dB loss in fiber optics is the reduction in light signal strength as it travels through a fiber cable, measured in decibels. Every fiber link loses some light along the way, and that loss is expressed in dB because the decibel scale makes it easy to add up small losses across long. 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. A. dB is a relative unit of measurement used to express the ratio between two values, typically power or intensity. In the case of fiber optic cable, we are comparing the power injected at one end of the cable to the power received at the other end. 10 is different from the Neparian.