page_title: Free spectral range
section_title: Fabry–Pérot interferometer
hierarchical_section_title: Free spectral range / Fabry–Pérot interferometer
caption_attribution_description: Etalon transmission plot
context_page_description: Free spectral range is the spacing in optical frequency or wavelength between two successive reflected or transmitted optical intensity maxima or minima of an interferometer or diffractive optical element.
  The FSR is not always represented by or, but instead is sometimes represented by just the letters FSR. The reason is that these different terms often refer to the bandwidth or linewidth of an emitted source respectively.
context_section_description: In a Fabry–Pérot interferometer or etalon, the wavelength separation between adjacent transmission peaks is called the free spectral range of the etalon and is given by
  where λ₀ is the central wavelength of the nearest transmission peak, n is the index of refraction of the cavity medium,  is the angle of incidence, and  is the thickness of the cavity. More often FSR is quoted in frequency, rather than wavelength units:
  The FSR is related to the full-width half-maximum δλ of any one transmission band by a quantity known as the finesse:
  where  is the coefficient of finesse, and R is the reflectivity of the mirrors.
  This is commonly approximated (for R > 0.5) by
caption: The transmission of an etalon as a function of wavelength. A high-finesse etalon (red line) shows sharper peaks and lower transmission minima than a low-finesse etalon (blue). The free spectral range is Δλ (shown above the graph).
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