Return loss measures how much optical power is reflected back toward the transmitter due to imperfections at connectors, splices, or interfaces. In modern networks running at 10G, 100G, or even 800G speeds, poor RL can increase bit errors, reduce system reliability, and shorten. Stable optical power is the foundation of every high-capacity optical transport system. Even minor deviations—whether too high, too low, or unstable—can impact signal integrity, trigger service alarms, or interrupt traffic on DWDM, OTN, or long-haul optical line systems. The article Digital Diagnostic Function (DDM) For Optical Modules describes that DDM function can be used for real-time monitoring and fault location of the module's working status, in which the optical module's transmitting optical power and receiving optical power are the key parameters for. SMSR is the ratio of the average optical power of the main mode to the optical power of the most significant side mode under the worst transmission conditions. A. ues related to optical transmitters. An optical transmitter acts as the interface between the electrical and optical domains by con-verting e ectrical signals to optical signals.