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In optics, a Fabry-Perot etalon (or interferometer) is typically made of two parallel highly-reflecting plates. Its transmission spectrum as a function of wavelength exhibits peaks of large transmission corresponding to resonances of the etalon.

The basic characteristics of a Fabry-Perot etalon are:

  • the reflectivity R of the plates (mirrors)
  • the distance D between the plates
  • the index of refraction n of the medium in between the plates

The fundamental properties of a Fabry-Perot etalon are:

  • the free spectral range[?] (FSR), which is the wavelength separation between two consecutive transmission peaks
    • FSR = c/(2nL), where c is the speed of light
  • the full-width at half-maximum (FWHM) of a transmission peak
  • the finesse F = FSR / FWHM

A Fabry-Perot interferometer differs from a Fabry-Perot etalon in the fact that the distance D between the plates can be tuned in order to change the wavelengths at which transmission peaks occur.

Fabry-Perot interferometers or etalons are used in spectroscopy, lasers, and astronomy.

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