The FOA Reference For Fiber Optics - Testing Fiber Optic Patchcords And Cables Topic: Testing Fiber Optic Patchcords And Cables Patchcord And Connectorized Cable Testing After connectors are added to a cable, testing must include the loss of the fiber in the cable plus the loss of the connectors.On very short cable assemblies (up to 10 meters long), the loss of the connectors will be the only relevant loss, while fiber will contribute to the overall losses in longer cable assemblies. In an installed cable plant, one must test the entire cable from end to end, including every component in it, such as splices, couplers, and connectors intermediate patch panels. Obviously one cannot test cable assemblies in the same manner as, since those tests are destructive. Instead of using a cutback test, one uses a source with a launch cable attached to calibrate the power being launched into the cable under test. This method was not used at first; a cable substitution test was used. ( A cable substitution test is still used for connectors that are not designed to be mated to like connectors, such as some POF connectors.) In this method, one attaches a reference cable of short length (about 1 m) and high quality to the source and records the power. After removing this cable from the source, the cable to be tested is then attached to the source and the power measured.
The loss of the cable is them referenced to the loss of the first cable. This method was abandoned, since it often lead to confusion when the cable under test was better than the reference cable and had a 'gain' not a loss, the coupling to the source was highly unrepeatable, and the test did not adequately test the centering geometry of the fiber in the ferrule of the connector.
A better test, FOTP-171 was developed along the lines of FOTP-34 for connectors, covered in. One begins by attaching a launch cable to the source made from the same size fiber and connector type as the cables to be tested. The power from the end of this 'launch cable' is measured by a power meter to calibrate the launch power for the test. Then the cable to test is attached and power measured at the end again.
One can calculate the loss incurred in the connectors mating to the launch cable and in the fiber in the cable itself. Since this only measures the loss in the connector mated to the launch cable, one can add a second cable at the power meter end, called a receive cable, so the cable to test is between the launch and receive cables. Then one measures the loss at both connectors and in everything in between. This is commonly called a 'double-ended' loss test and is identical to the test for installed cable plants used in and or OFSTP-7 for singlemode. OTDRs are not recommended for cable loss testing as the test method is subject to too many potential errors. If the cable is long enough, the OTDR can be used for reflectance testing (see below.) See the for information on how OTDRs work and their measurement uncertainty. Finding Bad Connectors Since single-ended testing only tests the connector attached to the reference cable, it is a powerful test for determining which connector is bad on a terminated cable.
If a test shows a jumper cable to have high loss, there are several ways to find the problem, starting with visual inspection. If you have a microscope, inspect the connectors for obvious defects like scratches, cracks or surface contamination. If they look OK, clean them before retesting. Retest the launch cable to make certain it is good. Then retest the jumper cable with the single-ended method, using only a launch cable. Test the cable in both directions. The cable should have higher loss when tested with the bad connector attached to the launch cable, since the large area detector of the power meter will not be affected as much by the typical loss factors of connectors.
Mode Power Distribution Effects on Loss in Multimode Fiber Cables The biggest factor in the uncertainty of multimode cable loss tests is the mode power distribution caused by the test source. Choosing a Launch Cable for Testing Obviously, the quality of the launch cable will affect measurements of loss in cables assemblies tested against it. Good connectors with proper polish are obviously needed, but can one improve measurements by specifying tight specifications on the fiber and connectors?
If the fiber is closer to nominal specifications and the connector ferrule is tightly toleranced, one should expect more repeatable measurements. However, it seems that the large number of factors involved in mating losses makes controlling these tolerances impossible. Therefore, it is recommended that launch cables be chosen for low loss, but not specified with tighter tolerances in the fiber or connector characteristics. It is probably much more important to carefully handle the test cables and inspect the end surfaces of the ferrules for dirt and scratches regularly. Optical Return Loss (Reflectance) Testing of Cable Assemblies Testing the optical return loss of cables and cable assemblies is very important for singlemode laser systems, since light reflected back into the laser may cause instability, noise or nonlinearity. While testing the ORL of a cable assembly is similar to that of a connector, using either FOTP-107 or the OTDR method, several factors should be noted to minimize errors. First, be certain that the launch connector is of the finest quality obtainable, and inspect it often for dirt, contamination and scratching.
