buffer? Splicing or
termination? Loose tube or tight buffer cable? Generally
loose tube cable with 250 micron fiber needs no fan out
kit for splicing - tubes go to splice tray and bare fibers
are protected in the tray - but probably needs it to
terminate if the fibers are exposed, for eample with SOCs
- splice on connectors. Tight buffer - 900 micron fiber -
does not need fan out kits.
We've got a discussion going in our department about
whether an APC connection is required at both ends of an
electronics link or just in one spot when trying to
control reflection for broadcast equipment.
A: It seems you could use APCs on the transmit end
to prevent reflectance bothering the laser source, but it
would get reflectance from the receiver end, probably not
an issue unless you are on short links, like in a data
center where the far end reflectance would not be
attenuated substantially. If you put the APC at the
receiver end, you would still have problems with
reflectance at the transmitter end. Perhaps the biggest
problem is managing to keep patchcords straight. We’d vote
for the simple solution, all APCs, which is what many
people are doing today. Alternatively, use the best UPC
connectors which have only about 10dB worse performance as
long as they are kept clean.
Q: Can I test multimode over 1310 wavelength?
A: Certainly you can, but why? In fact, multimode fiber
has been tested at both 850 nm and 1300 nm for most of its
history. Some standards still call for testing at both
When network speeds were 100Mb/s or less, sources were LEDs at
850nm used for shorter links - a few hundred meters - and LEDs
at 1300nm were used for longer wavelengths, up to 2km for
networks like Ethernet, FDDI and ESCON.
When Ethernet jumped to 1 gigabit/s, LEDs could not be used;
they were limited to ~200 Mb/s. The new fast VCSEL sources
(vertical cavity surface-emitting lasers) were adopted for most
links. Some networks still offered a 1300 nm option using 1310
nm Fabry-Perot lasers since VCSELs are limited to about 950nm
wavelength max. The 1310 lasers available were generally
pigtailed with singlemode fiber and required some special launch
cables to prevent modal problems, and even though they offered
longer range, they were not really cheaper than using singlemode
fiber and never gained much popularity. (See Specifications for
fiber optic links and systems, including FTTx)
So multimode fiber became almost exclusively used at 850nm. In
standards committees we discussed dropping the requirement for
testing at 1300nm, but some argued that since the fiber is more
sensitive to stress/bending losses at 1300nm, testing at 1300nm
provided information on the stress in the fiber. Once multimode
fiber became almost exclusively bend-insensitive fiber, that
argument lost validity.
While some standards still call for 1300nm testing and many test
sets offer 850nm and 1300nm LED sources, it’s probably not worth
Fiber Backscatter Coefficient
I was wondering if you have an idea of how accurate the numbers
from a fiber datasheet are for the stated backscatter
coefficient of the fiber. When looking at these numbers
reported on datasheets, they are nice round numbers like -77dB,
-82dB etc., which tells me that those aren’t actual
measurements. They are more ballpark values. Would
you happen to know roughly what sort of accuracy those would
have for a given length of fiber?
A: We believe is the number is an average from some tests
done whenever the fiber was designed and first manufactured and
is +/- several dB. Remember it’s not a guaranteed spec like
attenuation coefficient which they do measure.
Remember it is dependent on the pulse width. Corning in the
SMF-28 data sheet quotes the numbers you have for a 1ns pulse
You can probably surmise that 1 ns pulse is hardly a square wave
and there will be lots of variations in the integrated power in
a 1ns pulse created by different instruments. And who knows how
you relate that power to a 5, 10, 20 ns or whatever pulse.
This paper by Corning talks about reflectance and backscatter
and has some interesting points:
The key is backscatter is inversely proportional to mode field
diameter, so small changes in fiber diameter can cause changes
in backscatter - and MFD variations of several percent are
Q: For overhead installation, can snow shoes, or other
service loop devices, hold two separate cables? I imagine
the bend radius is a factor.
A: Snowshoes come in many sizes and are often sized for
holding several cables, for example loops around dome splice
closures with 2 cables being spliced.
Q: If I
have 50 micron test leads for my OLTS and I used them to test a
62.5 micron fiber link what can I expect in terms of
results? Will the 50 micron leads give me (generally)
higher or lower loss values?
A: Yes, you will see higher and or lower loss depending on which
way you test. See this
page in the FOA Guide on Mismatched Fibers.
Pull ADSS Cable In Ducts Underground?
Q: Our city is installing a 1.5 mile run, mostly aerial
and we want to use ADSS cable. There are two or three road
crossings where we want to go underground in conduit installed
by directional boring. Can the ADSS cable be dead-ended, brought
to the ground, figure-8ed and pulled through conduit then
continue the aerial installation?
A: The answer is yes this is not an issue and is done all
the time. It is standard procedure. (Thanks to Pat Dobbins, FOA,
the expert on ADSS cables.)
Q: I recently read an article you wrote in April of last
year about micro trenching..Currently, I am employed with an
underground construction company. Something we have never been
involved with is micro trenching and would like to possibly get
some equipment and training scheduled in the near future. In
saying that, it has seemed to be almost impossible to find
numbers on the price per foot. Essentially, I am asking if you
have any resources to some up with those numbers or models to
maybe use for pricing purposes.
A: Microtrenching is becoming another tool that
contractors are adopting because like directional boring is is
less disruptive than regular underground construction. I’m
working with one group that’s using microtrenching in CA cities,
installing microducts and a 288 fiber about the size
of a #2 pencil. Cost is difficult to generalize other than “more
than aerial and less than trenching.” Cost is very dependent
on where you are working and what the local geography
looks like. We know one contractor who claimed to do 5 miles a
day in rural Washington at costs near that of aerial. It’s
especially good in areas with lots of base stone where trenching
or boring is near impossible or cluttered utilities downtown.
Here are a couple of pages on the FOA website about
Plant Fiber Optic Cable Plant Construction
Ditch Witch sells equipment for trenching and trains users.
Condux has the equipment for blown cable and offers training
several times a year.
Old Multimode Fiber
Q: We have old multimode fiber and we are still connecting
more equipment to it over greater distances and need some way to
insure the equipment will work.
A: I do not know of any simple formula for figuring this
out. Last time I remember such a formula from around 2000 done
for Gigabit Ethernet with VCSELs and the equation reminded me of
the quantum mechanics course I took in physics. The problem is
you have two bandwidth factors, modal dispersion and chromatic
dispersion. Modal dispersion is highly dependent on mode fill,
e.g. the metric “encircled flux” was developed to define the
mode fill of multimode fiber with VCSEL sources for simulations
to estimate bandwidth. Chromatic dispersion is dependent on the
fiber spec and the spectral width of the source which is better
with lasers and LEDs.
What generally happens is the standards group developing the
network standard, especially IEEE 802.3 for Ethernet, runs the
numbers and specifies a maximum distance for the particular
network and its speed. FOA has a table of these specs here: Specifications
for fiber optic links and systems, including FTTx
Can A Fiber Optic Cable
Q: While working on a cut-over of a dwdm circuit
something has happened that I am now looking for an explanation.
A transmission fiber emitted "fire" on that same fiber! We did
not see any optical light we saw fire. About 5 cm of fiber
burned and remained smoke. What phenomenon occurred? Are the
dwdm amplifiers so powerful enough to generate fire?
Unfortunately I could not get into the station to take the model
of the equipment.
A: We’ve heard of high power WDM systems exploding dirt
of the endface of fiber connectors and damaging them, but this
is a new one. We contacted several technical people in fiber
companies and found that this can happen if there was a crack in
the fiber in the cable near the connector or lots of reflection
perhaps caused by a very dirty connector that allowed the very
high power to heat the cable enough for combustion. DWDM with
many multiplexed signals and a fiber amplifier creates a lot of
power confined to a very small core of the singlemode fiber.
That power can ignite the acrylate coating on the fiber.
ADSS Cable Underground
Q: I'm installing an aerial run of ADSS cable and wonder
if I can pull the cable under roads in duct installed by
directional boring without splicing when I need to go
A: Yes you can pull ADSS cable in ducts installed by
directional boring without splicing. Just install the ADSS cable
link to the pole where you need to go underground and "dead end"
it at that pole. Drop the cable down, "figure-8" the cable to
prepare for the pull, then pull the cable in the duct. If you go
aerial on the other side, just continue the ADSS installation
Reflective Events Causing Transmission Problems
Q: I have a technical question about reflective events. I
recently assisted to troubleshoot an intermittent SM fiber link
for a customer. The cable was dug up a few years ago and a fiber
contractor has (fusion spiced) a different chunk of cable into
the link to repair it. When troubleshooting the link, I checked
the cable with the otdr. I found that each of the 12 fibers had
a reflective event at the fusion splice. This was only at the
splice tube closest to me. The other fusion splices in the other
tube were virtually invisible (as they should be). I'm a little
puzzled as to why there are reflections at the fusion splices. I
did a little research, but couldn't come up with a good answer
as to what is a possible cause of the reflections. (The OTDR
also showed a lot of ghosting on every fiber tested) (in some
cases it recorded over 40 ghost events) Although I haven't been
able to confirm that there is high Bit error rate due to the
transceiver not providing these statistics, (except for 3 out of
10 pings fail) I am suspecting that High reflectance is possibly
the cause of their unreliable fiber link.
