Optical Return Loss (ORL) and Insertion Loss (IL)
In optical networks, return loss is the loss of light energy resulting from discontinuity of refractive index through the transmission media. In practice, the discontinuity is often most pronounced at an air-glass interface such as at a fiber endface. Insertion loss is defined as the loss of light energy resulting from the insertion of a device in an optical link.
Measuring ORL and IL is typically accomplished with a light source / power meter combination, or by using a dedicated instrument such as an OTDR, Return Loss Meter or Backscatter Reflectometer.
Standards bodies with recommendations for ORL testing include Telecordia (GR-1312-CORE) the International Engineering Consortium (IEC-61300-3-6) and the International Telecommunications Union (ITU-T G.691, G.692, G.957 and G.959)