Repolishing is possible for most keyed, ceramic ferrule connectors and will often improve measurements. Also insure the splice bushing used is kept clean and does not show wear. Remember to terminate the connector on the far end of the cable assembly, otherwise it will reflect light and give false readings. Dipping the connector into index matching fluid or gel will usually do, but putting several tight turns in the fiber to create attenuation will also minimize the reflection effects. OTDRs are limited in their usefulness in testing reflectance of jumper cables, since the jumpers are often too short for the resolution of the OTDR. If using an OTDR, make certain that the OTDR reflectance spike does not exceed the dynamic range of the OTDR (the reflectance peak will be flat-topped), or the measurement will underestimate the reflection. Recommended reading:.
More on optical return loss is on the page about. Videos on cable testing on the FOA Channel on (C)1999-2008, The Fiber Optic Association, Inc.
The FOA Reference For Fiber Optics - 5 ways to test loss Topic: 5 Standard Ways To Test Fiber Optic Cables 5 Standard Ways To Test Installed Fiber Optic Cable Plants Abstract: We often are asked questions about testing installed fiber optic cables that indicate the industry standards are confusing, have little information on measurement accuracy and no guidelines for troubleshooting. This web page is an attempt to clear up some of this confusion. But remember, as Bob Metcalfe, co-inventor of Ethernet, says, 'Standards are wonderful, because we have so many to choose from!' It has been said occasionally that there are in foct 6 ways - if you include. That's true, and this method is used to troubleshoot bad connectors on cables by testing each end separately, but the loss of a cable plant must be done 'double ended' as described here.
Update: The old TIA OFSTP-14 was replaced by a new ISO standard. The TIA has adopted IEC 61280-4-1 as the replacement of OFSTP-14. Most of the two documents is the same, with some important exceptions. For insertion loss, three reference methods are are still approved, but the nomenclature is different - no more 'Method A, B or C' designations- it's now 1, 2 or 3 reference cables. OTDR testing is now an approved second tier test method as long as you use both launch and receive cables.
Reference test cables with 'reference grade connectors' are recommended. Methods are given for testing and verifying the loss of reference test cables.
For multimode modal control, CPR with a mandrel wrap is gone, at least for 50/125 fiber at 850nm, replaced by ',' a complex - and not completely debugged - method of measuring the source output. Most of the changes are nomenclature. In the meantime, continue testing as usual.
There are five ways listed in various international standards from the EIA/TIA and ISO/IEC to test installed cable plants. Three of them use test sources and power meters to make the measurement, while the fourth and fifth use an OTDR. The best way to understand them is to look at the diagrams below.
The source/power meter method, generally called 'insertion loss,' approximates the way the actual network uses the cable plant, so one would expect the loss to be similar to the actual loss seen by the network, which is preferable. The OTDR is an indirect method, using backscattered light to imply the loss in the cable plant, which can have large deviations from insertion loss tests. OTDRs are more often used to verify splice loss or find damage to cables. The differences in the three insertion loss tests are in how we define '0 dB' or no loss. All three tests end up with the same test setup (Figure 1), but the reference power can be set with one, two or three cables as shown in the three setups below. All four methods have measurement uncertainty.
After we explain the methodology, we will examine the measurement uncertainty. Insertion Loss Per TIA OFSTP-14 (Multimode) and OFSTP-7 (Singlemode) (and similar international standards) Insertion loss testing with a test source and power meter simulates the way the cable plant will be used with an actual link.
The test source mimics the transmitter, the power meter the receiver. But insertion loss testing requires reference cables attached to the source and meter to connect to the cable under test.
This insertion loss test can use 1, 2 or 3 reference cables to set the “zero dB loss” reference for testing. Each way of setting the reference gives a different loss. Generally network 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 may assume a 1 cable reference, which can affect the acceptance of the cable plant. Reference per TIA OFSTP-14 (Multimode), 1 Cable Reference, Formerly Called Method B This method uses only one reference cable attached between the test source and the power meter.
The meter, which has a large area detector that measures all the light coming out of the fiber, effectively has no loss, and therefore measures the total light coming out of the launch reference cable. Once the reference is set, the launch reference cable should not be removed from the source, as it may have a different coupled power when reattached. When the cable is tested as shown above, the loss measured will include the loss of the reference cable connection to the cable plant under test, the loss of the fiber and all the connections and splices in the cable plant and the loss of the connection to the reference cable attached to the meter.
