Laser materials are usually contained between mirrors. This produces an optical resonant cavity.   In the early stage of the lasing process, spontaneous photons are emitted in every direction, as are stimulated photons.  All of these, with the exception of those propagating nearly along the axis of the cavity, quickly pass out of the sides of the laser.  In contrast, the axial beam continues to build up as it bounces back and forth across the active medium.  This accounts for the degree of collimation of the laser beam. Though the medium acts to amplify the wave, the optical feedback provided by the cavity converts the system into a light generator.

The light in the cavity is in the form of a resonant standing wave. There must be a node at each mirror and this can only happen when the distance between the mirrors L is equal to an integral number of half wavelengths i.e. when L = n( /2) and these standing waves have frequencies f = n(c/2L).

These modes of oscillation are the ones that will be sustained in the cavity and the emerging beam is restricted to a region close to these frequencies.

In the laser cavity, there are therefore an infinite number of modes of oscillation separated by a frequency f = (c/2L). However, only those optical frequencies supplied by the radiative transitions of the active medium can give light. Thus, the cavity will select and amplify certain narrow bands (the resonant modes of oscillation) within this optical emission. This is the origin of the quasi-monochromaticity of laser beams.

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