EXFO - Bidirectional OTDR Testing: Multimode vs. Singlemode Fibers

Our thanks to EXFO for allowing us to reprint the following article.

By Caroline Hamel, B.Eng., OTDR Product Manager, Portable and Monitoring Division


As competition heats up among Europe’s new and established telecommunications companies, network reliability is emerging as the vital factor in the battle to secure a long-term customer base. Quite simply, with the era of zero-downtime networks fast approaching, carriers can no longer tolerate service outages on cables, or even single fibers, designed to transport numerous gigabit-per-second optical channels.


Once an optical cable has been installed, network managers need to be certain that each separate fiber span matches or exceeds the carrier’s specifications. The optical time domain reflectometer (OTDR) remains the only instrument available to characterize fibers at the required level of detail, generating distance versus attenuation data, as well as insertion loss measurements for all splices, defects, kinks, or breaks.


Singlemode OTDR users are reminded that, sometimes, as light crosses from one fiber to the next, backscatter may increase. That way, combined with the actual loss at that location, the slope of backscatter may remain unchanged.


At this point, because of the difference in backscatter, the OTDR trace will therefore show neither loss nor gain. No OTDR will detect the 0 dB event. An acquisition from the opposite end of the fiber will show an event with an overestimated loss measurement. Averaging the two unidirectional results will provide the true loss for that particular event.


Also, on singlemode fibers, OTDR acquisitions taken from opposite directions can be compared and the loss for each event averaged to alleviate the dreaded gainer, or positive event - this is often an issue of concern for the untrained eye. In fact, this averaging procedure removes the error due to different fiber backscatter ratios and provides better loss accuracy for each event.


One of the OTDR’s principal attractions is that it can provide detailed analysis with a single-ended test, requiring just one technician and one test set. However, this approach is really only sensible in multimode premises networks. In the singlemode world, where operators require extremely tight control of overall loss budgets, bidirectional OTDR analysis is very much the order of the day.


Optical Probes


Careful data acquisition is the single most important factor for reliable OTDR characterization of singlemode links. Unfortunately, many field technicians expect miracles from their test instrument. Since the OTDR can see the end of the fiber, it is often assumed that everything along the fiber span will be characterized and measured correctly.


In many cases, though, this is an ill-founded assumption, mainly because the quality of event detection depends heavily upon the signal-to-noise ratio (SNR). This means that when an acquisition becomes noisy - because of insufficient averaging time or inappropriate test settings, for example - the OTDR will struggle to measure losses and, eventually, to locate events with an acceptable level of accuracy.


An OTDR injects...

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