https://doi.org/10.1351/goldbook.08607
For a Fabry-Perot or laser cavity, wavenumber interval between successive longitudinal cavity modes of vibration; \(\Delta \tilde{\nu} = 1/(2nl)\), where \(l\) is the cavity spacing and \(n\) is the refractive index of the material in the cavity.
Note: SI unit: \(\pu{m-1}\).