In optical communications, intensity modulation (IM) is a form of modulation in which the optical

power Power most often refers to: * Power (physics), meaning "rate of doing work" ** Engine power, the power put out by an engine ** Electric power * Power (social and political), the ability to influence people or events ** Abusive power Power may ...

output of a source is varied in accordance with some characteristic of the modulating signal. 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 usually by direct detection, not

heterodyning A heterodyne is a signal frequency that is created by combining or mixing two other frequencies using a signal processing technique called ''heterodyning'', which was invented by Canadian inventor-engineer Reginald Fessenden. Heterodyning is us ...

. However,

optical heterodyne detection Optical heterodyne detection is a method of extracting information encoded as modulation of the phase, frequency or both of electromagnetic radiation in the wavelength band of visible or infrared light. The light signal is compared with standa ...

is possible and has been actively studied since 1979. Bell Laboratories had a working, but impractical, system in 1969. Heterodyne and

homodyne In electrical engineering, homodyne detection is a method of extracting information encoded as modulation of the phase and/or frequency of an oscillating signal, by comparing that signal with a standard oscillation that would be identical to the si ...

systems are of interest because they are expected to produce an increase in sensitivity of up to allowing longer hops between islands for instance. Such systems also have the important advantage of very narrow channel spacing in optical

frequency-division multiplexing In telecommunications, frequency-division multiplexing (FDM) is a technique by which the total bandwidth (signal processing), bandwidth available in a communication channel, communication medium is divided into a series of non-overlapping freque ...

(OFDM) systems. OFDM is a step beyond wavelength-division multiplexing (WDM). Normal WDM using direct detection does not achieve anything like the close channel spacing of radio frequency FDM.Ananth Selvarajan, Subrat Kar, T. Srinivas. ''Optical Fiber Communication: Principles and Systems'', page 129, Tata McGraw-Hill Education, 2003 .

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