A: Reflectance is a big problem in SM links, especially
short links. If you are seeing lots of ghosts, I suspect the
link is very short. Fusion splices can have reflectance if the
splicer is improperly set and the fusion is incomplete or has
bubbles. Those splices should have not only have reflectance but
higher loss. The solution is to open up the closure, use a VFL
to find the reflective events and redo the splices.
Blasting Near Fiber Optic Cables
have a project where blasting is planned near fiber optic
cables. We find no standards for this. Is it safe for the fiber
or should we treat it like other utilities like gas and water?
recommends considering fiber optic cables to be similar to gas
lines when blasting nearby. We know of no standards for this
but there are some descriptions of projects requiring blasting
near fiber optic cable installations. Here is a pipeline
company's guidelines for blasting. The guidelines seems
to focus on staying 5m from the fiber optic cable and using
careful blasting techniques.
Connector Protective Caps
Q:How do you clean LC Fiber Optic end caps (the cap that
covers the cleaned fiber cable)? Is there a tool for that?
A: We assume you are talking about the small plastic
protective caps on the connector ferrule. There is a joke in the
industry that goes “there’s a reason they call them "dust caps’”
they’re often full of dust.” The problem is these are plastic
molded parts that are made by the billions for various purposes
- some just fit fiber optic connectors. They come out of the
molding machine and are dumped in barrels. No provision is made
to keep them clean, plus they will have some mold release
chemicals inside them that can attract or hold dust. Even static
electricity is a problem.
We know no way to clean them nor to keep them clean. We
recommend using them to protect the connector ferrule - in fact
we’re trying to get people to call them “protective caps” - but
after they are removed and before use (connecting to another
cable or a transceiver or testing them) they need inspection and
See these pages in the FOA Guide: Microscope
Inspection And Cleaning of Fiber Optic Connectors Cleaning
Fiber Optic Connections
Directional Splice Loss
I have a customer that is splicing a fiber distribution hub to
their fiber plant. The fiber distribution hub utilizes
100FT long fiber stubs of SMF G.657.A1 and the fiber plant uses
SMF G.654.D. The project has a contract fusion splice
passing spec of 0.2dB loss, averaged bi-directional and also a
one-way <0.3dB loss (either direction) specification; using
an OTDR for measurements.
From my research, if the splices OTDR’s test results for the 2
directions are -0.2dB / +0.6 (average of +0.2), the network is
not actually seeing a +0.6dB loss; but this is how the OTDR
interprets the backscatter information… the OTDR being somewhat
confused due to the bend insensitive fiber characteristics.
A: Correct - the directional differences are due to the
mode field diameter variations in the two fibers. G.654 is
a large MFD fiber, ~12.5microns, compared to ~9 microns for
G.657.A fiber. The OTDR measures based on backscatter which will
be very different for the two fibers.
Markers For Underground Fiber Optic Cables
Q: I have a general question about above ground
markers for fiber optic cable in conduit. Is there a
recommended spacing for the markers? Is there a standard
to reference for this?
A: We asked some people who make them and they said the
guideline is “line of sight.” The rules for markers are
mainly what information needs to be on them. Of course we also
recommend adding marker tape about a foot above the conduit. I
was curious if there were any legal issues and I found this
interesting page from Cornell Law School: https://www.law.cornell.edu/cfr/text/49/192.707
So I might add to line of sight any crossings of roadways,
rail ways and some markers for bridge crossings.
We have a new section on the FOA Guide: Outside
Plant Fiber Optic Cable Plant Construction that may be
And Return Loss
Q: Help me understand measuring reflection little better.
Why do we consider -55dB to be a better reading than, say,
-25dB? If reflection and return loss are inverse readings and we
had a 55dB return loss, would that positive reading for return
loss be considered good?
A: Reflectance is measured as the ratio of reflected to
incoming signal at a connection. The confusion comes because
reflectance and return loss are inverse readings. Consider this:
If we have 1/1000 of the light reflected, the reflectance would
be -30 dB (1/1000 = -30 dB) but the return loss would be 30dB
since it is defined as 1000/1, the inverse, and is described as
Likewise, an APC connector would have a reflectance of -50 dB or
a return loss of 50 dB.
However, return loss as tested by all OTDRs is not be the
reflection from a single event but the total of all reflectance
events plus total backscatter from the length of fiber being
tested in the trace.
This is where most people are confused and misuse the terms.
Reflectance Testing Expanded Beam Connectors
Q: We have a contract where we are required to test
assemblies that vary from 0.5m to 800m in length with connectors
of either PC to Expanded beam or expanded beam to expanded beam
LC/SC/ST -----------to------------- MIL-DTL-83526
MIL-DTL-83526 -------to---------- MIL-DTL-83526
We currently have a back reflection ‘power meter’ on loan but
have been told that the results for assemblies above 5m lengths
may not be reliable to measure the connector termination due to
the nature of the expanded beam termination and cumulative
scatter reflection in the cable.
Therefore we are considering OTDR equipment for our back
reflection testing but trials we have done so far show that the
‘standard’ OTDR testers are not able to separate events that
occur within a 1 – 2m length so we are not able to separate
connections on short lengths.
We are coming to the conclusion that we will now need to
purchase both a back reflection meter and OTDR to cover the 0.5m
to 800m range but would appreciate any industry knowledge we can
A: We took some time to try to research measuring
reflectance on expanded beam connectors and was surprised how
little I could find. FOA has a web page summarizing reflectance
and a full chapter in our book on testing (http://thefoa.org/FOArgTest.html)
but I don’t remember ever being asked about reflectance on
expanded beam connectors.
The best information I have found indicates that reflectance is
fairly high because of all of the optical surfaces. It’s
possible to test all the cables - short to long - by using a
mandrel wrap attenuator after the connectors under test. But if
these are multi-pin connectors with multi-fiber cables, that
Generally OTDRs are not good for short length testing but
fortunately there are specialized instruments designed for your
application. Here is the Luciol OTDR from Switzerland which can
do the job (http://www.amstechnologies.com/fileadmin/amsmedia/downloads/3035_vOTDR.pdf)
I believe there are others like this also.
Having both types of instruments is problematic, as the readings
will not agree between the two measurement methods. I suggest
using the Luciol instrument with the concurrence of your
customer so the tests can be comparable. Otherwise, I would not
be surprised at 3-5 dB differences or more.
The Lifetime Of Fiber?
"The utility I work for has some FO cable installed, some
20 years+ and I am wondering what is industry standard for the
useful life of a cable? This is from an asset management point
of view. I realize that FO cables can and do last for decades,
especially if the work on them is minimized (ie. splicing for
repair or relocation), but what would you consider a good book
value for useful life?"
A: Cable manufacturers have generally made fiber optic
cable for a lifetime of 20+ years and in the last decade or so
we’ve been told that 40 years is reasonable for a cable. But
that means the cable will retain its specifications for that
time frame. Networks, however, do not stand still. In the last
20 years, network speeds have increased by up to 100 times. In
the same time period, the fiber has been engineered to
accommodate longer and faster links. 20+ year old fiber was
installed when speeds were around 1Gb/s, where dispersion was
not an issue, nor was dense wavelength division multiplexing
(DWDM) being widely deployed. So if you are using fiber at lower
speeds, the current cables are probably fine. You might have
trouble splicing older fibers in closures because the exposed
fibers do tend to get brittle.
If you want to upgrade to faster speeds or DWDM, the older
fibers will need testing - we call if fiber characterization -
and here is a page in our FOA
Guide about it: Fiber
Characterization and Testing long haul networks (CD, PMD,
Terminate All Fibers Or Just Some?
are currently running fiber which will be 12/24, my question is
do we need to terminate every pair even if we aren't going to be
using them or is there an alternative?
you do not need to terminate all of them and leaving some bare
fibers is often done when there is no planned use for the fibers
or to save money. However, there are some other issues to
consider. You do need some spare fibers ready to use, either in
case of problems or for upgrades. For small fiber counts, the
cost of terminating them all at once will be cheaper than having
to come back to the site and doing it in the future. The
economics are quite different if you have 144 or more fibers, of
course. If you leave bare fibers, be sure to leave enough length
to terminate or splice later - about a meter for termination and
2m for splicing. And protect them from damage so they can be
used in the future.
Q: I know user who want to send their OTDR’s in for
calibration, especially those that do government work. Are
you aware of a check list of the required tests to comply to
what would amount to an “OTDR Calibration”?
A: ORDR calibration is covered on FOA
Guide page on OTDR FAQs - OTDR calibration is
a unsolved problem. I was first involved with it at NBS in
Boulder (now NIST) in the mid to late 1980s. They could not
justify having a set of “golden fibers” to send around because
they could not figure out how to make a “better" OTDR to use as
a standard as we/they did on fiber optic power meter
Later some tried building electronic calibrators that could
calibrate the timebase and linearity of the receiver, but that
ignored the laser transmitter - you would need to calibrate it’s
wavelength also. Wavelength variation could cause up to 3%
difference in backscatter and loss measurements alone. Then the
OTDR allows for changing the index of refraction to different
fibers which affects length measures.
At one time some manufacturers looked at OTDR cal systems but
the only electronic cal units for OTDRs I know that made
production came from Belarus. There is a single system in the
Navy at Corona, CA however.
