OPTICS  AND  CAVITIES

       Some small ion lasers have internal optics [notably, Cyonics/Uniphase argon tubes (Uniphase holds the patent)] and resemble overgrown HeNe tubes (with extensive cooling fins), but most lasers, even many small air-cooled tubes, feature external optics. In almost all cases the laser tube has two Brewster windows protruding from the ends of the tube (on quartz stems sealed to the laser tube), so most ion lasers have a polarized output. Like a HeNe, ion lasers have very low gain, so low-loss windows are necessary for operation. Cavity mirrors are mounted on a frame which keeps these aligned. For a longer laser the design of the frame becomes very important, since thermal expansion and mechanical movements can easily misalign the cavity.

       Cavities are frequently of plane-spherical type, the OC being spherical and having a radius of curvature slightly longer than the cavity length. This arrangement allows the use of interchangeable flat optics at the HR end. For multiline use a broadband reflector can be installed in the HR position. Selective reflectors may also be used to allow only certain wavelengths to oscillate, as is frequently done with krypton lasers to select only the red line and argon to select the powerful green or blue line. Wavelength selectors using a prism and an HR are also an option for single-line operation, and most tunable lasers also allow the addition of an intracavity etalon, allowing single-frequency, narrow-spectral-width operation. To reduce losses at the mirrors, mirrors are made from multiple layers of thin dielectric films.