Longitudinal cavity mode ($\Delta \nu =c/(2L)$)

A normal mode whose longitudinal phase satisfies the resonator's round-trip condition. Holding the transverse mode fixed, an empty nondispersive two-mirror cavity of length L has adjacent optical frequencies separated by $c/(2L)$. An offset from mirror or transverse phases changes the position of the mode sequence without changing this ideal spacing. A dispersive cavity instead uses its round-trip group delay.