Bottom line, test it’s operation according to manufacturer's
Variations In OTDR Length Measurements
Q; I am performing OTDR testing using 3 different
wavelengths (1310, 1550, and 1625) on the same fiber but I get 3
different results for fiber length. The test set and
testing parameters do not change.
A: The OTDR calculates length by measuring the transit
time to an event (half the total time since it measures the
pulse time both down and back) and multiplies it by the speed of
light in the fiber. Speed = C(speed of. Light in a vacuum)
divided by N(the index of refraction of the glass). Most fibers
have an N~1.46.
In an OTDR that speed of light is set using index of refraction
in the setup parameters. In the fiber, the light actually
travels at different speeds according to the wavelength of light
- that is what causes chromatic dispersion. For Corning SMF -28
SM fiber, effective refractive index at 1310nm is 1.4674,
and at 1550nm is 1.4679, not much difference but adds up at 10km
So your OTDR probably has the wrong setup.
Connector Mating Adapters
Q: I am looking for a standard that describes the value of
parameters (IL, RL) and class (if we can talk about class) for
FO mating adapters.
A: Mating adapters are part of the connection but like
each connector they only contribute to the total loss of the
connection and cannot be separated from the other two when
talking about loss. They can be specified by mechanical
dimensions and materials. For example on 2.5mm ferrule
connectors (SC, ST, FC and the obsolete FDDI and ESCON duplex
connectors) the mating adapters have had alignment sleeves made
of molded glass-filled thermoplastic, phosphor-bronze and
ceramic. The plastic ones are cheap but wear out quickly - 10
insertions will leave plastic dust all over the mating
connectors. Phosphor-bronze mating adapters last longer - maybe
500 cycles. The ceramic sleeve ones last almost indefinitely. We
know this because we were in the test equipment business for 20
years (we started FOTEC in 1980 and sold it to Fluke in 2000)
and we tested these mating adapters for longevity with reference
test cables used in insertion loss testing. We had many calls
from techs with problems caused by the adapters with plastic
sleeves. So the way we know the mating adapters are graded is by
alignment sleeve materials.
Removing Old Fiber
Q: I have several 1000 feet of old 62.5/125 armored fiber
optic trunks under a raised floor that I am replacing/upgrading
to 50u MM and SM trunks. Is there any guidance on ‘Best
Practices’ to follow when cutting these trunks into more
manageable lengths for removal?
Use a jaws-type cutter to cut the cable into reasonable lengths
and remove it. There should be no danger in cutting the cable up
as long as your workers only cut the right cable. Raised floors
often have large numbers of cables - often including power
cables - so its important to ensure the proper cables are being
cut an removed.
Q: With a fiber optic pathway that has multiple patching
points...if the end user requires APC connections, isn't it only
important to have those angled connectors at the end/equipment
connections with UPC being acceptable throughout the middle part
of the link?
Reflectance at the connection is the issue, of
Reflectance near a transmitter can affect the laser transmitter
causing nonlinearities or noise in the device. That’s always
been a major concern.
The second issue is reflectance causing background noise in the
link. If you have ever seen a ghost on an OTDR, you have seen a
reflectance at a connection that is bouncing back and forth in
the fiber and is of high enough amplitude that you can see it at
the source. Of course if it reflects back and forth in the fiber
link, it will also show up at the receiver end, becoming noise
and/or distorting the receiver pulses. In a bidirectional
single-fiber network like a PON, it affects receivers at both
Some refer to this as multipath interference. It is being
studied by international standards groups but nothing has been
published on it as far as I know.
We are familiar with a link that was ~1km of SM fiber with
hand-polished ST connectors at several connections. The link had
acceptable loss for all the fibers in the cable but none would
work with electronics. Replacing the connectors with fusion
spliced pigtails cleared the problem up immediately. Was the
With that background, I would answer you question this way. APC
connections at each end of the link will effectively stop any
reflectance issues going back and forth in the whole link. Using
UPC or PC connectors in the link with reflectance better than
-40dB are unlikely to cause problems. (Keep them clean of course
since dirty connectors show high reflectance.) If the links are
very short (<1km), the fiber will not attenuate any
reflectance substantially, so short SM links (FTTH and passive
OLANS for example) often use APC connectors everywhere.
And a final practical issue - mixing APCs and PC connectors is
very bad, perhaps damaging the surfaces. If you do mix them in a
link, you must train personnel how to handle them. If you have
patch panels with PCs and equipment with APCs, for example, you
have to ensure the patch cords are color coded properly (blue =
PC, green = APC) and everybody knows not to mix them
What Do Testing Results Indicate?
Q: I was told the other day by a network technician that
it is possible that a fiber optic strand that is tested to
standard, 850, 1300, mm and 1310-1500 SM bi-directional can pass
a test but when connected to an optic it doesn’t work. I told
him that the optic is the variable but if a strand passes the
testing its qualified to “work” or pass light.
A: There are several reasons it can be true.
Either MM or SM
-The installed cable plant is OK but the patchcords are bad. Or
mixed up - we know instances where systems did not work because
MM systems were connected with SM patchcords and vice versa -
instant 17-20dB loss.
-The polarity is wrong so the transmitter does not go to the
proper receiver. For MPO networks, this is a major problem since
there are so many different polarities used. (See MPO
array/parallel connectors and how to test them). This is often a
-Post testing, the connectors get contaminated and not cleaned
or are damaged.
-The link meets the loss budget but the length is too long for
the fiber type to support the transmission bit rate - e.g. OM2
fiber on a 10G system that is near 300m long (see
Specifications for fiber optic LANs and Links for the list of
-Mixing PC and APC connectors. Bad for loss - may cause serious
-Reflectance problems. Interestingly this question was asked
this morning by the tech boss at a giant university. Here is the
question and my answer:
Connector Loss At Patch Panel
Q: If I have two SC connections in a cabinet eg one
incoming cable jumperd to an out going cable. Should I be
looking for a loss of no more than .75db across the two of them
as per TIA-568
TIA 568 has included a connector loss of 0.75dB for decades.
Even the committee is aware that this is a bogus number for most
connectors but they leave it in because the manufacturers of MPO
connectors need it to comply with the standard.
SC connections should be ~0.2-0.3dB if the connectors are good
and properly cleaned. Now in the patch panel you describe, each
of the two connections should be in that range for a total loss
of 0.2-0.6dB. TIA would allow 0.75dB for each connection or
Differences In Fibers
Q: What is the difference between OM3 and OM4 type fibers
and G.654/G.655? They seem to be rated for the same amount
of GBs (10-400) and the only difference seems to be the
multi-mode nature of OM3/4 vs. the single mode nature of
G.654/655. Can they both be used in long haul communications if
OM3 and OM4 fibers are both 50/125 micron fiber but have
different bandwidth capability. OM3 is rated at 1500MHz-km while
OM4 is rated 3500 MHz-km. OM4 is an evolution of OM3 where
design and manufacture allow more bandwidth. More bandwidth
translates into slightly longer link lengths in faster networks,
~1-10Gb/s. For example, Ethernet at 10Gb/s will go 300m on OM#
and 450m on OM4, which can be important if it is being chosen
for a enterprise network backbone.
The differences in G.654 and G.655 are more complicated. G.654
is singlemode fiber optimized for use at 1550nm for long
distance use. G.655 is “non-zero dispersion shifted” fiber
tweaked for dense wavelength division multiplexing (DWDM) to
prevent secondary problems with high power and closely spaced
wavelengths of DWDM and fiber amplification. It’s the kind of
fiber used in long submarine cables.
Testing At 820 or 850nm
am working on calibration of an optical power meter. It is an
old Photodyne 2285XQ and I need to test it at 820nm, I believe
this wavelength is (or used to be) fairly common in military
applications. The problem is the only equipment I have available
is all based around 850nm/1310nm/1550nm. Our optical power
meters and optical spectrum analyser can certainly operate at
820nm, but I’m having real difficulty finding a commercially
available light source at 820nm.
A: That Photodyne meter was probably built before the US
National Bureau of Standards agreed around 1983 to create the
first calibration standards for fiber optic power meters. NBS
created standards for the three primary wavelengths of fiber
optics - 850, 1300 and 1550nm - based on the available laser
wavelengths for calibration with their standard ECPR -
electrically calibrated pyroelectric radiometer. The 850nm
range was never considered a problem because in the early days
LEDs were sometimes called 820nm and sometimes called 850nm, but
the spectral width is quite wide and the variation in actual
peak wavelength, even as called out in standards, is 850+/-30nm
or 850+/-20nm, making it a very “broad” standard.
I know the rules for MIL standards and calibration so here are
several solutions. Your solution will require a transfer from
some transfer standard power meter.
1. Use the 820nm laser and calibrate the output with the H-P set
at 820 if that is possible. Possible error due to calibrating
only a small spot on the Photodyne detector.
2. You can also make the cal above with a high-intensity source
with a filter around 820nm.
3. Use an 850 LED and calibrate the output with the H-P set at
820 if that is possible. This assumes the photodetector
sensitivity curves are similar.
were sent this OTDR trace and asked why some traces showed
negative loss - gains.
issue with this OTDR test of a factory-manufactured patchcord
was not straightforward. To summarize, the patchcord was plugged
directly into the OTDR port, with no launch cable. The OTDR has
no reference for measuring the loss of the first connector on
the cable nor the second connector since neither launch or
receive cables were used. Without a launch cable, the OTDR is
trying to get information from the connection to the instrument
itself which is basically impossible since it’s suffering from
overload caused by the test pulse - even with APC connectors.
