Reference Guide To Fiber Optics
|Topic: Fiber Optic Testing||Table of Contents: The FOA Reference Guide To Fiber Optics|
Fiber Optic Testing
Testing is used to evaluate the performance of fiber optic components, cable plants and systems. As the components like fiber, connectors, splices, LED or laser sources, detectors and receivers are being developed, testing confirms their performance specifications and helps understand how they will work together. Designers of fiber optic cable plants and networks depend on these specifications to determine if networks will work for the planned applications.
For the purposes
of this particular page, we will focus on the
installed cable plant, but other pages on this
website will cover many more aspects of fiber
optic testing. See the Test
section of the FOA Online Guide for much more
optic cables are installed, spliced and
terminated, they must be tested. For every fiber
optic cable plant, you need to test for continuity
and polarity, end-to-end insertion loss and then
troubleshoot any problems. If it's a long outside
plant cable with intermediate splices, you will
probably want to verify the individual splices
with an OTDR test also, since that's the only way
to make sure that each splice is good. If you are
the network user, you may also be interested in
testing transmitter and receiver power, as power
is the measurement that tells you whether the
system is operating properly.
Testing is the subject of the majority of industry standards, as there is a need to verify component and system specifications in a consistent manner. A list of TIA fiber optic standards is on the FOA website in Tech Topics. Perhaps the most important test is insertion loss of an installed fiber optic cable plant performed with a light source and power meter (LSPM) or optical loss test set (OLTS) which is required by all international standards to ensure the cable plant is within the loss budget before acceptance of the installation.
Testing fiber optics requires special tools and instruments which must be chosen to be appropriate for the components or cable plants being tested. See Jargon and Test Instruments to see a description of these instruments.
Even if you're an experienced installer, make sure you remember these things.
1. Have the
right tools and test equipment for the job.
2. Know how to
use your test equipment
3. Know the
network you're testing...
A note on using a fiber optic source: eye safety. Fiber optic sources, including test equipment, are generally too low in power to cause any eye damage, but it's still advisable to check connectors with a power meter before looking into it. Besides, most fiber optic sources are at infrared wavelengths that are invisible to the eye, making them more dangerous. Connector inspection microscopes focus all the light into the eye and can increase the danger. Some telco DWDM and CATV systems have very high power and they could be harmful, so better safe than sorry. Read our page on Safety.
A good example of how it can save time and money is testing fiber on a reel before you pull it to make sure it hasn't been damaged during shipment. Look for visible signs of damage (like cracked or broken reels, kinks in the cable, etc.) . For testing, visual tracers help also identify the next fiber to be tested for loss with the test kit. When connecting cables at patch panels, use the visual tracer to make sure each connection is the right two fibers! And to make certain the proper fibers are connected to the transmitter and receiver, use the visual tracer in place of the transmitter and your eye instead of the receiver (remember that fiber optic links work in the infrared so you can't see anything anyway.)
You can also use this gadget to visually verify and optimize mechanical splices or prepolished-splice type fiber optic connectors. By visually minimizing the light lost you can get the lowest loss splice. In fact- don't even think of doing one of those prepolished-splice type connectors without one. No other method will assure you of high yield with those connectors.
A note on VFL eye safety. VFLs use visible light. You will find it uncomfortable to look at the output of a fiber illuminated by a VFL. That's good, because the power level is high and you should not be looking at it. When tracing fibers, look from the side of the fiber to see if light is present.
Connector Inspection by Microscope
The magnification for viewing connectors can be 30 to 400 power but it is best to use a medium magnification. The best microscopes allow you to inspect the connector from several angles, either by tilting the connector or having angle illumination to get the best picture of what's going on. Check to make sure the microscope has an easy-to-use adapter to attach the connectors of interest to the microscope.
Video readout microscopes are now available that allow easier viewing of the endface of the connector and some even have software that analyzes the finish. While they are much more expensive than normal optical microscopes, they will make inspection easier and greatly increase productivity.
remember to check that no power is present in the
cable before you look
at it in a microscope to protect your eyes! The
concentrate any power in the fiber and focus it
into your eye with
potentially hazardous results. Some microscopes
have filters which
remove infrared light from sources to be safe.
More on Visual Inspection.
Optical Power - Power or Loss? ("Absolute" vs. "Relative" Measurements)
Practically every measurement in fiber optics refers to optical power measured in dB. The power output of a transmitter or the input to receiver are "absolute" optical power measurements, that is, you measure the actual value of the power. Loss is a "relative" power measurement, the difference between the power coupled into a component like a cable, splice or a connector and the power that is transmitted through it. This difference in power level before and after the component is what we call optical loss and defines the performance of a cable, connector, splice, etc. Take a minute and read about "dB," the measurement unit of power and loss in optical fiber measurements.