It is important to note with all these tests the quality of the reference cables is important in the uncertainty of the measurement. If one uses reference cables with bad connectors, the losses when mated to the cable under test will be higher than they should be, not a good result if you want the installed cable to show the quality of your installation processes! Installers should test all reference cables using FOTP-171 single-ended cable tests to ensure their quality. Cables with losses higher than 0.5 dB per end should be cleaned and retested, then discarded if they do not meet the 0.5 dB max loss. Dirt is always another issue.
If any of the connectors are dirty, measurements will show higher loss and more variability. Reference per TIA OFSTP-14 (Multimode), 2 Cable Reference, Formerly Called Method A This method probably evolved from the telcos where lengths were long and the instruments were brought together only once a day for calibration, plus instruments often had connectors different from the cable plant (e.g. Biconics on the cable plant and SMAs on the instruments.) Here the launch reference cable is attached to the source, the receive reference cable to the meter, then the two cables are mated to set the reference. Once the reference is set, the launch reference cable should not be removed from the source, as it may have a different coupled power when reattached, and one should probably do the same at the meter. Setting the reference this way includes one connection loss (the mating of the two reference cables) in the reference value. When one separates the reference cables and attaches them to the cable under test, the dB loss measured will be less by the connection loss included in the reference setting step.
That's approximate, by the way, since the variations in mating alignment make the loss slightly different each time two connectors are mated. Anyway, this method gives a loss that's less than the 1 cable reference, Method B (above) and because one connection is included in settting the reference, it has a higher variability. Dirt is always an issue. If any of the connectors are dirty, measurements will show higher loss and more variability. If the connectors are dirty when setting the reference but cleaned afterwards before testing the cable, one gets a lower loss, or even a gain! Why would you use this method? If your test equipment has different connectors than the cable under test, but the connectors are all mateable with proper mating adapters, this method can be used.
Reference per TIA OFSTP-14 (Multimode), 3 Cable Reference, Formerly Called Method C Some of the newer connectors are male/female or plug/jack, not two males that use a mating adapter to create a connection. One connector is used on the jack in the wall or patch panel and one is used on a patchcord. Examples are the MTP and MT-RJ. These connectors cannot be mated to test equipment nor can two similar ( plug or jack ) be mated with a mating adapter. Reference cables generally will be patchcords with plugs while the cable under test will have jacks on either end. The only way to get a valid reference is to use a short cable of known good condition as a 'stand-in' for the cable to be tested to set the reference. To test a cable, replace the reference cable with the cable to test and make a relative measurement.
Obviously this method includes two connection losses in setting the reference, so the measured loss will be less by the two connection losses and have greater uncertainty. Dirt is again an issue. If any of the connectors are dirty, measurements will show higher loss and more variability.
If the connectors are dirty when setting the reference but cleaned afterwards before testing the cable, one gets a lower loss, or even a gain! Why would you use this method? If your test equipment has different connectors than the cable under test and the connectors on the cable to test are not mateable, this method can be used.
Since this method works with any connector style, it has been chosen for several international test standards. Since there is some confusion about what each method actually measures, the FOA has a web page that explains it in simple diagrams and math.
OTDR Testing OTDRs (right) test from one end of the cable using the backscatter signature of the fiber to make an indirect measurement of the fiber. OTDRs 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, since a tech is required to attach the receive cable to each fiber as it is being tested.
OTDR Test With Launch Cable Only When testing with an OTDR using only the launch cable, the trace will show the launch cable, the connection to the cable under test with a peak from the reflectance from the connection, the fiber in the cable under test and likely a reflection from the far end if it is terminated or cleaved. Most terminations will show reflectance that helps identify the ends of the cable. Using markers A (set at the end of the launch cable) and B (set at the end of the cable under test), the OTDR can measure the length of the cable under test and the loss of the connection to the cable under test plus the loss of the fiber in the cable under test and any other connections or splices in the cable under test (not shown.) This method does not, however, test the connector on the far end of the cable under test because it it not connected to another connector, and connection to a reference connector is necessary to make a connection loss measurement. OTDR Test With Launch And Receive Cable If a receive cable is used on the far end of the cable under test, the OTDR can measure the loss of both connectors on the cable under test as well as the fiber in the cable and any other connections or splices in the cable under test (not shown.) The placement of the B marker after the connection to the receive cable means some of the fiber in the receive cable will be included in the loss measured. Most OTDRs have a 'least squares' test method that can subtract out the cable included in the measurement of a single connector, but this will not work on a double ended cable loss test. Different Results From Different Reference Methods So just how much does the loss of a cable plant change with the different methods?