The OTDR (Yokogawa) is one of the cleanest OTDRs at the
interface (we have one on loan at FOA right now) but it’s still
not designed to measure that loss. Furthermore, using the
instrument interface to plug in every connector will wear that
connector in the unit out quickly and require servicing by the
factory. The second issue is the difficulty of measuring on
short cables like this. Note the vertical digital resolution of
the display and think about the location of the second marker -
it’s measured dB value will jump around as it goes from digital
segment to the next digital segment. When you use a launch cable
and measure the loss of the connectors using the “4-point”
measurement - also called “least squares approximation” in the
FOA Guide to OTDRs. That will overcome the digitization errors
as well as the settling times of the pulses.
POTS over Fiber
Q: I would like to know if there is information on your
website that explains "POTS OVER FIBER"?
A: POTS - the acronym for “plain old telephone service” -
is digitized to transmit over fiber. In the early days (late 70s
and 80s) it was simply T-carrier with a fiber converter. By the
end of hte 80s it was ATM and SONET. More recently, it’s all
going to carrier Ethernet since 99%+ of the traffic is data not
voice or PONs (passive optical networks) for fiber to the home.
Re-routing Old Fiber Optic Cables
Q; I have a questions about the re-routing of fiber optic
lines that have been in place for a number of years. Is it
a standard transaction in the fiber optic business to have to
re-route fiber that has been in service for a long period of
time. (e.g. >20 years) If so, is there a best
practice for removal from conduit for re-rerouting?
A; There is no way we would recommend removing and
reinstalling 20-year old fiber cable. First of all, old cable
may be damaged in removal. Then cable and fiber technology has
improved over the years so you can get much better components
today at greatly lower prices. (One industry analyst I know
likes to say that fiber is cheaper than kite string and fishing
line!) Today’s cable designs allow for much smaller cables with
many more fibers (288 fibers in 9.7mm - just over 3/8”) and new
conduit designs allow for more cables in a conduit (microducts
and cloth ducts) and easier installation - blowing in cables and
microtrenching are perfect for metro areas.
More fibers, especially in a big city, is a must. Smart cities,
small cells, FTTH (fiber to the home), ITS (intelligent traffic
systems), V2X (vehicle to vehicle, infrastructure, etc.) and
many other services need lots of fibers.
Our recommendation is to pull it out and dump it. Install new
ducts and the fiber you need (x10 maybe?) and have new ducts for
future use. Are you familiar with “Dig Once”?
APC Connectors On Power Meters
Q: We need to test a fibre link terminated with APC SC
pigtails. I am using SC-APC Ref Leads to interface the LSPM to
the fibre link. I am using the 1 Test Cord Method. Step 1 means
connecting the LS to the PM via one Ref Lead. That means I have
an APC Green SC connector plugged into the PM. Is that
A:The SC APC connector should only be mated to another SC
APC connector to prevent potential damage to the fiber/ferrule
end. But most power meters have adapters for the connector that
have an air gap above the detector to prevent contact to the
detector window. Plus, the detector should be large enough to
capture the light from the SM fiber exiting at a small angle.
Thus you can plug the connector into the power meter directly.
Some meter manufacturers make SC APC adapters for their power
meters that angle the connector toward the detector but that is
generally not necessary unless the meter has a very small
However if the power meter has a pigtailed detector - that is
the meter has a fiber>fiber interface, you will need to add
an adapter patchcord to mate the APC connector to it. When you
set a 0dB loss reference all those connections will be zeroed
Q: I have taught for several institutions and throughout
all my years of doing this I was always taught that when testing
for insertion loss and back reflection for singlemode cable
links that testing bi-directional is an imperative.
Recently when I was attending a meeting involving members of the
military along with folks involved in the development of the
military manuals, it was mentioned that with singlemode testing
that bi-directional testing is not necessary.
A; First of all, there is a directional difference in
splice or connector loss - and maybe reflectance (that term is
now almost universally used in place of “back reflectance” which
is a poor term since a reflection is always back) - as long as
two different fibers are being spliced. Fiber geometry is the
main difference - mode field diameter in SM and core diameter in
MM - but it can also be a matter of the fiber composition.This
happens if two different manufacturers’ fibers are joined or
bend-insensitive (BI) fiber with differences in depresssed
cladding geometry are joined.
If you test bidirectionally with sufficient accuracy, you can
see the difference. It’s an OTDR trace that most people are
familiar with - if you see a gainer, you shoot the
other direction and average to get the “actual” splice loss.
When splicing or connecting different types or manufacturers SM
fiber you may see directional differences of up to 0.3dB or
more. Same for MM, not only for differences in core diameter but
also for connecting BI to non-BI fibers. While those OTDR
measurements are actually differences in backscatter levels,
they are indicative of real differences in connector loss or
splice loss in opposite directions.
Are those differences enough to be of concern? That’s a judgment
We’ve found most people do their bidirectional testing all
wrong. With an OTDR, you should disconnect the instrument, not
your launch and receive cables, and take just the instrument to
the other end. Disconnecting the launch and receive cables
changes the fibers at the connections to the cable under test
and you lose the connection you want to test from the opposite
Bidirectional testing with a test source and power meter is more
confusing. You have to do your “0 dB” reference, check the
launch and receive reference cables, measure the cable under
test, then move just the source and meter to the other end, test
the cable under test again, then disconnect and then measure the
output of the source to get the “0 dB” reference used in that
direction. That’s confusing!
Even if you do test bidirectionally, you do not get the “actual”
connection or splice loss like everybody says - you get the
average of the two directions. Unless you are willing to do a
lab setup and some careful testing, that’s the best you can do.
Now we are into measurement uncertainty. If the measurement
uncertainty is around the same as the typical bidirectional
difference, does bidirectional testing gain you that much?
That’s a judgement call.
The new FOA book on
testing goes into this - a complete chapter is devoted to
When To Test Fiber
Q: Should testing of the fiber plant be done before the
Optical Network Terminal is installed?
A: Fiber optic testing is generally done when the cable
plant is installed to confirm proper installation and check that
the performance is adquate for the electronics planned for use
There are several processes - first an overview of testing:
Check cable before installing - continuity if it looks OK, OTDR
testing if the reel is damages
Test installed cable when splicing - check the fusion splicer
estimate of loss and do OTDR testing if questionable and inspect
every connector as termination is done to confirm the connector
Test cable plant after splicing and termination - end-to-end
insertion loss and OTDR testing for longer OSP (outside plant)
New singlemode cable plants for high speeds may need "fiber
characterization” - adding in CD (chromatic dispersion), PMD
(polarization mode dispersion) and SA (spectral attenuation for
DWDM wavelengths) testing. If one is considering upgrading a
cable plant that is already installed or has been used, these
same tests should be done - inspection/cleaning, insertion loss,
When the system is installed, one should know it should work
because of the testing done during installation. One should
inspect and clean patchcords before installation and test them
if suspect. In fact any connector needs inspection/cleaning
before hooking up equipment. Dirt is the biggest problem with
fiber optic systems.
Between Singlemode or Multimode Mating Adapters
Q: What is the difference between singlemode and multimode
bulkhead/adapters (mating adapters). My understanding is you
cannot use the singlemode with the multimode and visa versa.
A: There are 3 types of adapters - rated for SM or MM -
based on the alignment sleeve material.
-Plastic (glass filled thermoplastic) alignment sleeves are
cheap, not very precise and wear quickly (you can see ceramic
ferrules get dirty using them) - only good for multimode and one
or two insertions - not recommended
-Metal (phosphor bronze) alignment sleeves are better with good
alignment but still wear some - OK for MM, some are rated for
singlemode (check before you buy), and are OK for most uses but
will wear out if used for repetitive testing
-Ceramic alignment sleeves are the best and most expensive. They
are very precise in alignment and last for a long time.
Recommended for all singlemode and all testing purposes.
Don’t use MM adapters for SM but SM adapters are OK for MM.
Why A Figure 8?
Q: What is the reason for wrapping the cable in a figure
A: When you need to do an intermediate pull, you have to
pull the fiber and coil it on the ground. A simple coil will put
a twist in the cable. Figure-8 coils put in twists of opposite
directions on each side of the 8 making for no overall twist in
the cable. See How
To "Figure 8" Cable For Intermediate Pulls in the FOA
Can I Build A GPON Network With "Taps"?
Q: Can I build a GPON network where I do a drop to one
subscriber then continue to the next subscriber for another drop
and so on?
A: There have been examples of this type of “tap” drop proposed,
for example in rural areas for drops to widespread
subscribers on a longer network than is typical for FTTH. It’s
just a version of a cascaded splitter network. with taps that
just do a 2 way split. The taps used are typically 90/10 taps,
where 10% of the power is tapped off for the drop.
There are some important issues to consider - Since you are
dropping 10% of the power at each tap, you are limited by how
many drops you can have. If you calculate the loss
budget - after the first tap, you have 90% power left less the
excess loss of the splitter (~0.3-0.5dB). The tap power is down
~10.3 dB and the through power is down ~0.6 dB. At the next tap,
you use the same formula plus you add the loss of the
fiber to that tap and so on until you reach the GPON
limit. It’s a pretty complicated process to design, but
you can see that with these power losses you will not get a
large number of drops in a GPON network with 28dB max power
budget. We did a rough calculation and 20-24 drops may be
possible depending on the fiber lengths.
