output frequencies of a laser are determined by several factors.
First, the gross wavelength is determined by the energy uncertainty
(broadening) of the laser transition, which determines the
wavelength and overall linewidth. Nonetheless, at any given
instant, only a relatively few frequencies within this overall
envelope are allowed to oscillate. These "longitudinal modes"
result from the boundary conditions that, in a conventional
two-mirror lasers, the amplitude of the wave must be zero
at the mirror surface (i.e., that the oscillating wave is
a standing wave). This means only those laser frequencies
that meet the criteria
operate, where c is the speed of light, L is the effective
cavity length, and N is an interger. Adjacent modes are typically
illustration below shows the lasing envelope of a helium neon
laser operating at 632.8nm with a cavity spacing of 23cm.
This results in a mode spacing of 640MHz. Since the width
of the gain curve (FWHM) is only 1400MHz, only two longitudinal
modes can operate at any given time. If the laser were twice
as long, four longitudinal modes could operate simultaneously.
the allowable longitudinal modes are a function of cavity
length, the frequency will change as the cavity length changes.
In lasers where only a few longitudinal modes can operate,
these changes will cause outpout power to fluctuate as the
modes sweep under the gain curve