Power in a fiber
optic system is like voltage in an electrical
circuit - it's what makes things happen! It's
important to have enough power, but not too much.
Too little power and the receiver may not be able
to distinguish the signal from noise; too much
power overloads the receiver and causes errors
Measuring power requires only a power meter (most come with a screw-on adapter that matches the connector being tested), a known good fiber optic cable (of the right fiber size, as coupled power is a function of the size of the core of the fiber) and a little help from the network electronics to turn on the transmitter. Remember when you measure power, the meter must be set to the proper range (usually dBm, sometimes microwatts, but never "dB" - that's a relative power range used only for testing loss! Read about "dB") and the proper wavelength , matching the source being used in the system (850, 1300, 1550 nm for glass fiber, 650 or 850 nm for POF). Refer to the instructions that come with the test equipment for setup and measurement instructions (and don't wait until you get to the job site to try the equipment, try it in the office first!)
measure power, attach the meter to the cable
attached to the source
that has the output you want to measure (see
the right). That can be at the receiver to
measure receiver power,
or using a reference test cable (tested and known
to be good) that is
attached to the transmitter to measure output
power. Turn on the
transmitter/source and give it a few minutes to
stabilize. Set the
power meter for the matching wavelength and note
the power the meter
measures. Compare it to the specified power for
the system and make
sure it's enough power but not too much.
More on measuring Power.
In addition to a power meter, you need a test source. The test source should match the type fiber ( generally LED for MM or laser for SM) and wavelength (850, 1300, 1550 nm) that will be used on the fiber optic cable you are testing. If you are testing to some standards, you may need to add some mode conditioning, like a mandrel wrap, to meet the standard launch conditions.
generally need one or two reference cables,
depending on the test we
wish to perform. The accuracy of the measurement
you make will depend
on the quality of your reference cables, since
they will be mated to
the cable under test. The quality and cleanliness
of the connectors on
the launch and receive cables is the most
important factor in the
accuracy of loss measurements. Always test your
reference cables by the
patchcord or single ended method shown below to
they're good before you start testing other
groups have never been able to successfully
specify the quality of
reference cables in terms of tightly toleranced
components like the
fiber and connectors. The best recommendation
for qualifying reference
cables is to choose cables with low loss, tested
Reference cables and mating adapter
In order to mate the reference cables to the cables you want to test, you need mating adapters. Mating adapters are as important to low connection loss as the quality of the connectors since the mating adapter is responsible for aligning the two connector ferrules correctly. Mating adapters must be kept clean, like connectors and discarded after some number of uses as they wear out from repeated matings. Mating adapters may have alignment sleeves made from plastic, metal or ceramic. Plastic alignment sleeves used on the cheapest mating adapters should not be used for testing as they wear out in only a few insertions, leaving dusty residue on the connectors. Metal adapters are good for many more insertions and provide a better alignment, so they are acceptable. Ceramic alignment sleeves are the best, providing the best alignment and practically never wearing out.
In order to measure loss, we need to set our reference power for loss our "0 dB" value. Correct setting of the "0 dB' reference launch power is critical to making good loss measurements!
Clean your connectors and set up your equipment like this:
Turn on the source and select the wavelength you want for the loss test. Turn on the meter, select the "dBm" or "dB" range and select the wavelength you want for the loss test. Measure the power at the meter. This is your reference power level for all loss measurements. If your meter has a "zero" function, set this as your "0" reference.
Some reference books and manuals show setting the reference power for loss using both a launch and receive cable mated with a mating adapter or even three reference cables. Industry standards, in fact, include all three methods of setting a "0dB loss" reference. See this explanation on the options in measuring fiber optic cable loss . The two or three cable reference methods are acceptable for some tests and are the only way you can test if the connectors on the cable plant are not the same as your test equipment, but it will reduce the loss you measure by the amount of loss between your reference cables when you set your "0dB loss" reference. Also, if either the launch or receive cable is bad, setting the reference with both cables hides the fact. Then you could begin testing with bad launch cables making all your loss measurements wrong. EIA/TIA 568 and OFSTP-14/7 allows any method as long as the method is reported with the data.