The table below shows test results for a 520 meter simulated cable plant with multimode 62.5/125 fiber tested four ways at 850 nm using several different sets of reference cables to see the results. Unfortunately, we did not use a receive cable to get data for the fifth method mentioned above, but we did take data in both directions. Using 10 different sets of reference cables and making multiple measurements allowed calculating an average of each measurement, comparable to what several different test crews might find, plus calculating the standard deviation, a statistical indication of the repeatability of the measurement. Test Method Results, Loss in dB, Standard Deviation 1 Cable Reference 2.96 dB, +/-0.02 dB 2 Cable Reference 2.66 dB, +/-0.20 dB 3 Cable Reference 2.48 dB, +/-0.24 dB OTDR Launch Cable Only 1.91 dB / 2.05 dB (reverse directions) Note how the loss of our test cable plant reflects the comments we made above. The one cable reference method has higher loss than the other methods, but it also has much less measurement uncertainty (standard deviation.) The 2 and 3 cable reference methods have less loss because we have subtracted the connector loss(es) included when we set the reference for 0 dB loss, and the uncertainty is higher because of the greater variance when connected to the reference cables. And the OTDR measurement is significantly lower than the other three methods plus is 0.14 dB different according to which direction we are testing in. Note that if we added to the OTDR loss the average connector loss difference in the other tests, about 0.25 dB, the loss would still only be about 2.1 - 2.2 dB, significantly less than any of the insertion loss tests.
If you want to read more about what each insertion loss method actually measures and why the uncertainty is different for each method, the FOA has a web page that explains it in simple diagrams and math. Interpreting Test Data What is the loss of this cable plant? Well all four of those measurements are strictly according to international standards, so you can take your choice! However, it might cause a problem if you attach some network electronics.
Most network specifications have been written around a loss test that uses a one cable reference method, as does the EIA/TIA 568B test method. If your cable has been tested with any other method, the lower loss you measure will give you a false measurement of system margin. If you are dealing with new, fast networks like Gigabit Ethernet or Fibre Channel which have much smaller operating power margins, you could be in trouble.
The different loss obtained in OTDR measurements comes from it’s completely different methodology. OTDRs use backscattered light to imply the measurement while insertion loss is measured directly by transmission. All OTDRs use laser sources which inject light generally only in the center of the core of the fiber where loss is lower. Some claims for using mode conditioners to make OTDR and insertion loss data correlate has not proven reproducible in independent tests conducted by the FOA (.) A single-ended OTDR measurement, which most tech use, does not test the connector on the far end. Recent standards mention and sometimes require a receive cable for OTDR tests, which would have added 0.2-0.3 dB to the loss measured in the test detailed above.
But having to use a receive cable with an OTDR negates its advantage of being able to be tested from one end by a single technician. Which Test Method Should Be Used? For insertion loss testing, it depends on the types of connectors on your test equipment and the connectors on the cables you need to test. If your cable plant has the same connector styles as your test set, or all the connectors use the same 2.5 mm ferrule style common to ST, SC, FC and some other connectors, you can use the one cable reference method. If your test equipment has ST or SC connectors and you must test LCs on the cable plant, you may have no choice but to use a 2-cable reference or 3 cable reference.
To compare these test results with a 1 Cable Reference, you must add an estimated loss for the connector included in the reference measurement, say 0.3-0.5 dB for a typical connector on a high-quality factory-made patchcord. If your test equipment has ST or SC connectors and you must test duplex male/female connectors like the MT-RJ on the cable plant, you may have no choice but to use a 3-cable reference. To compare these test results with a 1 Cable Reference, you must add an estimated loss for the connectors included in the reference measurement, say two times 0.75 dB for a typical MT-RJ connector on a high-quality factory-made patchcord. Reference methods for fiber optic loss testing. Reference Method (OFSTP-14) Reference Cables Connectors Included in Reference Measurement Estimated reduction in measured loss Estimated increase in errors 2 Cable Reference 2, launch and receive 1 0.2-0.75 dB +/-0.2 dB 1 Cable Reference 1, launch 0 0 dB 0 dB 3 Cable Reference 3, launch, receive and “golden cable” 2 0.4-1.5 dB +/-0.25 dB Table 2. Recommended reference methods for connectors Connector Type Mates to test equipment Does not mate to test equipment Plug to Plug with mating adapter 1, 2 or 3 Cable Reference 1 or 3 Cable Reference.
