In optical transmission media, the optical return loss is increased by changing the light reflection at the FOC end face. This results in return loss values of 60 dB and is referred to as High Return Loss (HRL).
In the case of optical waveguides with flat-ground end faces, part of the light energy is reflected back into the optical waveguide at the end faces, which has a direct effect on the return loss. If the reflected light is reflected so that the light rays exit the optical waveguide, then the return loss increases. This is the case with oblique-ground coupling and APC coupling. These coupling types are characterized by a high return loss, High Return Loss (HRL).