There are two methods that are used to measure loss, a "patchcord test" which we call "single-ended loss" (TIA FOTP-171) and an "installed cable plant test" we call "double-ended loss" (TIA OFSTP-14 (MM) and OFSTP-7 (SM).) Single-ended loss uses only the launch cable, while double-ended loss uses a receive cable attached to the meter also.
Single-ended loss is measured by mating the cable you want to test to the reference launch cable and measuring the power out the far end with the meter. When you do this you measure the loss of the connector mated to the launch cable and the loss of any fiber, splices or other connectors in the cable you are testing. Since you are aiming the connector on the far end of the cable mated to the power meter at a large area detector instead of mating it to another connector, it effectively has no loss so it is not included in the measurement. This method is described in FOTP-171 and is shown in the drawing. An advantage to this test is you can troubleshoot cables to find a bad connector since you can reverse the cable to test the connectors on the each end individually.
double-ended loss test, you attach the cable to
test between two reference cables, one attached to
the source and one to the meter. This way, you
measure two connectors' loses, one on each end,
plus the loss of all the cable or cables,
including connectors and splices, in between. This
is the method specified in OFSTP-14 (multimode,
the singlemode test is OFSTP-7), the standard test
for loss in an installed cable plant.
Note there are three methods of setting the reference, using one, two or three reference cables. The method originally called for in TIA-568, is the one cable method, but that method doesn't work with every type of connector and test equipment interfaces. Therefore most standards now allow for using either one, two or three reference cables as long as the test method is documented along with the test data. The use of reference cables is explained here.
Should You Get When Testing Cables?
(0.5 dB X # connectors) + (0.2 dB x # splices) + fiber loss on the total length of cable
OTDRs are powerful test instruments for fiber optic cable plants, if one understands how to properly set the instrument up for the test and interpret the results. When used by a skillful operator, OTDRs can locate faults, measure cable length and verify splice loss. Within limits, they can also measure the loss of a cable plant. About the only fiber optic parameters they don't measure is optical power at the transmitter or receiver.
are almost always used on OSP cables to verify the
loss of each splice and pinpoint stress areas
caused by installation.
They are also widely used as OSP troubleshooting
tools since they can locate problems in the
cables. Most ODTRs lack
the distance resolution for use on the shorter
cables typical of
In order to use an OTDR properly, it's necessary to understand how it works, how to set the instrument up properly and how to analyze traces. Most OTDRs offer an "auto testing" option. Using that option without understanding the OTDR and manually checking its work often leads to problems. Let's look at how an OTDR works and see how it can help testing and troubleshooting. (Read more about the OTDR)
Since the pulse
is attenuated in the fiber as it passes along the
fiber and suffers loss in connectors and splices,
the amount of power in the test pulse decreases as
it passes along the fiber in the cable plant under
test. Thus the portion of the light being
backscattered will be reduced accordingly,
producing a picture of the actual loss occurring
in the fiber. Some calculations are necessary to
convert this information into a display, since the
process occurs twice, once going out from the OTDR
and once on the return path from the scattering at
the test pulse.
Actual OTDR Trace
Diagram of OTDR trace with events shown
There is a lot
of information in an OTDR display. The slope of
the fiber trace shows the attenuation coefficient
of the fiber and is calibrated in dB/km by the
OTDR. In order to measure fiber attenuation, you
need a fairly long length of fiber with no
distortions on either end from the OTDR resolution
or overloading due to large reflections. If the
fiber looks nonlinear at either end, especially
near a reflective event like a connector, avoid
that section when measuring loss.
Connectors and splices are called "events" in OTDR jargon. Both should show a loss, but connectors and mechanical splices will also show a reflective peak so you can distinguish them from fusion splices. Also, the height of that peak will indicate the amount of reflection at the event, unless it is so large that it saturates the OTDR receiver. Then peak will have a flat top and tail on the far end, indicating the receiver was overloaded. The width of the peak shows the distance resolution of the OTDR, or how close it can detect events.
OTDRs can also detect problems in the cable caused during installation. If a fiber is broken, it will show up as the end of the fiber much shorter than the cable or a high loss splice at the wrong place. If excessive stress is placed on the cable due to kinking or too tight a bend radius, it will look like a splice at the wrong location.
More good reading on fiber optic testing- The JDSU Reference Guide to Fiber Optic Testing: The JDSU Reference Guides to Fiber Optic Testing (Volumes 1 and 2) are written for fiber optic installers, project managers, telecom technicians, and engineers who need to understand, apply, and correctly measure and record the performance of fiber infrastructures. It is the best guide available for fiber optic testing and it's free. Download yourself a copy and read it!