Plug to Jack 1 or 3 Cable Reference 3 Cable Reference.The 2 Cable Reference can be used if the connector under test can be adapted to the connector interface on the test set – e.g. A tester with SC interface, but ST and FC can be mated to SC with a hybrid mating adapter, so SC reference cables can be used. Additional Reading. (C)1999-2014, The Fiber Optic Association, Inc.
Fiber Optic Testing After the cables are installed and terminated, it's time for testing. For every fiber optic cable plant, you will need to test for continuity, end-to-end loss and then troubleshoot the problems. If it's a long outside plant cable with intermediate splices, you will probably want to verify the individual splices with an OTDR also, since that's the only way to make sure that each one is good. If you are the network user, you will also be interested in testing power, as power is the measurement that tells you whether the system is operating properly. You'll need a few special tools and instruments to test fiber optics.
See Jargon in the beginning of Lennie's Guide to see a description of each instrument. Getting Started Even if you're an experienced installer, make sure you remember these things. Have the right tools and test equipment for the job. You will need: 1.
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Source and power meter, optical loss test set or test kit with proper equipment adapters for the cable plant you are testing. Reference test cables that match the cables to be tested and mating adapters, including hybrids if needed. Fiber Tracer or Visual Fault Locator. Cleaning materials - lint free cleaning wipes and pure alcohol. OTDR and launch cable for outside plant jobs. 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!
Know the 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. 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 a good idea to check connectors with a power meter before looking into it. Some telco DWDM and CATV systems have very high power and they could be harmful, so better safe than sorry.
Fiber optic testing includes three basic tests that we will cover separately: Visual inspection for continuity or connector checking, Loss testing, and Network Testing. Visual Inspection Visual Tracing Continuity checking makes certain the fibers are not broken and to trace a path of a fiber from one end to another through many connections. Use a visible light 'fiber optic tracer' or 'pocket visual fault locator'. It looks like a flashlight or a pen-like instrument with a lightbulb or LED soure that mates to a fiber optic connector. Attach a cable to test to the visual tracer and look at the other end 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 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.) Visual Fault Location A higher power version of the tracer uses a laser that can also find faults. The red laser light is powerful enough to show breaks in fibers or high loss connectors. You can actually see the loss of the bright red light even through many yellow or orange simplex cable jackets except black or gray jackets. You can also use this gadget to optimize mechanical splices or prepolished-splice type fiber optic connectors.
In fact- don't even think of doing one of those connectors without one no other method will assure you of high yield with them. Visual Connector Inspection Fiber optic microscopes are used to inspect connectors 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. And remember to check that no power is present in the cable before you look at it in a microscope protect your eyes! Optical Power - Power or Loss? ('Absolute' vs. 'Relative') Practically every measurement in fiber optics refers to optical power.
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 or a connector and the power that is transmitted through it. This difference is what we call optical loss and defines the performance of a cable, connector, splice, etc. Measuring power 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) 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!) and the proper wavelengths matching the source being used. 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)! To measure power, attach the meter to the cable that has the output you want to measure. That can be at the receiver to measure receiver power, or to a reference test cable (tested and known to be good) that is attached to the transmitter, acting as the 'source', to measure transmitter power.
Turn on the transmitter/source 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. Testing loss Loss testing 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 requires measuring the optical power lost in a cable (including connectors,splices, etc.) with a fiber optic source and power meter by mating the cable being tested to known good reference cable.
In addition to our power meter, we will need a test source. The test source should match the type of source (LED or laser) and wavelength (850, 1300, 1550 nm). Again, read the instructions that come with the unit carefully. We also need one or two reference cables, depending on the test we wish to perform. The accuracy of the measurement we make will depend on the quality of your reference cables.
Always test your reference cables by the single ended method shown below to make sure they're good before you start testing other cables! Next we need to set our reference power for loss our '0 dB' value. Correct setting of the 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.
