Intensity modulation

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

The recovery of the modulating signal is typically achieved by direct detection, not heterodyning. However, optical heterodyne detection is possible and has been actively studied since 1979. Bell Laboratories had a working, but impractical, system in 1969.<ref>T. Okoshi, Coherent Optical Fiber Communications, pages 2-3, Springer, 1988 Template:ISBN.</ref> Heterodyne and homodyne systems are of interest because they are expected to produce an increase in sensitivity of up to Template:Nowrap<ref>T. G. Hodgkinson, D. W. Smith, R. Wyatt, D. J. Malyon, "Coherent optical fibre transmission systems", in Bishnu P Pal (ed), Fundamentals Of Fibre Optics In Telecommunication And Sensor Systems ,page 470, Bohem Press, 1992 Template:ISBN.</ref> allowing longer hops between islands for instance. Such systems also have the important advantage of very narrow channel spacing in optical frequency-division multiplexing (OFDM) systems.<ref>Chinlon Lin, "Opto-electronics and the information age: a perspective", in Bishnu P Pal (ed), Fundamentals Of Fibre Optics In Telecommunication And Sensor Systems ,page 20, Bohem Press, 1992 Template:ISBN.</ref> 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.<ref>Ananth Selvarajan, Subrat Kar, T. Srinivas. Optical Fiber Communication: Principles and Systems, page 129, Tata McGraw-Hill Education, 2003 Template:ISBN.</ref>

Intensity modulation with direct detection

Intensity Modulation / Direct Detection (IM/DD) is a scheme is simple and cost-effective in fiber optic communication, making it a suitable for various optical communication applications. It involves modulating the optical power of the carrier signal to represent the transmitted data. This modulation can be achieved using techniques, such as on-off keying (OOK). The intensity-modulated optical signal is generated by modulating the amplitude or the current of the light source, typically a laser diode with one or two cavity designs such as Fabry-Perot or distributed feedback (DFB).<ref name=":0">Template:Cite journal</ref>

At the receiver end, direct detection (DD) is used to recover the modulated signal. The modulated optical signal is detected by a photodetector (most commonly PIN or APD photodiode), which converts the optical power variations into corresponding electrical current or voltage variations. The output of the photodetector is then processed and decoded to retrieve the original information.<ref name=":0" />

See also

• Photoacoustic Doppler effect

Further reading

• William Shieh, Ivan Djordjevic, OFDM for Optical Communications, Academic Press, 2009 Template:ISBN.