Measurement using interfering light fields. A phase difference changes the detected intensity, allowing measurements of displacement, optical path length or a spectrum. The round-trip phase in a Michelson interferometer gives one explicit path-to-phase calibration; Fourier transform spectroscopy recovers spectral information from an interferogram.
A nearly uniform wavefront phase over one collector is called its piston. Differential atmospheric piston changes the pairwise optical phase without requiring a change in the source. Delay tracking and closure phase address different consequences of this phase error; intensity correlations are unchanged by an ideal pure phase multiplication at either detector.
fringe tracking estimates changing optical delay or phase and adjusts delay compensation to keep an astronomical interferometer near a useful interference condition. The delay must first lie inside the finite-bandwidth coherence envelope; rapid tracking then controls atmospheric piston. This is optical-path stabilization, with sensitivity and reference-source limitations.
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