This network will probably be much more expensive and more
distance limited than simply running a cable with many fibers
and dropping fibers from that cable with midspan entry. Couplers
are expensive, fiber is cheap. We also do not know the issues
with the large differences in transmission times between the
first connections and the last ones, which depends on the length
of fiber in the systems. That may require some programming at
OM1 MM Fiber
Q: We are an automation system integrator in South Africa. We
have a client that has multimode 62.5/125 fibre optic plant
wide. None of the runs between components are longer than 2km.
We intend to upgrade the technology from a proprietary
communication protocol to a standard ethernet protocol at 100
MHZ. The fibre to copper convertors we will be using are using
1300nm light source and have a Fibre Optic Link Budget of 12.8dB
for 62.5/125 um and 9.8dB for 50/125 um. The client has been
advised to replace the multimode 62.5/125 with multimode 50/125
cabling and we need to know if this is really a requirement.
A: Do you know how old the fiber is? It should be what we called
FDDI grade 62.5/125 fiber with a loss of ~1dB/km and a bandwidth
of 500MHz-km at 1300nm. A 2km link should have a loss of 2dB for
the fiber and ~0.5dB/connection - well under the power budget of
the link. 100Mb/s Ethernet variants were designed for 2km or
more on this fiber. There is no reason to upgrade at this time,
50/125 fiber would not be needed until Gigabit Ethernet was
Do We Need Repeaters For 30 Mile Link?
Q: I need to design a 30 mile (~50km) link. Will regeneration
like a fiber amplifier be necessary?
A: It depends on the comms equipment but I doubt you need
regeneration. 30 miles is 50km, only 10dB of loss for the fiber
at 1550nm, maybe 10 splices at <0.1dB adds only 1dB loss and
another dB for connectors on each end. I think you probably can
find equipment that runs on 12dB loss budget. That said, most
new high speed systems (>10G) have 20km versions then go to
expensive long haul coherent systems. So talk to the
communications equipment manufacturers and see what they say. If
you do need a EDFA, they are not that expensive but the site is
expensive and requires power (+ backup). See if it’s possible to
put the EDFA in the end facilities to get enough power for the
Testing PCS Fiber
We Get Many Questions From Our Instructors Also
Q: I recently did a CFOT
training class for a government agency. They use 200
micron core fiber in short sections (50 ft. the longest) within
aircraft. The connectors are SMA 905s. The cable specs
state that there is 8dB of loss in 1 km at 850nm. I have
searched but cannot find any info regarding how to test the
cable/connector links. The OLTS they have uses 62.5/125
jumpers. Is testing these cables with the above OLTS setup at
850 nm a valid testing method?
A: That’s 200/240 PCS or HCS fiber - plastic clad silica or hard
clad silica step-index fiber - that has a glass core and plastic
cladding. It has been used on a lot of platforms because it’s
large core makes connection alignment easy. The SMAs are used
because the connectors are air gap connections so vibration will
not cause scuffing that you can get with PC - physical contact
connectors. On aircraft, the 1/4-36 nuts on the SMAs are usually
Testing can be done just like any other cable plant with a
double-ended test (OFSTP-14) for the whole cable or a
single-ended test (FOTP-171) to check the connection on either
end. You need a 850nm LED source and a meter with a large
detector (>2mm) to pick up all the light and an adapter for
SMA connectors. You also need launch and receive cables of
matching fiber and connectors about 2m long. No worries about
mode conditioning since the step-index fiber is a mode mixer
Fiber attenuation coefficient of 8 dB/km sounds about right.
Connection losses ~0.5-1.0 dB are normal. A typical patchcord
would have a loss of ~1-2 dB.
To get a microscope to view it might be difficult.
Q: I have
10 kilometers of singlemode cable installed that was not
labeled. It has been suggested that we shoot a VFL down the
fiber and label it. I am having trouble finding a VFL that will
shoot this far. Any ideas?
A: Occasionally we see some imported VFL that claims to go 10km
or more. That tells us the company is clueless about fiber
optics. VFLs work at ~650nm in the visible red spectrum while SM
is optimized for 1300-1600nm in the infrared where it has a loss
of ~0.3dB/km. At 10km it has a total loss of ~3dB or half the
input signal. At 650nm, singlemode fiber has a loss of ~10dB/km
which means it loses 90% of its power per km. At 10km, you have
100dB of loss - leaving you with 0.00000001% of the input power
- not much!
VFLs have enough power for 2-3km max. To identify fibers at
10km, you need a 1310nm laser source and a power meter to do
continuity. Or a gadget called a fiber identifier. For more
WiFi Access Points In a Passive Optical LAN
Q: If we
install GPON passive fiber optical LAN in a new hotel, would one
need to run fiber to every AP? Since every hotel room needs an
AP this gets expensive. Any suggestion on the simplest and less
expensive way of connecting Fiber Cable to an AP in the
A: You do not need a fiber to every wireless AP in a GPON
passive optical LAN (POL). The AP needs a UTP (Cat 5e/6) cable
with Gigabit Ethernet and POE (Power over Ethernet)
capability. The POL fiber should terminate in a multiport
switch that has a fiber input and then 4 or more UTP/POE outputs
for the wirelses APs. That’s the cost saving architecture of a
GPON POL. See this page in the FOA Guide:
You should also check out the APOLAN website (http://www.apolanglobal.org)
for more information on hospitality applications with POLs.
Q: I am
wondering how the landscape will change as the nation moves from
4G-LTE to 5G. Will it use the same network as currently, or will
the network need to be updated or replaced? To what extent will
5G be dependent on wireless vs fibre optic? Will the
infrastructure nationally move more toward an underground wired
one, rather than a Radio Access Network?
A: The wireless network is totally dependent on fiber optics for
it’s communications backbone. The “wireless” part is the
connection from an antenna to the mobile device. From that
point, the network is cabled, mostly fiber already and soon to
be all fiber.
4G/LTE and soon 5G in urban areas is moving to “small cells”
with about 10X as many cell sites covering much smaller areas.
Every small cell site needs a couple of fibers. Metro backbones
will require very much larger fiber counts, especially with
C-RAN (centralized radio access network) architectures now being
An example is Santa Monic, CA where we live. It has about 200K
citizens, 8.9 square miles(about 23 sq km), but has planned for
600 small cell sites, spread over multiple service providers.
At FOA we see wireless as one of the most active areas for
fiber, along with data centers.See http://www.thefoa.org/tech/ref/appln/wireless.html
Q: What are
good OTDR settings for a 300-500m fibers? We’re using a 1.5 m
launch cable and sometimes got (-) loss,
A 1.5m cable is not a launch cable. A launch cable must be long
enough to allow the OTDR to settle down after the test pulse.
THe negative loss is because the OTDR has not settled down
Generally the minimum launch cable for testing short cables
would be 10-20m for MM, 100m for SM. Then use the shortest test
pulse, ~1km range, average enough to reduce the noise.
Q: I was told a contractor installed arsenic
coated fiber optic cable because they didn’t want animals to
chew through it. Is this true?
A: Some cable has
chemicals put in the jacket to make it taste bad to rodents. We
have not found any one who claims to use arsenic, in fact, we
could find no references to what kinds of chemicals are used.
Do Cables "Go Bad"?
Q: It’s been my observation over ~15 years of building
and managing fiber channel storage area networks that from time
to time cables will fall out of transmission spec. In
terms of communicating with non-storage people, they in essence,
“go bad”. Other than possible damage due to physical
disruption of a cable, or contamination at the connectors
usually caused by a human being unplugging/replacing, has it
been your observation that MM cables can “go bad”?
A: There are some
possible causes of problems over time. We know of connectors
that fail for several reasons.
say fiber requires no maintenance - set it up right and lock it
up. As you pointed out human intervention is often the issue.
biggest cause is with prepolished/splice connectors with
mechanical splices. the assumed problem is the index
matching get goes bad, but that’s highly unlikely. It’s
usually the crimp fails and the fiber pulls out, especially
if it has any stress on the fiber.
connectors can have a bond between the connector and fiber
fail, more likely on anaerobic connectors.
stress on the fiber at the connector is bad. Patchcords
should not be left hanging on racks but dressed into
horizontal racks below each patch panel.
stress in cables can be a problem - tension or tight bends -
and they may get worse over time.
is always a worry. It takes years to show up, but indoor
cables are not protected from moisture like OSP cables.
course, transceivers fail too - electronics are generally
very reliable but do deteriorate over time and cause
Ports Or Media Converters?
Q: Should I Buy A Switch With Fiber Ports Or Use Media
A: I’m assuming you are
thinking of using a switch with copper Ethernet ports and a
media converter instead of a switch with fiber ports. The
downside is that it adds complexity and increases the chance of
failure. My analogy is something my primary flight instructor
told me many years ago - multiengine planes are not safer
because having two engines doubles your chance of having an
engine failure. IBM still says that most network problems are
cabling problems. Using media converters adds more electronics,
more power supples and more cabling connections.
PON Meters And Sources
Q: We're evaluating PON power meters and test sources.
How should we test them? Do we need a PON network?
