The primary factors that contribute to return loss in mated fiber optic connections, as explained in the sources, are:•Refractive index mismatches at the mating interface1 . These mismatches can occur even when the fibers are perfectly clean and have ideal geometries2 .◦Differences in refractive indexes between the two mated fibers1 .... While ideally these would be the same, slight variations can exist4 . The return loss due to this difference in refractive indexes is defined by the formula: (RL(dB) = 10 * LOG((\frac{n_1 - n_2}{n_1 + n_2})^2))5 . However, in practice, measured return loss rarely achieves the theoretically high values predicted by this simple model alone4 ....◦Refractive index changes due to the polishing process1 .... Polishing the end face of connectors creates a "damage layer" on the fiber's end face with a different refractive index7 .... This damage is a significant factor in refractive index mismatches1 .◦Refractive index changes due to fiber deformation from the connector spring force (Hertzian contact stress)1 .... The pressure exerted by the springs in the connectors to maintain physical contact can deform the fibers at the mating point, leading to a change in the refractive index in that region11 . This effect is more pronounced in connectors with fiber protrusion or undercut12 ....