Longitudinal mode

A longitudinal mode pattern has its nodes located axially along the length of the cavity.

Transverse modes, with nodes located perpendicular to the axis of the cavity, may also exist.

In practice, the separation distance of the mirrors L is usually much greater than the wavelength of light λ, so the relevant values of q are large (around 105 to 106).

The frequency separation between any two adjacent modes, q and q+1, in a material that is transparent at the laser wavelength, are given (for an empty linear resonator of length L) by Δν: where c is the speed of light and n is the refractive index of the material (note: n≈1 in air).

The frequency spacing of longitudinal modes in the cavity is then given by: where ni is the refractive index of the i'th element of length Li.