Free Spectral Range
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
Due to chromatic dispersion, the group index can deviate from the refractive index, and can be frequency-dependent.
The free spectral range of a Fabry–Pérot interferometer (or a Lyot filter) is the frequency spacing of its transmission peaks. It 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.