Optical return loss is the loss of intensity between the input and output of an optical fibre link due to back-reflections. Significant sources of return loss include fibre bends, misaligned fibre cores, and poor optical hygiene. The loss of optical power can degrade network capacity and reduce signal integrity.
A bend beyond an optical fibre’s specified bend radius (“macrobend”) can cause light to reflect off the core-cladding interface such that the direction of propagation is reversed (back-reflected). Light may also leak into the fibre cladding if the incident angle doesn’t satisfy the requirement for total internal reflection.
Imperfections in the fibre created during manufacturing can result in microscopic bending of the core-cladding interface (“microbend”). Microbends can also arise from lateral stresses or strains exerted on the fibre.
Fibres can be shielded from bending, stresses, and strains by using protective jacketing, or by moving the fibre above or below ground to avoid unwanted bending.
Fibre optic components and splices align fibre end-faces to allow light to transmit from one fibre core to another. In most cases there will be some misalignment of the fibre cores, and this results in back-reflections and/or light leakage. Alignment techniques are available to align fibre end-faces and allow the maximum possible transmission through components and splices.
Optical fibre hygiene
"Optical fibre hygiene" is a term used to describe the cleanliness of fibre end-faces in an optical network. Microscopic particles of dust, lint, or residue present on the core of a fibre can cause significant return loss. The mating of dirty connectors can also embed particles into the glass surface of the fibre, creating scratches or pits that cause signal loss. Inspection and cleaning protocols can be used to increase transmittance.
Test and measurement
Return loss can be measured using optical time domain reflectometry (OTDR) or optical continuous wave reflectometry (OCWR).
Optical time domain reflectometry (OTDR) identifies and locates sources of loss along a fibre link. This is achieved by launching pulses of light into optical fibre at recorded times, and detecting back-reflected light. The pulse travel time (“time-of-flight”) is used to calculate the distance travelled, and by inference, the location of sources of return loss.
Optical continuous wave reflectometry (OCWR) is used to directly measure the strength of reflected power as a result of launching continuous wave (CW) light into a fibre.