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High average power picosecond lasers have been studied intensely because of their suitability for industrial applications such as laser micromachining [1–3], drilling precise holes for fuel injection [4], thin film patterning for solar cells [5]. Neodymium doped yttrium vanadate (Nd:YVO4) crystal is a promising material for high average power picosecond lasers, because of its large stimulated emission cross-section and broad emission band, in comparison with conventional Nd:YAG [6, 7]. However, the power scaling of Nd:YVO4 lasers is limited by thermally induced optical distortions that are quite significant in these lasers due to poor thermal properties of active medium leading to severe degradation of beam quality. The spherical component of thermal lens can be compensated by proper laser cavity design, i. e. choosing right radius of curvature and position of spherical resonator’s components. Nevertheless, the non-spherical component cannot be compensated in the same manner. This leads to reduced efficiency and multi-mode oscillation as the heat load increases. Spherical aberrations of thermal lens of the active media are quite severe when the solid state lasers are strongly pumped. The aberrations influence the diffraction loss, the output power, the mode profile, and other laser beam properties [8]. Diffraction limited output has been demonstrated in high average power fibre lasers [9]. However, this technology has limited applications in high peak power lasers due to limited aperture of amplifying media that causes optical damage problems and appearance of nonlinear effects at relatively low laser energies. In this case, traditional end-pumped rod laser still holds its importance due to technical simplicity and virtually unlimited clear aperture. However, the performance of bulk-crystal based high power lasers is always limited by a thermal loading which causes thermo-optical and thermomechanical perturbations. This is particularly important for longitudinally pumped systems where large thermal gradients arise from deposition of heat within a small volume near the pump facet of the laser crystal. Therefore, thermal lens mitigation is crucial for efficient diode end pumped laser design. Effective thermal lens compensation in diode-pumped and flash-lamp lasers employing phase conjugation has been demonstrated [10, 11]. Due to added complexity, this approach, however, is not desirable for industrial application aimed laser design. Alternative technique for thermal lens aberration correction is the use of adaptive and deformable mirrors [12, 13]. However, mirrors based on MEMS or piezocrystals are relatively expensive and sometimes also lack in lateral resolution. Recently a thermooptically driven adaptive mirror based on thermal expansion has been proposed

Thermal lens compensation in high average power diode pumped Nd:YVO4laser using aspheric mirror

Thermal lens compensation in high average power diode pumped Nd:YVO₄laser using aspheric mirror