Accurately Testing Fiber Optic Cables
Note: You need to know what we mean when we say “accurate” – that the
measurement made gives a value close to the “real” value. Standards
people prefer we refer to the “uncertainty” of the measurement because
it’s practically impossible to know what the real value is, but it is
possible to determine how much error is likely in any given
measurement. With apologies to those people, I’m going to use the term
accuracy because everyone uses it more commonly.
The customer for a fiber optic cable installation will require
documentation of test results before accepting and paying for the work.
This obviously leads to certain but often conflicting requirements on
the part of the contractor doing the installation. Testing takes time,
so completing all the tests in the minimum time means more profit.
Testing, however, needs to be done carefully to ensure the measurements
are accurate1, and that can take time. Accurate testing, however, will
ensure that no good cables are rejected and no bad ones missed, so the
contractor will not have to repair what are really good cables and get
callbacks on bad ones.
Lots of time – and cost - can
be saved if the contractor and installers know the proper measurements
that need to be made, understands how to make those measurements
correctly, has the proper tools, keeps them in good condition, has them
calibrated regularly and knows how to use them efficiently. It is also
the duty of the contractor to convey to the customer what is being done
is in line with industry convention and standards. Learning the
background and the issues concerned with making accurate measurements
can save lots of problems – and money.
Industry
committees spend massive amounts of time and energy developing
standards that ensure accurate testing. However, those standards are
generally written for manufacturers, not users, so the task of
translating “standardese” – the language they are written in – into
understandable English is left to the manufacturers themselves and
technical educators in articles like this. This tutorial will give you
insight into what tests are required, what problems are inherent in
testing multimode fiber, how measurement techniques differ and how to
interpret the results of testing and document them.
What Tests Are Available, Needed and Performed?
All fibers in a cable plant should be tested at least
for continuity, proper end to end connections and, most importantly,
loss. How each of these tests are performed depends on the installation
type, required standards and the actual layout of the components in the
cable plant.
Actually, there are five industry standard ways of
testing the loss of a fiber optic cable – three for insertion loss and two
for OTDRs – depending on how you use reference test cables for your
setup. Insertion loss testing with a test source and power meter with reference cables (right) can use 1, 2 or 3 reference
cables to set the “zero dB loss” reference for testing and each way
gives a different loss. Generally standards prefer the 1 reference
cable loss method, but it requires that the test equipment uses the
same fiber optic connector types as the cables under test. If the cable
has different connectors than the test equipment (e.g. LCs on the cable
and SCs on the tester), it may be necessary to use a 2 or 3 cable
reference, which will give a lower loss since connector loss is
included in the reference and will be subtracted from the total loss
measurement. Any of the three methods are acceptable, as long as the
method is documented. Be careful, however, as most network link losses
assume a 1 cable reference, which can affect the acceptance of the
cable.
OTDRs (Figure 2) always require a launch cable for the
instrument to settle down after reflections from the high powered test
pulse overloads the instrument. OTDRs have traditionally been used with
long distance networks where only a launch cable is used, but this
method does not measure the loss of the connector on the far end.
Adding a cable at the far end allows measuring the loss of the entire
cable, but negates the big advantage of the OTDR, that it makes
measurements from only one end of the cable.
First of all, to look at test requirements, we'll divide the topic by installation type: Outside Plant (OSP) or Premises.
Testing Outside Plant Cables
OSP cables are typically long distance singlemode cables that
are installed in short sections, usually 5-12 km max depending on the
cable size, since the bulk and weight of the cable determines how long
is the longest cable that can be installed. Shorter lengths may be
common in urban or campus networks, as cable is installed between
junction points which are determined by the geography of the cable
plant. Since shorter lengths of cable are spliced together,
verification of the splices is important and is usually done with an
OTDR test during the installation process. Once installation is
complete, end-to-end insertion loss is done with a test source and optical power meter, sometimes called an OLTS (optical loss test set) and reference test cables. Certain ultra-long distance cables may require more complex testing for chromatic or polarization-mode dispersion.
The accuracy of testing these long singlemode fibers with multiple
splices depends on many factors. Since the fibers are long, the
attenuation of the fiber is an important part of the measured loss.