A: There is no
requirement for having a PON to test the meters. I would check
it against a meter you trust to test
1) if the reading of absolute power (dBm) agrees - should be
within +/-0.2dBm. Compare at several power levels, as high as
possible (~0dBm with a laser), medium, (~ -20dBm) and very low
2) make some loss tests of cables and attenuators over the range
of 1-5-10-15-20dB and compare to a meter you trust.
3)the extra calibration at 1490 is not an issue - the difference
between 1490 and 1550 is very small and providing that
calibration can be more a confusion factor since there are no
transfer standards for that wavelength.
3) The big issue with sources is stability. Connect the source
with a short cable to a trusted power meter, connect it to its
power supply, turn it on and monitor the output over time. There
should be a short warm-up period and then it should be stable
within a few 0.01s dB. Let it run on batteries until the
batteries run down to ensure that the source has a proper power
supply that keeps the light output stable over time as the
Loss At 1383nm?
Q: We tested one link of 90.8 km at 1310/1383/1550nm
and we got high loss at 1383 while other wave lengths have good
results. What's up?
A: That wavelength is
the center wavelength of the OH+ water peak, so you are seeing
the extra attenuation there. Older fibers will have attenuation
of 2-3 dB/km at that wavelength but new “low water peak” fibers
will be <1dB/km. See “Low Water Peak Fibers” here http://www.thefoa.org/tech/ref/OSP/fiber.html
Q: Are there different grades (micron) polishing
films/papers for multimode and single mode fiber cables in ODF
termination ? If yes, What are the grades polishing papers for
multimode 50/125 um and 62.5/125 um fibers.
A: The polishing of MM
and SM fiber is indeed different. Both start with an “air
polish” with 12micron alumina polishing film to remove the
protruding fiber. Then the polishing continues on a soft
polishing pad (3mm 80 durometer rubber).
MM uses a 3micron alumina polishing film polished dry then a
final 0.3micron alumina film polish. See
SM is usually done with a wet polish using as special polishing
slurry and diamond polishing film. The diamond film will polish
both the ferrule and the fiber to get the best end finish. See http://www.thefoa.org/tech/ref/termination/sm.html
There are even more pages of information on the FOA Guide at http://www.thefoa.org/tech/ref/contents.html#Components
Q: Can I get an indoor armored 8 core fiber optic
A: Most cable
manufacturers make indoor armored cable using corrugated wrap
armor to protect cables from crushing loads from other cables
especially in under floor installations.
Q: Is there a current standard, for maximum allowable
loss, for MPO fiber connectors? If so… what is the standard #
from EIA/TIA? (Was it amended in 568B, since they were
introduced?) Would it be similar to standard connectors @ 0.75dB
Max allowable loss?
A: The MPO is covered
under the TIA 568 standard. All fiber optic connectors are the
same - 0.75dB.
There are discussions being held at TIA and ISO/IEC on using a
different method of specification, statistical in nature, that
says X% would be less than YdB in several stages from 0.1-0.2 to
over 1dB, but it’s led to some headed discussions.
MPOs for MM are probably no less than 0.5dB and SM are near the
0.75dB mark. At least the SM ones are APC (usual 8 degrees, but
still a flat polish).
I’ve recently learned that MPOs are polished for fiber
protrusion to try to get fiber contact, but the evenness along
the line of fibers is harder to control.
image from SUMIX
showing protruding fibers in MPO connector
More Than “Single” Mode?
Q: We're now using SM fibre so it looks like we don't
need mandrels in the Ref Lead at the Light Source. The
info I have is that we need to make a couple of air coils 35mm
to 50mm in diameter. Why?
A: When you launch from
a pigtial laser source through a connector into a reference
cable, you do have several modes being propogated. It usually
takes 100m or so for the second or third order modes to
attenuate. So the coil causes them to be attenuated by the
stress enough to no longer be significant - it’s a mode filter
just like MM. If you do not do this, you will measure higher
loss in the fiber and at connections near the source. Since most
SM has traditionally been long distance, the effect was small or
ignorable, but with short links, it can be significant.
Followup Q: But how do we explain multiple modes in Single Mode
A: When you get the core of the fiber down to ~5-6 times the
wavelength of the light, it no longer acts like geometric optics
(like MM fiber). Some of the light can travel outside the core
(see the note on “waveguide
At launch, significant amounts of power are at higher angles
creating short lived modes that are highly attenuated.
Replacing OM1 MM Fiber
Q: We are an automation system integrator in South
Africa. We have a client that has multimode 62.5/125 fibre optic
plant wide. None of the runs between components are longer than
2km. We intend to upgrade the technology from a proprietary
communication protocol to a standard ethernet protocol at 100
MHZ. The fibre to copper convertors we will be using are using
1300nm light source and have a Fibre Optic Link Budget of 12.8dB
for 62.5/125 um and 9.8dB for 50/125 um. The
client has been advised to replace the multimode 62.5/125 with
multimode 50/125 cabling and we need to know if this is really a
A: Do you know how old
the fiber is? It should be what we called FDDI grade 62.5/125
fiber with a loss of ~1dB/km and a bandwidth of 500MHz-km
at 1300nm. A 2km link should have a loss of 2dB for the fiber
and ~0.5dB/connection - well under the power budget of the link.
100Mb/s Ethernet variants were designed for 2km or more on this
fiber. There is no reason to upgrade at this time, 50/125 fiber
would not be needed until Gigabit Ethernet was desired.
Bi-Directional OTDR Testing
Q: Should the testing be done with the same piece
of equipment from both ends then merge the results or does that
not matter - can you use traces from two OTDRs as long as the
test equipment is compatible and settings are adjusted properly.
A: Yes, you should use
the same test set from each end but this way - take a trace,
disconnect the OTDR from the launch cable and go to the far end
of the receive cable and connect it there to take the second
trace. The usual way people do bi-directional tests is to
disconnect the launch cable and take it to the far end and shoot
back up, often not using a receive cable at all, figuring they
get the far end connector on the second test. But when you
disconnect the launch cable (and/or the receive cable) you lose
the connection you want to test in the other direction! As for
using the same OTDR, every OTDR is different and the results you
get may be significantly different, esp. if they are not
calibrated recently - and few OTDRs are ever calibrated.
OLANs in Hotels And Resorts
Q: Are passive OLANs a good choice for hotels or
A: Passive Optical LANs
are enterprise networks based on fiber to the home (FTTH)
technology not Ethernet over structured cabling. The FTTH
network is usually using GPON standard equipment over one
singlemode fiber with passive optical splitters that provides
basic Level 1 and 2 network functionality. This is not Ethernet
but carries Ethernet over the GPON protocols at 2.5G downstream
and 1.25G upstream.
Passive OLANs offer several advantages over conventional
Ethernet switches and structured cabling, including much less
cost (~50% capital expense and ~20% operating expense),
much lower space requirements (see the link to the library
photos below and note the two racks of equipment that support
4000 drops), longer distance requirements (to 20km), easy
expansion (these are systems designed for hundreds of thousands
of users) and easy management (when you have hundreds of
thousands of users, that’s important.)
For hotels, convention centers and similar facilities, the ease
of upgrading to a passive OLAN is a big advantage - one fiber
goes from the computer room to a splitter where it can serve 32
switches of 4 ports each. That’s right, one fiber can support
128 users! It can support anything that a network can - wireless
access points, security cameras, secure entry systems, VoIP
phones or POTS phones - anything that will run over a
Q: I have a question about the OLTS - do you have to
recalibrate it every day ?
A: Any optical loss
test set needs to be calibrated for “0dB” whenever anything
changes - the launch cable - source output - or even every few
tests to ensure the connector is clean and undamaged - plus they
wear out. See 5
different Ways To Test Fiber Optic Cables.
Q: I have not been able to find a good definition of
“optical insertion loss” or “insertion loss” or “optical loss.”
A: Insertion loss was
the term originally used for the loss of a connector tested by a
manufacturer. They would set up a source and length of fiber
connected to a meter, measure power, insert a pair of connectors
and measure the loss. Since it was an inserted connection, it
became known as insertion loss.
Over time, the term insertion loss became more widely used to
contrast with the loss measured by the OTDR, an indirect
measurement using backscatter that may not agree with the loss
with a light source and power meter.
Insertion loss, therefore migrated to meaning a loss measured of
a cable or cable plant inserted between the launch and receive
cables attached to a light source and power meter for double
ended testing used with installed cable plants. For patch cord
testing, you do not use a receive cable attached to the power
meter but connect it directly to the cable under test, making
the test just include the one connection to the launch cable.
Two other terms often mixed up are attenuation and loss, which
are essentially the same, except when discussing a fiber. In
fibers, attenuation is often used instead of attenuation
coefficient. Attenuation is the absolute loss i dB while
attenuation coefficient is the characteristic attenuation of a
fiber expressed in dB/km.
