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Definition: frequency spacing of the axial modes of an optical resonator
The free spectral range of an optical resonator (cavity) is the frequency spacing of its axial (Gaussian-shaped) resonator modes. It is therefore also called axial mode spacing. For an empty standing-wave resonator of length L, it can be calculated as
For a standing-wave resonator filled with a dispersive medium, the free spectral range is determined by the group index, rather than by the ordinary refractive index:
Due to chromatic dispersion, the group index can deviate from the refractive index, and can be frequency-dependent.
More generally, e.g. for an optical resonator containing different transparent media, the free spectral range is the inverse of the round-trip time (round-trip group delay) of an optical pulse.
The free spectral range of a Fabry–Pérot interferometer often limits the optical frequency range in which it can be used. A large free spectral range can thus be desirable. However, for a given finesse, a larger free spectral range also leads to a larger resonator bandwidth and thus a poor spectral resolution.
For a wavelength-tunable single-frequency laser, it often (but not always) limits the achievable mode-hop-free tuning range.
See also: cavities, resonator modes, etalons, group index, group delay
