For spectral quantities, dBm/nm means 'dBm in a 1-nm bandwidth'. Here you can submit questions and comments. 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. ” Optical loss is measured in “dB” which is a relative measurement, while absolute optical power is measured in “dBm,”. It focuses on decibels (dB), decibels per milliwatt (dBm), attenuation and measurements, and provides an introduction to optical fibers. This document is not restricted to specific software and hardware versions. Instead, it quantifies how much a signal has increased or decreased relative to another signal. Thus, a source with a power level of 0 dBm corresponds to 1mW. Instruments measuring in dB can be optical power meters or optical loss test sets (OLTS), with optical power meters usually reading in. This is the signal strength or power level. dBm stands for decibel-milliwatts. Think of it like a way to measure the brightness of a flashlight beam — but for invisible laser light.