Here is probably the best explanations: http://www.thefoa.org/tech/ref/basic/test.html
Q: We are looking at a company’s fiber network which
has been laid at various points in time over numerous
years. In this process, we are trying to identify the
changes that were made to either/both the glass fiber and the
cladding. Are there different generations of what was
industry standard in creating the fiber? For example, are
you able to identify the difference in a fiber that was laid in
1980 versus one laid today? Was the cladding the same
size/thickness etc. in 1980 as it is today or has this been
modified/improved upon over the years? In all, we are
trying to find what modifications have been over the years and
how this may improve the life of the network and its
A: This is a common
problem today. Many network operators are evaluating their fiber
networks for upgrades, hampered by the fact that few are
properly documented. Below is a timeline that should answer your
questions. What many network owners are doing now is testing
their cable plants - a process called Fiber
Characterization. There are contractors who do this
First field trials, US and UK, using multimode fiber at 850nm
First long distance networks still using multimode fiber at
850nm, planning to upgrade with wavelength-division
multiplexing at 1310nm
singlemode fiber becomes feasible, telecom drops multimode
fiber, all future installations are singlemode - this first SM
fiber with a 9 micron core and 125 micron cladding is still
available today but with better specifications. Really early
fiber may not have good environmental protection and degrades
over time. Early speeds were 145-405Mb/s, up to 810Mb/s by the
end of the decade.
around this time, modern fiber begins - better performance and
environmental protection. Fibers for wavelength-division
multiplexing in the 1500nm range appear allowing multiple
signals on a single fiber and fiber amplifiers allow long
- massive build-out of fiber backbone leads to glut of fiber -
WSJ ~2001 says 93% of all fiber is dark. Speeds grew from
1.2-10Gb/s over the 90s decade
- massive Internet growth and mobile device growth eats up
glut of fiber and demands many times more. Dense
wavelength-division multiplexing becomes the norm. Speeds
began at 1.2/2.5Gb/s, upped to 10, 40 and are now at 100Gb/s.
So most fiber installed after 1990 has the possibility of being
used at 10Gb/s, after 2000, it’s probably OK for 40Gb, and since
2010, you are probably OK for 100G and maybe more. To
verify performance, you test each fiber for connector condition,
loss, spectral attenuation, chromatic dispersion and
polarization mode dispersion. There are test sets that will do Fiber
Characterization in basically one step.
Fusion Splicing Live Fibers
Q: Is it safe to fusion splice a live fiber, or is
there a chance that
the light from the arc will damage the detectors in the modules
at the end (20km-rated SM for us).
A: I have never heard of
this being a problem. The amount of light coupled into the fiber
from the splicing would be very small compared to a properly
coupled laser. When a cable is broken you might be splicing the
fibers that are live without knowing which are live and not
caring. On your newer splicers this is not a problem. On the
older splicers with the LID system you would have to reduce the
power to get a good splice which they would do by putting a bend
in the Fiber.
Communications Over One Fiber
Q: Is true duplex over a single fiber possible, or is
more like a shared time-domain technique in a quasi-duplex mode?
I would guess that true duplex would lead to interference
A. Bidirectional links
are widely used - that’s how FTTH PONs work. They use splitters
to combine/split the signals and one wavelength downstream and
another upstream. See Fiber
Optic Datalinks and for FTTH FTTH
Hybrid 2.5-1.25mm Connector Mating Adapters
Q. Can I use the hybrid 2.5-1.25mm adapters for
connecting SC connectors to LCs or MU connectors. It would make
testing much more convenient.
A: We do not recommend
them for most uses, especially testing, as they can be highly
unreliable. Reserve them for emergencies and use hybrid patch
Test MM Fiber @ 1300nm?
What is your opinion about the need for testing at 1300
nm on OM3 and OM4 fiber especially now that bend insensitive
multimode fiber is taking over?
unnecessary and costly. It’s rooted in the FDDI/100M days 25
years ago when 1300 LEDs were used and is now obsolete. The
only actual uses at 1300nm I know are the extremely rare
systems using 1310 lasers which may be standards but simply
don’t seem to ever be used. As you say, BI fiber makes the
issue of finding stresses moot.
Fiber In Service Loops
Q: We designing a rural utility system that will be
expanded to FTTH (or FTTR - fiber to the ranch in this case).
We're wondering how much excess fiber in service loops to add.
One software package is asking for 12% but that seems excess.
have typically seen 100 feet on straight through boxes
(reserves), 35-50 feet on cut ends for splicing and anywhere
from 15-25 feet at the premise depending on how much is
required for the termination device, positioning, etc. When
rough estimating we have typically used 10% over linear
Maintenance of Fiber Networks
Q: Can you guide me how to prepare Optical Fiber
Cable Annual Maintenance Proposal?
A: Basically, the
network needs to be installed properly, fully tested and
everything carefully documented. Then no routine maintenance
is required. Most problems with fiber optic networks occurs
when techs are working with it, e.g. damaging cables or
getting connectors dirty when testing, so leaving it alone is
the best plan.
Electronic transmission equipment can be tested anytime to
ensure proper data transmission, but that does not involve
accessing the fiber.
We have several things which may be of help:
You Tube Video: FOA
Lecture 39 Maintaining Fiber Optic Networks
Web page: Maintenance
I have 4 questions about OTDRs:
Q: What is dynamic range I read many time but
can’t understand yet, whether it is a range of losses can be
measured by OTDR for example if an OTDR has 45 dB dynamic range,
it can read the losses of point up to 45 dB or what it means.
A: I do not believe there is a standard definition of
dynamic range, but it is generally accepted to be the highest
loss of the longest cable where you can see the end of the
cable. That usually means using the longest test pulse and most
averaging and assuming the end of the cable has a
Q: What is dead zone is it fixed in meters mean an
OTDR cannot measure up to initial 5, 10 or 20 meter
A: The dead zone is a function of the pulse width and
speed of the OTDR amplifier. For most OTDRs it’s about 2-3 times
the test pulse width.
Q: What Type of settings needed before launching a
A: See FOA
Lecture 18: OTDR Setup or the section "Modifying OTDR
Setup Parameters For Best Test Results” in
testing. A: Basically you set up wavelength(s), test pulse
width (long enough to reach end of cable but short enough for
best resolution), index of refraction or group velocity (a
function of the fiber type and wavelength) and the number of
averages (enough to mitigate noise but not take too long)
Q: Reading a test with 1310nm and 1550nm - why values
different for a same length of fiber.
A: The attenuation of the fiber will be different at
each wavelength and the index of refraction which is different
at each wavelength causes a difference in length. The OTDR
measures length by measuring time and then multiplying that by
the speed of light in the fiber (which is the inverse of the
index of refraction.)
The FOA page "Frequently
Asked Questions About OTDRs" answers these
questions and more.
Old Cables Out Of Conduit
Q: How do you get old cables out of a conduit when
they are stuck?
A: Usually we are concerned about reducing friction
when pulling cables through conduit, but sometimes you need to
get them out. Here
is a page from American Polywater the leading lubricant
company with advice on the subject.
Q: Is there a guide published by FOA that provides
insight as to the process of fiber optic manufacturing? It's my
understanding that the guide stresses quality and controls to
ensure performance and reduce product loss?
A: We do have a guide
for manufacturers. It is mostly aimed at communications systems
and components manufacture. Here
is a link to download it.
Long Does Termination Take?
FOA received a request from a consultant recently wondering if
we had information on the termination times for fiber optic
cables. After some looking in our archives, we realized we had a
document online that compared times for various fiber optic
termination processes. The paper was written after several FOA
instructors did a comprehensive time and motion study on
termination processes. The document is about 15 years old but
can read it here in the FOA Online Guide.
Connectors (From A Patchcord Maker)
Q: What are the chief defining standard(s) that
specifies connector and assembly IL (insertion loss) and RL
(return loss or reflectance) for both SM and MM fiber?
A: The description on
our Guide is here: http://www.thefoa.org/tech/ref/testing/test/conntest.html
FOTP-34 covers connector testing as a qualification test
for the type of connector - basically a "destructive" test for
Reflectance is described on that page and here also: http://www.thefoa.org/tech/ref/testing/test/reflectance.html
Testing an assembly like a patchcord is covered under
Tests For Fiber Optic Cable Plants
Q: I did some research and I noticed that there is a
bunch of tests that can be done to fiber optics and I was
wondering if there is a list of primary tests that can be done
as a basic test.
A: Fiber optic testing
does have a hierarchy of tests.
Here is a
link to a page on the FOA Guide site that explains the
the top of the list is "insertion loss" testing which uses a
light source and power meter to test the fibers in the
same way that a communications system transmits over the
fiber. It is a simple test and the equipment needed is
will also use a microscope to inspect the fiber optic
connectors for dirt and damage, a big issue for fiber.
instrument called an "OTDR" takes a snapshot of the fiber
using a technique like radar. Most outside plant cables are
tested with an OTDR and the data ( the snapshots are called
"traces") stored for future reference. OTDRs are more
expensive and require more training to use properly.
FOA also has information just for users of fiber optic networks,
to Clean POF (plastic optical fiber)
heard that plastic fibres such as PMMA can suffer damage from
cleaning from an alcohol solution. Are there alternate cleaning
solutions available for these types of fibres."
A: You can use a 10/90
mix of isopropyl alcohol/water. Typically use with a lint
free swab. (from out POF consultants)
Bare Fibers With OTDR
Q: We are starting to test some OPGW cables. We have an
OTDR but we don’t find some reusable connectors. If we have to
test an OPGW with 48 fibres, we can’t set up 48 SC connectors!
Are there some reusable connectors in the commerce?
