Generally, for single-mode connectors, the recommended insertion loss is below 0. Insertion loss and return loss are important parameters used to evaluate the performance of fiber optic connectors. A superior connector will exhibit minimal optical loss, thanks to precise alignment of th s, cost-efectiveness, and. Insertion loss is the loss of optical power that occurs when a fiber connector is inserted into a fiber optic link. It is the difference between the input power and the output power of the link, expressed in decibels (dB).