Optical cavity
Light confined in the cavity reflects multiple times, producing modes with certain resonance frequencies.
Optical cavities are designed to have a large Q factor, meaning a beam undergoes many oscillation cycles with little attenuation.
The most common types of optical cavities consist of two facing plane (flat) or spherical mirrors.
However, this problem is much reduced for very short cavities with a small mirror separation distance (L < 1 cm).
In these cases, rather than using separate mirrors, a reflective optical coating may be directly applied to the laser medium itself.
If the two radii are equal to half the cavity length (R1 = R2 = L / 2), a concentric or spherical resonator results.
A common and important design is the confocal resonator, with mirrors of equal radii to the cavity length (R1 = R2 = L).
By using methods such as ray transfer matrix analysis, it is possible to calculate a stability criterion:[16] Values which satisfy the inequality correspond to stable resonators.
A simple geometric statement describes the regions of stability: A cavity is stable if the line segments between the mirrors and their centers of curvature overlap, but one does not lie entirely within the other.
In addition, for most gain media, thermal and other inhomogeneities create a variable lensing effect in the medium, which must be considered in the design of the laser resonator.
Practical laser resonators may contain more than two mirrors; three- and four-mirror arrangements are common, producing a "folded cavity".
Examples include acousto-optic modulators for cavity dumping and vacuum spatial filters for transverse mode control.
Similarly the pointing stability of a laser may still be improved by spatial filtering by an optical fibre.
To compensate for this, a single pass delay line is also needed, made of either a three or two mirrors in a 3d respective 2d retro-reflection configuration on top of a linear stage.
The two lenses act as a telescope producing a flat phase front of a Gaussian beam on a virtual end mirror.
