Fiber optic connectors are produced by polishing their endfaces to create a smooth domed shape. If the shape or endface geometry is precisely controlled, the result will be physical contact between fiber cores when two connectors are mated together. The polishing process creates a very small damaged layer at the tip of the fiber that has a higher index of refraction (index layer) than the glass before the damaged layer. Since reflections in optical fiber are caused by changes in index of refraction, it is the index layer that causes the return loss associated with a mated pair of connectors | Connector Return Loss To a Master vs. Intramated white paper Source of Return Loss Fiber optic connectors are produced by polishing their endfaces to create a smooth domed shape. If the shape or endface geometry is precisely controlled the result will be physical contact between fiber cores when two connectors are mated together. The polishing process creates a very small damaged layer at the tip of the fiber that has a higher index of refraction index layer than the glass before the damaged layer. Since reflections in optical fiber are caused by changes in index of refraction it is the index layer that causes the return loss associated with a mated pair of connectors. Factory Return Loss Measurement to a Master After polishing insertion loss and return loss are measured on each connector by mating to a master or reference connector. The master connector is selected by characterizing the offset of the fiber core from the center of the ferrule to be sufficiently small so that the insertion loss will be the same regardless of the orientation of the offset on the mating connector. The master connector is also subjected to a special polishing process that removes or reduces the thickness of the index layer. The reflection from the master connector fiber endface is then measured by dipping it in index matching fluid to verify that the reflection is very low relative to the reflection level of the connectors that will be mated to it during testing. The return loss of the connector under test will include the reflections due to both the change in index of refraction as the light enters the index layer and the change in index of refraction as the light leaves the index layer Figure 1 . The light reflected from each of these interfaces has traveled different distances and when recombined will interfere with each other either constructively or destructively depending upon the wavelength and the thickness of the index layer. The equation EQ. 1 which describes the return loss
