2-D mandates a minimum return loss of 20 dB for multimode connectors and 26 dB for single-mode PC (physical contact) connectors. FiberLife is here to guide you through the causes of loss in fiber optic adapters and provide optimization methods to help you choose and use these adapters effectively, thereby enhancing network efficiency. What Is Loss in Fiber Optic Adapters? In fiber optic networks, “loss” refers to the. Insertion loss and return loss are important parameters used to evaluate the performance of fiber optic connectors. 2-D, the maximum allowable insertion loss. When measuring the attenuation effects of the fiber connectors, insertion loss (IL) and return loss (RL) are two essential parameter measurements.