optical communications Optical communication, also known as optical telecommunication, is communication at a distance using light to carry information. It can be performed visually or by using electronic devices. The earliest basic forms of optical communication date b ...

, intensity modulation (IM) is a form of

modulation Signal modulation is the process of varying one or more properties of a periodic waveform in electronics and telecommunication for the purpose of transmitting information. The process encodes information in form of the modulation or message ...

in which the optical

power Power may refer to: Common meanings * Power (physics), meaning "rate of doing work" ** Engine power, the power put out by an engine ** Electric power, a type of energy * Power (social and political), the ability to influence people or events Math ...

output of a source is varied in accordance with some characteristic of the modulating

signal A signal is both the process and the result of transmission of data over some media accomplished by embedding some variation. Signals are important in multiple subject fields including signal processing, information theory and biology. In ...

. 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 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 standard o ...

is possible and has been actively studied since 1979.

Bell Laboratories Nokia Bell Labs, commonly referred to as ''Bell Labs'', is an American industrial research and development company owned by Finnish technology company Nokia. With headquarters located in Murray Hill, New Jersey, the company operates several lab ...

had a working, but impractical, system in 1969. Heterodyne and homodyne 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 In fiber-optic communications, wavelength-division multiplexing (WDM) is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (i.e., colors) of laser light. This techn ...

(WDM). Normal WDM using direct detection does not achieve anything like the close channel spacing of

radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the u ...

FDM.Ananth Selvarajan, Subrat Kar, T. Srinivas. ''Optical Fiber Communication: Principles and Systems'', page 129, Tata McGraw-Hill Education, 2003 .

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 On-off or Onoff may refer to: * On-off control, a type of feedback controller * On-off keying, a type of line modulation * On-off relationship, a form of personal relationship * On-Off Singles, a type of tennis game * On-off switch, a type of e ...

(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). At the receiver end, direct detection (DD) is used to recover the modulated signal. The modulated optical signal is detected by a

photodetector Photodetectors, also called photosensors, are devices that detect light or other forms of electromagnetic radiation and convert it into an electrical signal. They are essential in a wide range of applications, from digital imaging and optical ...

(most commonly PIN or APD

photodiode A photodiode is a semiconductor diode sensitive to photon radiation, such as visible light, infrared or ultraviolet radiation, X-rays and gamma rays. It produces an electrical current when it absorbs photons. This can be used for detection and me ...

), 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.

Intensity modulation with direct detection

See also

References

Further reading