When it comes to optical fiber, dB loss (decibel loss) is a critical metric for determining the quality and efficiency of data transmission. Simply put, dB loss measures the reduction in signal strength as light travels through the optical fiber. 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,”. dB is a relative unit of measurement used to express the ratio between two values, typically power or intensity.