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Page 22 EDUCATION q Sometimes when an optical time domain reflectometer (OTDR) trace is performed, a technician may see what looks like a negative loss or power increase, i.e. a "Gainer." What is this? In most scenarios, it is simply an OTDR optical effect that has little to no impact on the transmission properties of the network. The OTDR works using backscattered light, and different amounts of light are backscattered with fibers of different mode field diameters. This can lead to a situation in which light is launched through a larger mode field diameter (MFD) into a smaller one—an OTDR will show an effective power gain and elevated loss when the direction of launch is switched. When splicing fibers from different fiber manufacturers and different fiber types (e.g., G.652D to G.657A2), these one-way gains and losses can look quite startling, but are not representative of the true loss in the network. The best estimate of the true splice loss is from a bidirectional OTDR measurement (a measurement from each end, averaged together). This way the optical effect is removed and the true splice loss is calculated. This method has been tried and tested for many years. TECH TIP: What is an OTDR Gainer? Mark Boxer, Applications Engineer, OFS With the requirement to install more fibers, faster, organizations are looking to reduce their testing times and practices. Although bidirectional measurements are preferred, one-way OTDR measurements are possible. However, if they're used, be sure to coordinate expectations of proper gains and losses with fiber and cable manufacturers. q Illustration of an OTDR splice "gainer." The figure on the left shows an increase in backscattered power or "gainer" at a splice approximately 15 km from end terminal "B." The figure on the right shows an exaggerated loss when the OTDR trace is measured from the opposite direction. The true splice loss is the bidirectional average of the "gainer" and exaggerated losses.

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