Since the attenuation coefficient of the fiber is dependent on the
wavelength of the light source, small differences in the wavelength of
a test source (in either an OLTS or OTDR) can lead to significant
differences in the measured loss. The only way to minimize this
variation is to use test sources as close to the nominal wavelengths as
possible (1310 and 1550 nm typically, althoght others may be specified.)
OTDRs depend on fiber backscatter for making measurements, so any
difference in fiber backscatter at a splice will lead to higher loss in
one direction and lower loss (or a gain) in the other direction. The
only way to accurately measure splice loss is to measure in both
directions and average, a tedious process in a long, large fiber count
cable. One can get an idea of the magnitude of the uncertainty of the
measurement by looking at the attenuation coefficient of the fiber
on either side of the splice. It the two fibers are nearly equal, the
directional variation will be small, but if they are large, big
differences may be found.
How Do You Test Premises Cables?
In
premises cabling systems designed for use with LAN backbones, fiber to
the desk, CCTV, industrial control signals, etc., there are three tests
that may be done, connection verification, insertion loss and OTDR. All
cables should be tested for continuity with a visual fault locator or
fiber tracer and the connections verified. In my experience, many fiber
optic cabling problems are caused by poor documentation or confirmation
of connections. Since each link consists of two fibers, one fiber must
connect a transmitter to a receiver and the other the complementary
pair. Documentation and markings should all these connections to be
made simply. This is easily confirmed with a visual light source
coupled into the fiber.
The
measurement needed for confirming the quality of the installation is
the optical loss or insertion loss of each of the fibers in the cable.
Loss measurements are made end-to-end on the permanently installed
cable plant, the equivalent of the UTP permanent link. Industry
standards call for making that measurement with a test source and
optical power meter, sometimes called an OLTS (optical loss test set)
and reference test cables.
Proposals
have been made to also allow testing installed cable with just an
optical time domain reflectometer (OTDR) but no accepted standard today
requires this. TIA-568 (both the B version and the soon to be published
C version) follows the industry convention, requiring insertion loss
testing (called Tier 1 testing in TIA-568) and permits OTDR testing
also (Tier 2) to provide additional information, but does not allow
OTDR only testing in lieu of insertion loss testing.
The
use of OTDR testing of premises cable plants instead of insertion loss
testing causes much confusion among contractors and customers. Hardly a
week goes by that the FOA does not get a call regarding this issue.
Misinterpretation of these requirements have led to some unhappy
instances in our experience, including misreading OTDRs causing the
removal and discarding of $100,000 worth of good cable and the
retesting of 1100 cables of 12 fibers each, as well as several
instances of customers returning OTDRs to distributors who sold them
the units.
Measurement Uncertainty
Two Types of Measurement Errors
Measuring a physical parameter always involves errors. Unfortunately
you never know the real value you are trying to measure to begin with,
so all you can do is to estimate the error based on the possible
sources of error in making the measurement. There are two types of
errors, random and systematic.
Random errors
are what you see when you make a measurement multiple times and get a
slightly different value each time. Hook up your instrument and make
the measurement, disconnect and try again. Each time, the result will
be slightly different. Generally one should make several measurements,
average them and use the data to calculate the random error, called
standard deviation, to understand the uncertainty of the measurement.
Systematic errors are how the average measurement differs from the real
value, which can be caused by some problem in setup or calibration.
Unfortunately, it’s hard to determine the systematic error, but some
possible ways exist sometimes. We'll look at systematic errors first.
Systematic Errors in Fiber Optic Measurements
Why would all measurements be slightly different from the "real"
value. Consider testing long lengths of singlemode fibers. The
attenuation coefficient of the fiber is measured by the manufacturer at
1310 nm, but your test source may have a wavelength slightly different.
If your source wavelength is shorter than 1310 (say 1290 nm, still
within the limits of wavelength standards for laser sources,) all
measurements of loss will be slightly higher than the manufacturer's
tests. It may only be 0.02-0.03 dB/km, but over 25 km, that makes a
difference of 0.5-0.75 dB loss. Likewise a test source at longer
wavelength (say 1330 nm) will measure lower loss.
In multimode fiber, LED test sources, which have wide spectral
output, may have not only a different wavelength, but different
spectral outputs. The measured loss will be an integration of all the
wavelengths. Different LEDs will measure different losses, but the
effect may not be large because most measurements are made on short
cables. A bigger problem is the way the output of the LED fills the
modes in the core of the multimode fiber, discussed below.
