OPTICS  AND  CAVITIES (YAG Lasers)

      Most YAG lasers, especially those used industrially or in the lab, feature separate optics with all components mounted on a rail for stability. The optics for YAG lasers are usually straightforward, consisting of two mirrors of which one or both are slightly spherical. Spherical mirrors are usually employed to compensate for the thermal lensing effect of the rod, which is quite pronounced in a CW arc lamp pumped YAG laser since pump energies are quite large (2 to 4 kW even for a small CW laser). The dielectric reflective coatings employed on cavity mirrors are frequently transparent to visible light (so much so that they can be mistaken for uncoated optical flats at first glance), allowing the use of a coaxial HeNe targeting laser to locate the infrared beam. The HeNe targeting laser can be mounted on the rail behind the laser, but frequently is mounted parallel to the rail (with mirrors used to steer the beam toward the laser axis) to make the entire assembly more compact. The red HeNe beam passes through both the HR, laser rod, any other components in the system, and the OC. Transparency of cavity mirrors in the visible region also helps facilitate alignment of cavity optics.

         YAG lasers frequently include a Q-switch, allowing the production of fast, intense pulses (many applications depend on these type of pulses). Q-switches are simply attached to the mounting rail between the rod and optics, and most have adjustment screws that allow alignment with the intracavity beam. Q-switches are usually of the acoustooptic (AO) type, using inexpensive quartz or similar glass (which is quite transparent at this wavelength). Another popular option is a second (or third) harmonic generator, generally a simple crystal in a holder which also attaches to the rail.

         Beam expanders are another common component found in YAG lasers. Placed between the rear optic and the rod, these components help fill the entire cross sectional area of the rod with the intracavity beam for higher power extraction from the lasing volume. Since the wavelength of the beam is in the near-IR region, optics may be fabricated using commonly available materials and coatings. Antireflective coatings are usually deposited on the faces of rods as well as on the surfaces of optical components in the system, such as Q-switches and intracavity beam expanding optics.