Optical power margin: In an optical communications link, the difference between (a) the optical power that is launched by a given transmitter into the fiber, less transmission losses from all causes, and (b) the minimum optical power that is required by the receiver for a specified level of performance. An optical power margin is typically measured using a calibrated light source and an optical power meter. Note 1: The optical power margin is usually expressed in dB. At least several dB of optical power margin should be included in the optical power budget.