# 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

$FSR = \dfrac{c}{2n_gL}$

where $c$ is the speed of light in vacuum, $n_g$ is the group index of the resonator material, $L$ 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.