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bandwidth is often explicitly specified, and thus a collection of wavelengths is understood

Coherence length

In physics, coherence is a property of waves, that enables stationary (i.e. temporally and spatially constant) interference. More generally, coherence describes all properties of the correlation between physical quantities of a wave.

When interfering, two waves can add together to create a larger wave ( constructive interference ) or subtract from each other to create a smaller wave ( destructive interference ), depending on their relative phase. Two waves are said to be coherent if they have a constant relative phase. The degree of coherence is measured by the

interference visibility, a measure of how perfectly the waves can cancel due to destructive interference.

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 will be strong 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 radio-band systems, the coherence length is approximated by

where c is the speed of light in a vacuum, n is the refractive index of the medium, and Δ f is the bandwidth of the source.

In optical communications, the coherence length L is given by

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.

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: