Variation of thermal lens curvature type between the convex and the concave lens for zigzag slab laser

摘要

We investigate the thermal lensing along the direction of heat removal in a zigzag slab laser by numerically computing its temperature distribution, the OPD profile and the focal length of the thermal lens. Particular attention is paid to the dependence of the curvature type of the thermal lens on the number of bounces. As the number of bounces increases consecutively, the curvature type of the thermal lens in the slab thickness direction oscillates between the concave lens and the convex one, but not in an alternate manner. The reason is disclosed that for the convex lens case, the average temperature along the route, through which the ray on the edge of the main lobe aperture travels, is much lower than that at the center. The formation of the concave lens can be well explained in a similar way. In addition, we conclude that the beamlet with a larger number of bounces experiences weaker thermal lensing but more serious wavefront deformation due to the large side lobe portion in the curve of optical path difference. The analysis and the discussion provide a good reference for the design of slab parameters and the selection of the number of bounces. Furthermore, multiple zigzag slabs with different curvature types of thermal lenses can be placed in a cavity or lined up as a multi-stage amplifier, which may compensate the thermal lensing and the wavefront deformation, thus improving the beam quality.