A: I assume you mean you
need to test with a bare fiber on the OPGW. For testing bare
fiber, use a splice, not a connector. Have a long pigtail on the
OTDR as a launch cable, long enough for the test pulse to
settle, say 100-500m, then use a splice for a temporary
connection. You can fusion splice the fibers then cut the splice
out or use a removable splice like the Corning Camsplice
If you use a mechanical splice, you need a high quality cleaver
just like with fusion splicing and after several uses, you need
to add more index matching gel or liquid - mineral oil works OK.
the FOA page on Testing
Is A Flashlight Test Adequate?
I contracted a firm to install an OM3 of 200 meters. On
one end I have an SFP 1000SX ,on the other a 1000SX
converter from optical to UTP. We made pings but they never
reached, and I didn’t see the laser at the extreme of the fiber.
They promised me to send me the certification they supposely
made ,though they assured me the fiber is ok, because WITH
A FLASHLIGHT THEY SENT WHITE LIGHT FROM ONE SIDE TO THE OTHER
AND IT WAS VISIBLE. I saw the light too, and I thought the
culprit was my switch or my SFP. I want to know: is this a good
demonstration that the fiber is ok?
A: A visual continuity
test is not adequate - your eye is not calibrated! The power of
the lamp is unimportant as each eye’s sensitivity is different.
And your eye probably cannot see the light from a 850nm VCSEL
source - most people’s eyes are not sensitive at that infrared
wavelength. The installer should have tested the link with a
light source and power meter (http://www.thefoa.org/tech/ref/testing/test/OFSTP-14.html)
and given you the loss in dB. The connectors should also be
inspected with a microscope to ensure proper polishing and
(http://www.thefoa.org/tech/ref/testing/test/scope.html). If the
SFP output is -6dBm, what is the power at the receiver?
1000base-SX is supposed to work with 4.5dB loss (see
http://www.thefoa.org/tech/Linkspec.htm). The fiber loss should
be ~0.6 dB, so you must have >4dB connector losses! That says
bad installation! The 1000SX link should work over 200m if the
fiber has been properly installed.
Q: I have some 62.5 mm and sm inside fiber plant over
20 years old. When is a good time to upgrade?
A: When you need to or
have to. If it's working OK, there is no need to upgrade!
"Connector Loss" or "Connection
Q: I have always counted the loss of a connector as .75
dB (568B-3) and 1.5 for a mated pair. Is that correct?
A: While the industry
always says "connector" loss, it is actually "connection" loss.
As we explain in the page on termination and splicing (http://www.thefoa.org/tech/ref/basic/term.html)
When we say "connector" loss, we really mean "connection" loss -
the loss of a mated pair of connectors, expressed in "dB." Thus,
testing connectors requires mating them to reference connectors
which must be high quality connectors themselves to not
adversely affect the measured loss when mated to an unknown
connector. This is an important point often not fully
explained. In order to measure the loss of the connectors
you must mate them to a similar, known good, connector. When a
connector being tested is mated to several different connectors,
it may have different losses, because those losses are dependent
on the reference connector it is mated to."
TIA spec of 0.75dB is for a mated pair of connectors. If you
have been passing connectors tested @ 1.5dB loss....you may have
some very bad connectors in your cabling!
Q: I am doing a lot of fiber optic jumpers for control
systems, either single mode or multimode. I want to get a
scope to inspect the ends after I clean them would you recommend
a 200X, 400X handheld or one similar to a Noyes OFS 300
A: We prefer to use
lower magnification and have a wider view so I can see more of
the ferrule to determine its condition. You can see the fiber
effectively at 100X but 200X may be better. 400X may be too much
for most tasks like inspecting for cleanliness, but may be good
if you are polishing SM for good reflectance. We've used the
Westover units for years because they offer two different
methods of illumination - direct and at an angle. If you are
doing a lot of patchcords, I recommend a video microscope. I've
used the Noyes unit that interfaces to a PC to create the FOA
Microscope Inspection YouTube video here: http://www.youtube.com/watch?v=IyumH8CiUPQ&feature=youtu.be
and it works well.
Q: Who can I contact regarding recycling cable I am
removing from a building?
A: Here are some people
who say they recycle fiber optic cable or at least know how to
Hint: Did You Know You Have A Fiber Optic Tester In Your
That old mobile phone has a camera which may be sensitive to
infrared light - lots more than your eye - and can detect
light in an optical fiber or from a transmitter. Chris
Hillyer,CFOT/CFOS/I, Master Instructor, Northern California
Sound & Communication JATC sent us some photos showing how
this works. See below
or the video
now on YouTube. Update: You should check out your old
cell phones before you recycle them. We've found older models
use sensors which are better at infrared than the newer ones
which take better pictures. This is a good use for your old
cell phones hiding in the drawer!
This is a topic we keep reminding everybody about, and here is
a contrator in the Middle East: Here some samples of the
connectors for SM fiber already installed in the system we were
you can see, the dirt is large compared to the size of the fiber
(dark gray), and the core (not visible here) is only 9/125 of
the overall diameter of the fiber!
Every Connector - A Lesson We Learned From Creating Lessons
curriculum, we got inputs from many experienced techs about the
testing requirements. Everyone we talked to made a big point
about cleaning and inspecting connectors before testing. Dirty
connectors are a major problem with errors in testing. We've
also seen that many installers think that if a connector,
especially new connectors, has a "dust cap" on the connector, it
does not need cleaning. WRONG!
The common name for the plastic caps on connector ferrules is
"dust cap" and a friend says they
are called "dust caps" because they are full of dust.
Those plastic caps are made by the millions, popped out of
plastic molding machines into barrels and stored until put into
plastic bags. Whenever you remove one of them, clean the
connector before testing or connecting it.
More on connector cleaning is here and here.
on cleaning. See Product
News below for links to vendors of fiber cleaning products.
You Need To Know About Fiber Optic Cleaning And More
Forrest, one of the industry experts on cleaning fiber optic
connectors, retired about a year ago. We encouraged him to put
down on paper what he knew about fiber cleaning and he took our
advice. He's now created 4 books on cleaning topics that cover
just about everything you need to know. And he added another
volume that's also important - maintaining fusion splicers. We
recommend these books highly.
How to Precision Clean All
Fiber Optic Connections
Understanding Cross-Contamination Points on Fiber Optic
Inspection & Test Equipment
Maintaining a Fiber Optic Fusion Splicer
Comparison Study of Precision Cleaning Methods for All Fiber
Whitepaper: The Significance to Optical Internconnect:
Properly Cleaning a Fiber Optic Connection
Information on Ed's books is at http://fiberopticprecisioncleaning.com/available-books-whitepapers/.
news about Fiber Optic Cleaning Videos on YouTube by ITW
Fiber Optic Cleaning Videos on YouTube
See news about Fiber Optic Cleaning Videos on YouTube by ITW
Chemtronics three fiber optic cleaning videos on YouTUbe
Cleaning. They are good explanations of cleaning processes
- the Wet-Dry is especially interesting.
Uncertainty: Everyone testing fiber optics should understand
that every measurement has some uncertainty - whether you are
measuring loss, length, wavelength, power, etc. Knowing that
uncertainty is very important to interpreting the measurement.
It's worthwhile to read and understand the issue of measurement
accuracy covered in this
page of the FOA Online Fiber Optic Reference Guide.
continuing quest to help people understand how to test fiber
optic cable plants and communications systems, we've created two
more "QuickStart Guides to Fiber Optic Testing." They are
simple, step-by-step guides on how to test fiber optic cable
plants, patchcords or single cables using insertion loss or OTDR
techniques and optical power from transceivers. It's as
straightforward as it can get - what equipment do you need, what
are the procedures for testing, options in implementing the
test, measurement errors and documenting the results.
It can't get much simpler.
Send anybody you know who needs to know about fiber optic
testing here to learn how it's done in a few minutes.
Fiber Optic Cable Plants And Patchcords
Fiber Optic Cable Plants With An OTDR
Optical Power In Communications Systems
In The US Do Contractors Need Licenses For Fiber Optics?
get asked where in the US do contractors doing fiber optic
installations need licenses. We found a good website for that
information, the NECA -NEIS website. You might remember
NECA-EIS, as they are the partner with the FOA in the NECA/FOA
301 Fiber Optic Installation Standard. NECA is the National
Electrical Contractors Association and NEIS stands for National
Electrical Installation Standards. They have a very easy to use
map and table that gives you data on every state in the US, so
mark these pages for future reference.
(See “State Regulations”)
(all electrical licensing)
Low Voltage: http://www.neca-neis.org/state/index.cfm?fa=specialty_licensing
Technical Website For Installers
has one of the best technical website for cable installers. Check
out their website, especially “Videos,” “Engineer’s Corner”
and “Calculators.” http://www.polywater.com/NNNBSL.pdf
Optic Safety Poster
We've had numerous requests to reprint our guidelines
on safety when working with fiber optics, so we have
created a "Safety Poster" for you to print and post in your
classroom, worksite, etc. We suggest giving a copy to every
student and installer.
An Old Cell Phonet? See the video on
The camera in your old cell phone is sensitive to infrared light
- lots more than your eye - and can detect light in an optical
fiber or from a transmitter. Chris Hillyer,CFOT/CFOS/I,
Master Instructor, Northern California Sound & Communication
JATC brought this to our attention.
you have an old cell phone, try it. Our experience is that older
cell phone cameras have better sensitivity at IR wavelengths
than newer phones, so you may want to toss that old flip phone
into the toolbox.