For all measurements, systematic errors can be caused by testing with
launch cables that have bad connectors, especially fibers not centered
in the ferrule or are made with fibers with different core sizes (62.5
micron fiber cores can vary from about 60 to 65 microns.) Test this
yourself, using a a light source and power
meter and two cables of 50 and 62.5 micron cores. Test loss
single-ended in one direction and then the other and note the enormous
difference and how it is directional.
And, of course, the test method used (Method A, B or C for insertion loss or use of reference cables with OTDRs)
causes a systematic difference in measurements depending on the
unknown connection loss(es) included in the process of setting
the reference for "0 dB." (For more details on this, read "5 Ways" and "Loss Math.")
The biggest and perhaps
most common systematic error in testing comes from setting the
reference power before testing. If a mistake is made during the
reference process or the launch cable is removed and replaced on the
test source, the changes in reference value will be reflected in every
measurement. This is especially important when testing at two
wavelengths, as references should always be set with the meter on the
wavelength of measurement. Meters are calibrated at various wavelengths
because of the wavelength sensitivity of their detectors. Changing the
meter calibration setting can cause errors of several dB.
Dirt can also cause systematic errors if the reference cables are dirty
when the reference is set and cleaned afterwards during testing. If the
dirt causes a big enough loss when the reference is set, it may even
cause measurements to show a "gain" during tests - a real surprise for
even experienced installers when they find "gainers" during insertion
loss tests, thinking that happens only during OTDR testing.
Random Errors in Fiber Optic Measurements
Random errors are errors that change with each measurement. Prove this
yourself. Using a light source and power meter (set on 0.01 dB
resolution) connected with a mating adapter and two patchcords tested
single-ended. Mate and unmate the cables numerous times and note the
different losses which can vary by tenths of a dB. These are random
errors. If you can find a mating adapter with a plastic alignment
sleeve, try that over 10-100 matings and watch how the loss reading go
up. Look at the end of the connector and you see how dirty it gets from
the plastic sleeve wearing against the connector.
Speaking of dirt, that is one of the biggest causes of error. You
should always clean both connectors when testing a cable. Between
testing, keep dust caps on the connectors to prevent further
contamination, but remember "dust caps" are often a source of dust, so
clean the connector before each measurement. Unless all the connectors
are carefully cleaned before each test, the condition of the end of the
fiber can cause large random errors.
Finally, all
connectors wear with multiple insertions as the connectors end faces
mate and wear the endface polish. Over the course of many measurements,
the loss of reference cable connectors will increase slowly. The way to
find this is to retest against each other periodically and replace when
loss gets unacceptable. Experienced installers can repolish their
connectors on diamond film like singlemode connectors, but it may be
more cost effective to replace the cables. And always keep a set of
spare reference cables in the field.
Multimode Fiber Measurement Uncertainty
All test methods have uncertainties when testing fiber optic cable. Making accurate loss measurements on fiber has
been a constant and confusing subject of discussion within the
standards committees, especially with respect to multimode fibers. We have tried to understand how light travels in
multimode cable plants and how components like connectors affect how
that light travels. Then we tried to understand how the losses of
fiber, connectors and splices were affected by the methods used for
testing.
We're going to explain, hopefully in
understandable terms, how this works, how it affects your measurements
and how you can try to control test conditions to enhance your test
accuracy. It’s going to take some careful reading on your part, but
when we’re finished, you are going to be more knowledgeable, test faster and with less measurement uncertainty.
How Light Travels In Multimode Fiber
The
most important component affecting loss in a multimode cable plant is
the source coupling light into the fiber. Light sources may be LEDs or
lasers. Lasers may be VCSELs (vertical cavity surface-emitting lasers)
or Fabry-Perot lasers (telecom style.) Each of these emits light in a
different pattern (right), with LEDs having the broadest beam, F-P
lasers a very narrow beam and VCSELs in between. The light coupled from
the source is transmitted in a multimode fiber in many rays or “modes,”
hence the name multimode. (below)
As
you can see, a laser couples light only into modes that travel near the
center of the fiber while a LED couples light into practically all the
modes. Look closely and you can see the modes near the center of the
fiber core (lower-order modes) travel shorter paths than the modes near
the edges of the core (higher order modes.) The shorter path of the
lower-order modes means that they travel through less glass and suffer
less loss than the ones traveling in the outside of the core. That
means a laser suffers less attenuation (loss per unit length, in dB/km)
in the same multimode fiber than a LED.
