L4-A

CW Nd:YAG LASER SYSTEMS
Components of a CW Nd:YAG Laser
CW Nd:YAG lasers are available in output from a few milliwatts to as high as a kilowatt in power. Although they vary considerably in size and complexity of design, all have the same basic elements shown in Figure 1. The active medium is an Nd:YAG laser rod. It is optically pumped by a continuous-pumping lamp and is placed between two external mirrors that form the optical cavity for the laser beam. This section discusses each of the basic subsystems of such a laser.

Laser Rod
The rods used for CW operation are usually from one to four millimeters in diameter and have lengths from one inch to about six inches. Smaller-diameter rods are preferred because they present fewer cooling problems than larger rods. The rod ends are usually antireflection coated from the Nd:YAG wave-length of 1.06 microns. The rod is mounted inside a quartz or glass water jacket. Cooling is provided by water flow directly across the rod surface.

Optical Cavity
The optical cavity of the Nd:YAG laser usually consists of two mirrors mounted separately from the laser rod. Several cavity configurations may be used, but all employ at least one spherical mirror. Both long radius and long radius hemispherical cavities are commonly employed. In some systems, shaping of the beam within the cavity is desirable, and two mirrors with different radii of curvature are used. The HR mirror has a reflectivity of about 99.9% and the output coupler transmission varies from less than one percent on small lasers to about eight percent on larger ones. The optical cavities of Nd:YAG lasers are often equipped with an adjustable or interchangeable aperture for selection of multimode or TEM00 mode operation.