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. Although they are closely related and often used together, they describe fundamentally different physical concepts. For example, you might use dB to express the amount of signal loss over a certain length of. dBm or dBmW (decibel-milliwatts) is a unit of power level expressed using a logarithmic decibel (dB) scale respective to one milliwatt (mW). The basic process is straightforward: turn the meter on, set it to the correct wavelength, clean your connectors, plug in, and read the.