Coherence length

Coherence length is the distance over which a wave maintains a predictable phase relationship with itself. It is related to the wave’s temporal coherence and spectral bandwidth.

Temporal coherence: Temporal coherence describes how predictable a wave’s phase remains at different times. A wave’s temporal coherence determines how far it travels before its phase becomes unpredictable.
Optical coherence tomography: Optical coherence tomography is an imaging technique that uses low-coherence light and interferometry to map tissue structure. Its axial resolution depends on the short coherence length of its light source.
Spatial coherence: Spatial coherence describes how predictable phase differences are between distinct points across a wavefront. It concerns separation across a wavefront, unlike coherence length along propagation.
Coherence area: Coherence area is the transverse area over which a wavefront maintains significant spatial coherence. It measures transverse coherence, while coherence length measures coherence along propagation.
Coherence time: Coherence time is the time interval over which a wave maintains a predictable phase relationship. Multiplying coherence time by propagation speed gives coherence length in a uniform medium.
White-light interferometry: White-light interferometry measures surface properties using interference from broadband light. Its short coherence length localizes interference to closely matched optical paths.
Mutual coherence function: The mutual coherence function measures correlations between a wave’s complex amplitudes at two spacetime points. Its decay with separation provides a precise way to define coherence length.
Monochromatic wave: A monochromatic wave has a single frequency and an indefinitely repeating sinusoidal phase in the ideal model. The ideal monochromatic limit has infinite coherence length, unlike finite-bandwidth sources.
Spectral linewidth: Spectral linewidth is the frequency spread of a spectral feature or emitted signal. A narrower linewidth generally corresponds to a longer coherence length.
Low-coherence interferometry: Low-coherence interferometry uses broadband light to detect small differences in optical path length. The coherence length determines the path-difference range that produces detectable interference.

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