Abstract

The heat generated in the laser host materials, from pumping of rare earth ions, is a limiting factor in scaling solid state lasers to high average powers with near diffraction limited beam quality. This is due to unavoidable temperature gradients produced by the extraction of the heat deposited by the pump in the laser host material. The heat generated induces thermo optic distortions, which result in degradation of the laser beam quality and efficiency. In the past decade, cryogenic cooling of the gain medium has been shown to be an attractive approach for power scaling of solid state lasers, simultaneously avoiding the complex thermo optic compensation systems required for operating high powers lasers at room temperature. Cryogenic solid state lasers have thus been demonstrated to operate successfully at higher powers, with better beam quality and at higher efficiency, due to the significant improvements of the thermo optical and mechanical properties of the laser gain medium when cooled at low temperatures. The lasers demonstrated so far are primarily based on thin disk [1] and rod [2] geometries. We have developed the first fully optimized end pumped Yb:YAG zigzag slab laser design that allows conduction cooling from both sides in a robust laser head design, when operating at cryogenic temperatures. Instead of using indium soldering of the gain medium [1, 3] we have designed and developed an approach that uses all metal components in the laser head. Our approach allows the thin slab to be mounted in direct contact with the metal heat sink, using very thin layers of indium between the laser slab and the metal heat sink to ensure low thermal resistance between the slab and the laser head itself and providing uniform clamping of the slab. The slab is pumped from both ends using two diodes, which are located outside the cryostat. This laser head has shown no permanent mechanical deformations over 100 cooling cycles, as well as providing uniform and efficient heat dissipation for the slab. A typical result of our system is shown in Fig 1, showing the M² measured for both axis, when operating up to 115 W output power as a function of the diode pump power. Fig 2 shows the interferogram when operating at 115W: (a) zero warfront distortion and (b) carrier fringes to show the contrast of the interferogram as well as the beam profile in the far field of the laser operating at this power level. This was achieved after carefully optimizing our system [4] to minimize and ultimately eliminate the wavefront distortions observed when the slab is cooled, resulting in negligible distortion when lasing at up to and very likely beyond 115 W. In our system, there are no thermally induced aberrations or birefringence up to 200W. The approach is readily scalable to higher powers. The latest results will be shown.

© 2013 IEEE