In optics, a Fabry-Perot interferometer or etalon 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 L between the plates
- the index of refraction n of the medium in between the plates
- 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
Fabry-Perot interferometers or etalons are used in spectroscopy, lasers, and astronomy.