|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 detail.
After fiber 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 fiber optic standards is on the FOA website in Tech Topics and the FOA's own test standards are available free here. 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 fiber optic tech, 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
Before you start, get together all your tools and make sure they are all working properly and you and your installers know how to use them. It's hard to get the job done when you have to call the manufacturer from the job site on your cell phone to ask for help. Try all your equipment in the office before you take it into the field. Use it to test every one of your reference test jumper cables in both directions using the single-ended loss test to make sure they are all good. If your power meter has internal memory to record data be sure you know how to use this also. You can often customize these reports to your specific needs - figure all this out before you go it the field - it could save you time and on installations, time is money!
3. Know the cabling network you're testing
This is an important part of the documentation process we discussed earlier. Make sure you have cable layouts for every fiber you have to test and have calculated a loss budget so you know what test results to expect. Prepare a spreadsheet of all the cables and fibers before you go in the field and print a copy for recording your test data. You may record all your test data either by hand or if your meter has a memory feature, it will keep test results in on-board memory that can be printed or transferred to a computer when you return to the office.
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.
Continuity checking with a visual fiber tracer makes certain the fibers are not broken and to trace a path of a fiber from one end to another through many connections, verifying duplex connector polarity for example. It looks like a flashlight or a pen-like instrument with a light bulb or LED source that mates to a fiber optic connector. Attach the fiber to test to the visual tracer and look at the other end of the fiber to see the light transmitted through the core of the fiber. If there is no light at the end, go back to intermediate connections to find the bad section of the cable.
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 correct 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 with your eye anyway - but you may be able to use your digital camera or cell phone camera.)
Visual Fault Location
A higher power version of the fiber tracer called a visual fault locator (VFL) uses a visible laser that can also find faults. The red laser light is powerful enough for continuity checking or to trace fibers for several kilometers, identify splices in splice trays and show breaks in fibers or high loss connectors. You can actually see the loss of light at a fiber break by the bright red light from the VFL through the jacket of many yellow or orange simplex cables (excepting black or gray jackets, of course.) It's most important use is finding faults in short cables or near the connector where OTDRs cannot find them.
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.
Visual Connector Inspection by Microscope
Fiber optic inspection microscopes are used to inspect connectors to confirm proper polishing and find faults like scratches, polishing defects and dirt. They can be used both to check the quality of the termination procedure and diagnose problems. A well made connector will have a smooth , polished, scratch free finish and the fiber will not show any signs of cracks, chips or areas where the fiber is either protruding from the end of the ferrule or pulling back into it.
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.
And remember to check that no power is present in the cable before you look at it in a microscope to protect your eyes! The microscope will 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 too.
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!)
To measure power, attach the meter to the cable attached to the source that has the output you want to measure (see diagram to 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.
Loss of a cable is the difference between the power coupled into the cable at the transmitter end and what comes out at the receiver end. Testing for loss (also called "insertion loss") requires measuring the optical power lost in a cable (including fiber attenuation, connector loss and splice loss) with a fiber optic light source and power meter (LSPM) or optical loss test set (OLTS.) Loss testing is done at wavelengths appropriate for the fiber and its usage. Generally multimode fiber is tested at 850 nm and optionally at 1300 nm with LED sources. Singlemode fiber is tested at 1310 nm and optionally at 1550 nm with laser sources. The measured loss is compared to the estimated loss calculated for the link, called a "loss budget."
The insertion loss measurement is made by mating the cable being tested to known good reference cables with a calibrated launch power that becomes the "0 dB" loss reference. Why do you need reference cables to measure loss? Why can't you just plug the cable to test into a source and power meter and measure the power? There are several reasons:
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.
You 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 make sure they're good before you start testing other cables!
Standards 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 "single-ended" per FOTP-171 below.
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.
In a 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.
What Loss Should You Get When Testing Cables?
Before testing, preferable during the design phase, you should calculate a loss budget for the cable plant to be tested to understand the expected measurement results. Besides proviiding reference loss values to test against, it will confirm that the network transmission equipment will work properly on this cable. While it is difficult to generalize, here are some guidelines:
(0.5 dB X # connectors) + (0.2 dB x # splices) + fiber loss on the total length of cable
If you have high loss in a cable, make sure to reverse it and test in the opposite direction using the single-ended method. Since the single ended test only tests the connector on one end, you can isolate a bad connector - it's the one at the launch cable end (mated to the launch cable) on the test when you measure high loss.
