The units dB and dBm stands for decibel and decibel milliwatt, respectively. Optical fibers transmit optical power from the transmitter to. Fiber Optic Measurement Units: "dB" and "dBm" Whenever tests are performed on fiber optic networks, the results are displayed on a power meter, OLTS or OTDR readout in units of “dB. This document is not restricted to specific software and hardware versions. The information in. In optical communications, dB (decibel) is a logarithmic unit used to quantify signal strength, power gain, or loss. It doesn't measure an absolute quantity; rather, it shows how one value compares to another.