Indicates the transmitter fiber optic module is outputting less optical power than expected. If you are using a fiber cable with less light loss than expected (for example, in a test environment. In fiber-optic networks, insertion loss (IL) and return loss (RL) are two critical metrics that every engineer must understand. While IL measures how much optical power is lost as it passes through a component, RL measures how much power is reflected back toward the transmitter. For details, see Collecting Information and Contacting Technical Support. While optical power meters are the primary power measurement instrument, optical loss test sets (OLTSs) and optical time domain reflectometers (OTDRs) also measure power in testing loss.