Choice of the reference method
Some reference books and manuals show that to establish the reference power for loss, only a reference launch cable, launch and reception cable connected with a coupling adapter or even three reference cables is used. In fact, industry standards include all three methods to establish a "0dB loss" reference. The two or three-wire reference methods are acceptable for some tests and are the only way you can test some connectors, but it will reduce the loss you measure in the amount of loss between your reference wires when you set your " You could start the test with faulty launch wires, which would cause all loss measurements to be incorrect. This means that the inspection and testing of the reference cables are very important, in order to ensure that they are in good condition. You could start the test with faulty launch wires, which would cause all loss measurements to be incorrect. This means that the inspection and testing of the reference cables are very important, in order to ensure that they are in good condition.
Mode conditioning for multimode fibers
Most standards for multimode fiber testing include some mode conditioning to ensure repeatable results. The usual method is to use a source whose output meets standard criteria, coupled to a reference launch cable, in which a mandrel roll is used to eliminate higher-order modes. The standards may have different methods, but the only one used in TIA 568 is the most frequent. For more information on modal effects in multimode fiber and mandrel measurements, see the FOA website.
What is the loss you should get when testing the cables?
Before performing the test, preferably during the design phase, you must calculate the estimated optical loss for the cable network to be tested, in order to understand the expected measurement results. In addition to providing reference loss values with which to contrast, you will confirm that the network transmission equipment will function properly on this cable. Although it is difficult to generalize, here are some guidelines:
-For each connector, calculate a loss of 0.3-0.5 dB for adhesive / polishing connectors, 0.75 for pre-polished / splice connectors (0.75 max of TIA-568)
-For each splice, calculate 0.2 dB (0.3 max of TIA-568)
-For multimode fiber, the loss is around 3 dB per km for 850 nm sources; 1 dB per km for 1300 nm. This translates approximately to a loss of 0.1 dB per 100 feet for 850 nm; 0.1 dB by 300 feet by 1300 nm.
-For single mode fiber, the loss is around 0.5 dB per km for 1300 nm sources; 0.4 dB per km for 1550 nm. This translates approximately to a loss of 0.1 dB by 600 feet for 1300 nm; 0.1 dB by 750 feet for 1300 nm.
Therefore, for the loss of a cable network, calculate the approximate loss as detailed below:
(0.5 dB x number of connectors) + (0.2 dB x number of splices) + fiber loss in the total cable length.
Tips for detection and troubleshooting
Most problems with high cable losses are caused by faulty or dirty connectors, splices with high loss or loss of voltage produced during installation. The connectors can be inspected with a microscope to detect dirt, scratches, cracks or other damage. Visual fault locators can verify continuity, proper connections and, if the cable jacket allows, verify that there are no breaks or curvatures of high loss.
If a cable exhibits a high loss, reverse it and test it in the opposite direction using the single-end method, if possible. Since the single-end test only tests the connector at a single end, you can insulate a defective connector as follows: it is the one at the end of the launch cable coupled to the launch cable in the test when measuring loss high.
The high loss in the two-end test should be isolated by performing the test again with the single-end method and reversing the direction of the test to see if the end connector is defective. If the loss is the same, you must test each segment separately to isolate the defective segment or, if it is long enough, use an OTDR.
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