Intensity modulation

Intensity modulation (IM): In optical communications, a form of modulation in which the optical power output of a source is varied in accordance with some characteristic of the modulating signal.

Note: In intensity modulation, there are no discrete upper and lower sidebands in the usually understood sense of these terms, because present optical sources lack sufficient coherence to produce them. The envelope of the modulated optical signal is an analog of the modulating signal in the sense that the instantaneous power of the envelope is an analog of the characteristic of interest in the modulating signal. Recovery of the modulating signal is by direct detection, not heterodyning.

Source: from Federal Standard 1037C and from MIL-STD-188