Furthermore,
as light travels down the fiber, the attenuation changes. The light in
the outside modes is attenuated, leaving mostly light in the modes near
the center. At a kilometer from a LED source, the light in the outer
modes is mostly attenuated and the light carried in the fiber looks
more like the light launched from a laser. This means the attenuation
at that point is less than at the beginning because its only in lower
order modes.
So
what is the loss of the fiber? The manufacturer’s spec for fiber is
around 3 dB/km at 850 nm and 1 dB/km at 1300 nm. That is for a test
using a calibrated source that is much closer to the launch of a laser
source than a LED. The difference in the attenuation coefficient of a
fiber tested with a laser or LED can be 1-2 dB/km. With a LED source,
the first hundred meters of fiber – representative of a premises
network – may have an attenuation of over 4 dB/km.
The same
factors hold for connector and spice loss. Most of the loss in
connectors is due to misalignment of the two fibers and the higher
order modes are much more likely to be lost at a connector than lower
order modes. A connector coupled to a LED source with a short cable
could have a loss of 0.5 dB while if it were coupled to a laser source
or were 1 km away could have a loss of 0.3 dB.
By
now, I suspect your head is swimming. If you still have your wits about
you, you may want to know how any standards body can solve this issue.
The answer is how everything is solved – compromise. Create a standard
launch condition that is more than a laser but less than a LED, which
today is appropriate, since it’s more like the VCSELs (vertical cavity
surface-emitting lasers) used in today’s Gigabit and faster multimode
links.
Manufacturers use special lensed sources in their labs that can
control the launch conditions exactly. The way to approximate this
launch for field testing is to use a LED source and a mode modifier,
usually a few turns of the reference launch cable wrapped around a
cylindrical mandrel that filters out the higher order modes. The
mandrel size must chosen according to the fiber and cable type being
used.(right, Table below) These devices are available from many test
equipment manufacturers.
It’s
highly recommended that you use this standard source method, as it will
produce more consistent test results and provide greater
reproducibility better if you ever have to retest. And the losses
measured are going to be lower so you are less likely to fail good
cables.
Even
so, the uncertainty of the measurement is likely to be several tenths
of a dB. The uncertainty comes from the coupling of your reference
cables to the fiber under test, which includes the quality of the
terminations on the reference cables, how clean they are and how many
times they have been used, since they degrade with use.
TIA-568 Specified Mandrel
Wrap five turns over the specified size mandrel | | Cable/Fiber Type | | Fiber Size | 3 mm Jacketed Cable | 900 micron Buffered Fiber | | 50/125 | 22 mm | 25 mm | | 62.5/125 | 17 mm | 20 mm |
Here are two other more technical articles on modal distribution and control in MM fiber for testing.
Modal Effects on Multimode Fiber Loss Measurements
Encircled Flux For Multimode Fiber Measurements
So Why Aren’t OTDRs Used?
Some people think everybody uses OTDRs for fiber optic testing, but
that’s only for outside plant (OSP) applications. Most OSP
installations involve splicing singlemode fiber to get longer runs and
the OTDR allows verifying the quality of the splice. But when that link
is finished, it must still be tested for insertion loss with a light
source, power meter and reference cables, just like premises cables.
Insertion
loss and OTDR testing use different methods. Insertion loss tests just
like the fiber will be used, with a source on one end and a detector on
the other, so tested insertion loss should be close to what the
communications link actually will see. OTDRs, however, make an indirect
measurement, based on fiber scattering, the major source of loss of a
fiber. It sends a very powerful pulse down the fiber and some of the
scattering comes back toward the instrument, where it is measured and
stored. As the test pulse moves down the fiber (right), it takes a
“snapshot” of the fiber illuminated by the test pulse from which
information about the fiber may be implied.
Everything
the OTDR learns about the fiber is dependent on the amount of light
scattered back toward it and how the instrument is set up for the test.
