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Fabry-Pérot interferometer

(FPI, Fabry-Pérot cavity)
(type of chromatic-dispersing interferometer)

A Fabry-Pérot interferometer (FPI or Fabry-Pérot cavity) is an alternate to a grating-based interferometer that can be used as a spectrograph's chromatic dispersion mechanism. Rather than a creating a source's interfering rays using diffraction due to multiple slits, it produces the interfering rays with two parallel partially-reflective surfaces (an etalon), such that a ray striking it (other than precisely perpendicular) reflects between them once, twice, three times, etc., each time displaced sideways a bit. (This light-path between reflecting mirrors is why it is referred to as a cavity.) Subsequently focusing the resulting rays with a lens results in converging light that interferes constructively or destructively based upon wavelength, the length set by the distance between the two partially reflective surfaces. The resulting interference pattern reveals incoming light at a certain wavelength. Like a grating spectrograph, by nature, it produces higher order interference, so the instrument design aims to eliminate or reduce this.

The spectrograph is tuned through the distance set between the parallel reflecting surfaces. I believe imaging versions of such spectrographs are accomplished through scanning.

Such spectrographs have been designed for infrared (including far infrared) and ultraviolet, as well as visible light.

(instrument type,interferometry,spectrography,optics)
Further reading: Interferometer

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