Longitudinal modes
are standing waves along the optical axis of the laser.
Standing waves
are formed when two waves with the same frequency and amplitude are moving
against each other.
Optical cavity
is created by two mirrors at both sides of the laser.
The standing waves
inside a laser are created when the electromagnetic radiation is forced
to move back into the cavity from the mirrors.
The allowed frequencies inside an optical cavity
are determined by the length of the cavity (L) and the index of refraction
of the active medium.
Only those frequencies which create nodes
at both mirrors are allowed. Thus, the cavity length
must be an integer multiplication of half their wavelengths.
The allowed frequencies are spaced
at constant interval, which is equal to the basic frequency of the cavity.