Return loss, also known as reflection loss or back reflection, is the measurement of the amount of light reflected back towards the source when it encounters a fiber optic connector. It is caused by variations in refractive index, mismatches in fiber core diameter, and surface. Return loss is also known as reflection loss. The lower the insertion loss, the better the performance of. Reflectance is primarily a problem with connectors but may also affect mechanical splices which contain an index matching gel to prevent reflectance.