gap loss

The power loss that occurs when an optical signal is transferred from one fiber to another that is axially aligned with it, but longitudinally separated from it. Note: The gap allows light from the “transmitting” fiber to spread out as it leaves the fiber endface. When it strikes the “receiving” fiber, some of the light will enter the cladding, where it is quickly lost. [After FAA] 2. An analogous form of coupling loss that occurs between an optical source, e.g., an LED, and an optical fiber. Note: Gap loss is not usually significant at the optical detector, because the sensitive area of the detector is normally somewhat larger than the cross section of the fiber core. Unless the separation is substantial, all light emerging from the fiber, even though it diverges, will still strike the detector. Synonym longitudinal offset loss. [FAA]