Improved resonator design for rod lasers and slab lasers

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Abstract
The properties of several resonator schemes, which provide high output power and good beam quality simultaneously were investigated both theoretically and experimentally. Experiments were performed with high power Nd:YAG rod systems and Nd:YAG slab systems. Best performance was achieved with unstable resonator schemes. Output power up to 230 Watts was obtained with 6 x 3/8 inch rod and unstable resonators with different reflectivity profiles of the output coupler. Destruction of the high-reflecting spot can be prevented by appropriate resonator design: the thermal lens must be exactly corrected for and a slight misalignment of the rod is necessary. The unstable resonator provides good focusability over the whole range of output power. Compared with optimized stable resonators, the output power was 25 % lower. Considerable improvement of beam quality of slab lasers was achieved by using folded stable resonators, plane- unstable resonators, on-axis unstable resonators and off-axis-unstable resonators. Common stable resonators operating near the limit of stability are too sensitive against the negative refractive power that occurs in the y-direction. Beam parameter products of 1 mm mrad and lower were measured with off-axis unstable resonators.

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