In physics, coherence length is the propagation distance from a coherent source to a point where an electromagnetic wave maintains a specified degree of coherence. The significance is that interference can exist within a coherence length of the source, but not beyond it. This concept is also commonly used in telecommunication engineering.

In long-distance transmission systems, the coherence length may be reduced by propagation factors such as dispersion, scattering, and diffraction.

In optical communications, the coherence length L is given approximately by L = λ²/(n Δλ), where λ is the central wavelength of the source, n is the refractive index of the medium, and Δλ is the spectral width of the source.

Coherence length is usually applied to the optical regime.

Source: from Federal Standard 1037C in support of MIL-STD-188

The expression above is a frequently used approximation. Due to ambiguities in the definition of spectral width of a source, however, the following definition of coherence length has been suggested:

The coherence length is the optical path length difference of a self interfering laserbeam which corresponds to a 50% fringe visibility, where the fringe visibility is defined as V = (I_max - I_min)/(I_max + I_min) and I is the fringe intensity.

Helium-neon lasers have a typical coherence length of 20 cm, while semiconductor lasers reach some 100 m. Fiber lasers can have coherence lengths exceeding 100 km.

Electromagnetic radiation | Optics | Waves

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