# Free spectral range calculator

Input

Output

Hz

nm

### Try also:

Round trip time calculator

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