Coherence length

In physics, coherence length is the propagation distance over which a coherent wave (e.g. an electromagnetic wave) maintains a specified degree of coherence. Wave interference is strong when the paths taken by all of the interfering waves differ by less than the coherence length. A wave with a longer coherence length is closer to a perfect sinusoidal wave. Coherence length is important in holography and telecommunications engineering.

This article focuses on the coherence of classical electromagnetic fields. In quantum mechanics, there is a mathematically analogous concept of the quantum coherence length of a wave function.

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 ${\displaystyle \Delta f}$ is the bandwidth of the source.

In optical communications, assuming that the source has a Gaussian emission spectrum, the coherence length ${\displaystyle L}$ is given by

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