This “backscatter” is a function of the materials in the fiber and the
diameter of the core. Joints between two dissimilar fibers that have
different backscatter coefficients will not allow one way measurements.
One way the loss is too high, the other way too low (maybe even a
“gainer” where the change in backscatter is more than the loss of the
connection.)
The
second problem with OTDRs on multimode fiber is the laser source. As
mentioned above, lasers couple light narrowly into multimode fiber and
will measure lower attenuation and connector or splice loss than
recommended by standards on the outward bound test pulse, but scattered
light probably overfills the fiber, even more than a LED on the return.
To date, we are unaware of anyone who has modeled this and can provide
guidance on the expected test results from an OTDR.
In
addition, there are problems in premises applications with OTDR
distance resolution. Light travels about 1 meter in 5 nanoseconds. The
width of the test pulse is usually 10-30 ns and the minimum resolution
of the OTDR is about 3 times that or 2-6 meters. Highly reflective
events like multimode connectors in premises cabling, cause instrument
overload and lengthen the minimum resolution of the instrument. Only a
few specialized OTDRs have the resolution needed for premises cabling.
OTDRs
are complicated instruments. Before the OTDR is used to make a
measurement, you have to set all these parameters correctly: range,
wavelength, pulse width, number of averages, index of refraction of the
fiber and the measurement method (usually 2 types for each
measurement.) OTDR manufacturers should teach you how to set up the
OTDR properly and how to interpret the rather complicated display.
(left). But few customers are willing to invest the day or two
necessary to learn how to use the instrument properly. So manufacturers
create an “autotest” function like a Cat 6 certifier that tests the
fiber and gives you a pass/fail result. Every debacle I have seen in
OTDR testing resulted from inadequately trained personnel using
autotest.
Unfortunately,
because of their indirect measurement technique, OTDRs do not easily
correlate with insertion loss tests, and that’s why they are not
allowed by industry standards to be use alone. Some users claim to have
been able to control modal power in multimode fiber and get correlation
between OTDRs and insertion loss tests, but results are hard to
duplicate. The FOA did a comprehensive comparison test ourselves using
special mode conditioners and were unable to get correlation. In fact,
some of our tests gave divergent results between two different OTDRs!
If
one considers the OTDR test to be a “qualitative” not “quantitative”
test, and one knows how to interpret the OTDR trace properly, one can
determine if connectors and splices are properly installed and if any
damage has been done to the cable during installation. If the user does
not have the experience and knowledge to do a proper analysis, the
device usually only causes problems.
Testing Efficiently And Accurately
The contractor and the user should agree on what documentation and
testing are required before the project is started. That documentation
should include the layout of the cabling, types and numbers of fiber in
each cable, connection diagrams and insertion loss test results. That
agreement should be part of the bid and the contract. If the customer
wants OTDR data, they should be quizzed on why they want it and be made
to understood that OTDR testing is time consuming and expensive (like
the instrument itself!)
Before beginning the installation, the contractor should calculate a loss budget
for each link based on the length of the link and the number of
connections. This confirms the equipment will operate over that link.
Then the expected loss will be known to allow a pass/fail decision by
the person doing the testing. The contractor should have the proper
test equipment and installers using the equipment should be familiar
with its use.
When terminating cables, each cable
should be tested with a source and power meter using high quality
reference cables. The accuracy of the measurements depends on having
properly operating test equipment, high quality reference cables with a
mandrel wrap, cleaning all connections before every measurement and
using a consistent measurement technique.
Reference
cables should be tested with the same test equipment they are used with
each day and cleaned carefully before each measurement. This also
provides good practice to the installers using the equipment. All
installers using the test equipment should be familiar with using the
mandrel wrap on the launch cables.
Since the
light source and power meter insertion loss test requires an instrument
at each end of the cable, two installers working together will speed up
the process. A visual tracer can be used to identify the next fiber to
test, making communication easier and cheaper than using cell phones.
Data
should be recorded in a spreadsheet alongside the loss budget
calculation used for pass/fail criteria so the contractor and customer
can verify the installation. Troubleshoot high loss links that fail
testing by testing “single ended” with only a launch cable. Bad
connections will show up as high loss when connected to the launch
cable but not when connected directly to a power meter so reversing the
cable test direction will usually find bad connectors.
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