Technicians now know if a fibre link passes or fails the instant they complete the bidirectional test . |
Although continuous fibre cable is recognised as a low loss material for high speed communications , the terminations at each access point are weak links that may result in loss or failure . While the biggest cause of signal loss across fibre optic connectors is contamination , poor installation practices in the pathways can also cause signal loss . For this reason , smart customers require fibre certification of all fibre optic cabling links . Basic Tier 1 tests determine insertion loss , length and polarity . When conducting Tier 1 testing , each fibre link is measured for loss and results are documented . This ensures that the fibre link exhibits less than the maximum allowable loss . Finer details
Enhanced fibre certification goes beyond Tier 1 testing using an OTDR to measure fibre performance in detail . An OTDR creates a graphical signature of a fibre ’ s condition along its length . It represents the fibre and link components ( cable , connectors and splices ) and the quality of the installation by highlighting abnormalities in the OTDR trace . An OTDR is used to ensure that the cable has not been damaged during installation and each splice is properly made . An OTDR certifies that the installation meets design and warranty specifications for current and future applications .
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Another important benefit is that OTDR installation test results can then be archived as a reference to speed troubleshooting for future problems . The OTDR trace helps technicians find locations of cable breaks when troubleshooting traces are compared to installation documentation .
Unlike Tier 1 testers , the OTDR ’ s loss results are inferred and not actual loss measurements . The OTDR uses a unique optical characteristic of fibre to indirectly measure loss . The biggest factor in optical fibre loss is scattering . In fibre , light is scattered in all directions , including some scattered back toward the source . This ‘ backscattered light ’, along with reflected light from connectors or cleaved fibre ends , enables the OTDR to make measurements .
To initiate a test , the OTDR uses a high power laser transmitter that sends a pulse of light down the fibre . Back scattered and reflected light return to the OTDR through the fibre and are directed to a sensitive receiver in the OTDR . For each measurement , the OTDR sends out a very high power pulse and measures the light coming back over time . The light the OTDR receives is reflected light scattered from the pulse passing through the fibre . The OTDR performs sophisticated calculations to associate the backscattered light with an actual location in the fibre .
OTDRs infer loss based on changes in backscatter and assume that it is consistent throughout the link . But that is not always the case . If fibres with different backscatter performance are mixed in a single link , the OTDR reports either more or less loss than what is actually occurring . The amount of this error depends on the test direction . Testing a fibre link in one direction can give different results compared to testing the same fibre link in the opposite direction . That is why bi-directional OTDR testing or testing from both ends of the link and averaging the results is required . It is the only way for an OTDR to accurately and reliably measure overall loss for a link and its components .
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Time issues
Because of the significant time and cost involved in testing from both ends , technicians often try to save as much time as possible by testing all links from one end before moving to the other end . Unfortunately , this method does not work , and it is contrary to test standards and best practices . To accurately test a fibre link in both directions , the launch and tail cords must remain in their initial measurement positions during both tests . But that is not possible if you test all the links from one end before moving to the other . So these technicians are simply substituting one type of error for another .
To solve this dilemma , it is possible to test two fibres at the same time and use a long patch cord to loop the two fibres together . This allows the two
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