High loss in the double ended test should be isolated by retesting single-ended and reversing the direction of test to see if the end connector is bad. If the loss is the same, you need to either test each segment separately to isolate the bad segment or, if it is long enough, use an OTDR.
If you see no light through the cable (very high loss - only darkness when tested with your visual tracer), it's probably one of the connectors, and you have few options. The best one is to isolate the problem cable, cut the connector of one end (flip a coin to choose) and hope it was the bad one (well, you have a 50-50 chance!)
FOA Tech Bulletin on troubleshooting.
Virtual hands-on tutorial on insertion loss testing
Videos on testing including insertion loss are on the FOA Channel on
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.
OTDRs 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 premises networks.
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)
How OTDRs Work
Unlike sources and power meters which measure the loss of the fiber optic cable plant directly, the OTDR works indirectly. The source and meter duplicate the transmitter and receiver of the fiber optic transmission link, so the measurement correlates well with actual system loss.
The OTDR, however, uses backscattered light of the fiber to imply loss. The OTDR works like RADAR, sending a high power laser light pulse down the fiber and looking for return signals from backscattered light in the fiber itself or reflected light from connector or splice interfaces.
At any point in time, the light the OTDR sees is the light scattered from the pulse passing through a region of the fiber. Only a small amount of light is scattered back toward the OTDR, but with wider test pulses, sensitive receivers and signal averaging, it is possible to make measurements over relatively long distances. Since it is possible to calibrate the speed of the pulse as it passes down the fiber, the OTDR can measure time, calculate the pulse position in the fiber and correlate what it sees in backscattered light with an actual location in the fiber. Thus it can create a display of the amount of backscattered light at any point in the fiber.
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.
Note the large initial pulse? That is caused by the high-powered test pulse reflecting off the OTDR connector and overloading the OTDR receiver. The recovery of the receiver causes the "dead zone" near the OTDR. To avoid problems caused by the dead zone, always use a launch cable of sufficient length when testing cables.
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.
The limited distance resolution of the OTDR makes it very hard to use in a LAN or building environment where cables are usually only a few hundred meters long. The OTDR has a great deal of difficulty resolving features in the short cables of a LAN and is likely to show "ghosts" from reflections at connectors, more often than not simply confusing the user.
Using The OTDR
When using an OTDR, there are a few cautions that will make testing easier and more understandable. First always use a long launch cable, which allows the OTDR to settle down after the initial pulse and provides a reference cable for testing the first connector on the cable. Always start with the OTDR set for the shortest pulse width for best resolution and a range at least 2 times the length of the cable you are testing. Make an initial trace and see how you need to change the parameters to get better results.
Read more on OTDRs.
Virtual hands-on tutorial on OTDR testing
We highly recommend you read the FAQs on OTDRs.
Long OSP Link Testing
Long distance fiber optic links have unique testing needs. Two factors can limit bandwidth on long links, chromatic dispersion and polarization-mode dispersion. These topics are important but complicated, beyone this basic explanation, so there is a complete discussion on the topic and how testing is done in the FOA Online Reference Guide. More on CD and PMD.
The time may come when you have to troubleshoot and fix the cable plant. If you have a critical application or lots of network cable, you should be ready to do it yourself. Smaller networks can rely on a contractor. If you plan to do it yourself, you need to have equipment ready (extra cables, mechanical splices, quick termination connectors, etc., plus test equipment.) and someone who knows how to use it.
We cannot emphasize more strongly the need to have good documentation on the cable plant. If you don't know where the cables go, how long they are or what they tested for loss, you will be spinning you wheels from the get-go. And you need tools to diagnose problems and fix them, and spares including a fusion splicer or some mechanical splices and spare cables. In fact, when you install cable, save the leftovers for restoration!
And the first thing you must decide is if the problem is with the cables or the equipment using it. A simple power meter can test sources for output and receivers for input and a visual tracer will check for fiber continuity. If the problem is in the cable plant, the OTDR is the next tool needed to locate the fault. More from FOA Tech Topics: Troubleshooting and Restoration
More on Testing & Troubleshooting Fiber Optic Systems
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!
Test Your Comprehension
Table of Contents: The FOA Reference Guide To Fiber Optics
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