We have built a cw, diode-laser-pumped, Nd;YAG slab laser that emits 72 W of multimode power when pumped with 235 W, or 40 W of TEM00 power when pumped with 212 W of diode laser power. The slope efficiencies are 36% for multimode operation and 22% for TEM00 mode operation. The laser uses the zig-zag slab geometry to reduce the thermal effects associated with high power operation, resulting in less than one wave of distortion at the full pump power. Reasonable efficiency for the side- pumped slab design was obtained by confining the pump power within a gold-coated box which surrounds the slab. TEM00 mode operation was obtained in a simple three-mirror folded cavity. The Nd:YAG slab acted as an aperture in the cavity and the astigmatism due to off axis incidence on a curved mirror corrected for a minor 1 meter cylindrical thermal lens. A significant advantage of our design over previous slab lasers is a new Teflon AF protective coating on the slab total internal reflection surfaces which greatly simplifies the mounting and cooling of the slab laser medium.
To achieve the design goal of 50 watts cw in a single-frequency, fundamental mode we have chosen a slab laser design. The slab geometry with a zig-zag optical path eliminates stress birefringence and thermal focusing to first order in a uniformly pumped, ideal slab. However, at the thermal loads we are considering, higher order effects become significant. We have developed a computer program to analyze the full three dimensional behavior of the thermally loaded slab and have used this code in an attempt to minimize the wavefront distortion of a beam as it traverses the slab. The resonator design is also critical in achieving single- frequency, fundamental mode operation with high extraction efficiency. Because of the rectangular geometry and size of the slab laser, we have chosen to build a stable-unstable resonator. Single frequency operation is obtained by injection locking a ring cavity. Mode selection is achieved in the wide transverse direction of the slab by using an unstable cavity with a super-Gaussian mirror. An unstable resonator supports large mode volumes and has large discrimination against higher order transverse modes. In addition, a super-Gaussian mirror profile provides efficient extraction in a high quality mode. Our resonator design should lead to efficient operation in a near TEM00 mode with a slope efficiency approaching those of slab lasers with multimode output.