This method is acceptable for some tests, but 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 calls for a single cable reference, while OFSTP-14 allows either method. Testing Loss There are two methods that are used to measure loss, which we call 'single-ended loss' and 'double-ended loss'. 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 1. The loss of the connector mated to the launch cable and 2.
The loss of any fiber, splices or other connectors in the cable you are testing. This method is described in FOTP-171 and is shown in the drawing.
Reverse the cable to test the connector on the other end. 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 in between. This is the method specified in OFSTP-14, the test for loss in an installed cable plant. What Loss Should You Get When Testing Cables? While it is difficult to generalize, here are some guidelines: - For each connector, figure 0.5 dB loss (0.7 max) - For each splice, figure 0.2 dB - For multimode fiber, the loss is about 3 dB per km for 850 nm sources, 1 dB per km for 1300 nm.
This roughly translates into a loss of 0.1 dB per 100 feet for 850 nm, 0.1 dB per 300 feet for 1300 nm. For singlemode fiber, the loss is about 0.5 dB per km for 1300 nm sources, 0.4 dB per km for 1550 nm. This roughly translates into a loss of 0.1 dB per 600 feet for 1300 nm, 0.1 dB per 750 feet for 1300 nm. So for the loss of a cable plant, calculate the approximate loss as: (0.5 dB X # connectors) + (0.2 dB x # splices) + fiber loss on the total length of cable Troubleshooting Hints: 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!) OTDR Testing As we mentioned earlier, OTDRs are always used on OSP cables to verify the loss of each splice. But they are also used as troubleshooting tools. Let's look at how an OTDR works and see how it can help testing and troubleshooting.
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 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.
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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. 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.
OTDR Limitations 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. Restoration 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. Pre-terminated fiber optic cable assemblies are gaining steam as organizations begin to favor the ease of use provided by factory termination.
Instead of purchasing raw fiber and necessary equipment and having to cut, terminate and test it in the field, organizations can use pre-terminated fiber assemblies to purchase exactly what they need and roll the new network cabling system out quickly. Pre-terminated fiber is a plug and play solution with factory tested and polished connectors ready for immediate installation. The information on this page is an original copyrighted article by VDV Works LLC which has been authored and licensed to Atcom Services, Inc. We welcome you to link this page from your website; however, copying this article in whole or in part is strictly prohibited. Disclaimer: We have provided this article as general installation advice to our customers.
We make no claims about the completeness or the accuracy of the information as it may apply to an infinite amount of field conditions. It is the responsibility of the person or persons using this information to check with all concerned parties, owners and local authorities, etc. Before doing an installation. Users of this information agree to hold Atcom Inc. Harmless form liabilities of any kind relating to the use of this information.
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Posted on 15th Sep 2017 @ 6:15 PM Testing methods: Method A - using two MPO / MTP Fiber jumpers and a connector (consider a direction, as shown in the upper half portion of the following picture ) to set the reference value. After setting the reference value, then the link to be tested come in (the lower half portion of the following picture) and to be tested. Method A test results for each direction including the loss of the fiber and the connector;Therefore, method A is used to test such fiber link: a fiber link with a connector at one end and the other end does not. Method B - only using one MPO/MTP fiber optic patch cord ( consider a direction, as shown in the upper half portion of the following picture )to set the reference value.After setting the reference value, then the link to be tested come in (the lower half portion of the following picture ) and to be tested. Method B test results including both ends loss of the fiber link and connection. Thus, Method B is used to test such fiber optic link: both ends of the link are with connector, which connectors loss is an important part of the entire loss.
Method C - using three optical fiber patch cords and two connectors (one-way)and the length between the two fiber optic patch cords are less than 1Meter; to set a reference value;using the fiber link being tested to replace the fiber optic patch cords between the connectors when testing the link. Thus, Method C test results just including the fiber loss,not including the loss of the both two ends.
Matters needing attention 1. Firstly, ensuring the testing patch cords and being tested patch cords connectors end faces are clean; 2. Ensuring the testing patch cord and the being tested patch cord are made and female separately, MPO / MTP patch cords are male connectors with female connectors during the testing and operation; 3. As the MPO / MTP patch cords are with A, B, C polarity distinction, please pay attention to select the appropriate test line when testing the link. The test equipment must be carried out with a multi-channel multi-function devices.
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