# Free spectral range calculator

## About this calculator

This calculator computes the free spectral range of an optical resonator, which is the frequency interval between its resonance frequencies defined as

where is the speed of light in vacuum, is the group index of the resonator material, denotes the physical length of the resonator and the factor 2 is included due to light's double-pass through the resonator in a single round-trip.

This calculator also gives the FSR in wavelength units if the correct center wavelength is supplied.