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. 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. Note 2: The amount of optical power launched into a given fiber by a given transmitter depends on the nature of its active optical source (LED or laser diode) and the type of fiber, including such parameters as core diameter and numerical aperture.