Sometimes when an OTDR trace is performed, a technician may see what looks like a negative loss or power increase — a “gainer.” 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 mold field diameters. This can lead to a situation in which light is launched through a larger mode field diameter into a smaller one. An OTDR will show an effective power gain and elevated loss when the direction of the launch is switched.
