Optical Cavity - Practical Resonators

Practical Resonators

If the optical cavity is not empty (e.g., a laser cavity which contains the gain medium), the value of L used is not the physical mirror separation, but the optical path length between the mirrors. Optical elements such as lenses placed in the cavity alter the stability and mode size. 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". Commonly, a pair of curved mirrors form one or more confocal sections, with the rest of the cavity being quasi-collimated and using plane mirrors. The shape of the laser beam depends on the type of resonator: The beam produced by stable, paraxial resonators can be well modeled by a Gaussian beam. In special cases the beam can be described as a single transverse mode and the spatial properties can be well described by the Gaussian beam, itself. More generally, this beam may be described as a superposition of transverse modes. Accurate description of such a beam involves expansion over some complete, orthogonal set of functions (over two-dimensions) such as Hermite polynomials or the Ince polynomials. Unstable laser resonators on the other hand, have been shown to produce fractal shaped beams.

Some intracavity elements are usually placed at a beam waist between folded sections. Examples include acousto-optic modulators for cavity dumping and vacuum spatial filters for transverse mode control. For some low power lasers, the laser gain medium itself may be positioned at a beam waist. Other elements, such as filters, prisms and diffraction gratings often need large quasi-collimated beams.

These designs allow compensation of the cavity beam's astigmatism, which is produced by Brewster-cut elements in the cavity. A 'Z'-shaped arrangement of the cavity also compensates for coma while the 'delta' or 'X'-shaped cavity does not.

Out of plane resonators lead to rotation of the beam profile and more stability. The heat generated in the gain medium leads to frequency drift of the cavity, therefore the frequency can be actively stabilized by locking it to unpowered cavity. Similarly the pointing stability of a laser may still be improved by spatial filtering by an optical fibre.