The return loss RL is a measure of the portion of light that is reflected back to the source at the junction. It is expressed in decibel. The higher the RL value in decibels, the lower are the reflections. Typical RL values lie between 35 and 50 dB for PC, 60 to 90 dB for APC and 20 to 40 dB for multimode fibres.
In the early days of fibre-optic plug-in connectors, the abutting endfaces were polished to an angle of 90° to the fibre axis, while current standards require PC (Physical Contact) polishing or APC (Angled Physical Contact) polishing. The term HRL (High Return Loss) is frequently used, but it has the same meaning as APC.
In PC polishing, the ferrule is polished to a convex end to ensure that the fibre cores touch at their highest point. This reduces the occurrence of reflections at the junction.
A further improvement in return loss is achieved by using the APC polishing technique. Here, the convex end surfaces of the ferrules are polished to an angle (8°) relative to the fibre axis. SC connectors are also sold with a 9° angle. They possess IL and RL values identical to 8° versions, and for this reason they have not established themselves worldwide.
Return loss due to reflection
As a result of the junction between two fibres, eccentricities, scratches, and contaminants, portions of the light or modes are diffused at the coupling point (red arrow). A well-polished and cleaned PC connector exhibits approx. 14.7 dB RL against air and 45-50 dB when plugged in.
With the APC connector, although modes are also reflected, due to the 8° or 9° angle they occur at an angle greater than the acceptance angle for total reflection. The advantage: these modes are not carried back in the fibre.
From the calculation of the acceptance angle according to it follows that all modes possessing an angle greater than 7.5° are decoupled after only a few centimetres and therefore cannot reach the source and interfere. A good APC connector exhibits at least 55 dB RL against air and 60-90 dB when plugged in.
By comparison: The fibre itself has a return loss of 79.4 dB at 1310 nm, 81.7 dB at 1550 nm and 82.2 dB at 1625 nm (all values at a pulse length of 1 ns).