A good dBm (decibel-milliwatt) level for fiber optic communication typically ranges from -3 dBm to -9 dBm. This range ensures optimal signal strength and quality for data transmission over fiber optic cables. 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,”. Fiber optic internet transmits data using pulses of light traveling through thin glass strands. If the optical power injected was -20 dBm and the power received at the other end -21 dBm, then the. Decibel or dB is a unit to measure the amount of signal strength or loss in a sound system or an amplifier.