Question

What is the physical significance (importance) of free spectral range (δνr) of the resonator in real laser systems?

Answer 1

Answer

The free spectral range of an resonator  is the frequency spacing of its axial (Gaussian-shaped) resonator modes.

The free spectral range is the inversely proportional of the round-trip time of an optical pulse.

And now further The free spectral range of a Fabry–Pérot interferometer is the frequency spacing of its transmission peaks in system.

For larger free spectral range is also leads to a larger resonator bandwidth and also a poor spectral resolution.