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 (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.
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