cm . __ e __ lli!l 15 January 1996 OPTICS COMMUNICATIONS EISEVJER Optics Communications 123 (1996) 115-120 Variable reflectivity mirror unstable resonator with deformable mirror thermal compensation N. Pavel, T. Dascalu, V. Lupei zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQP Institute zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA of Atomic Physics, IFTAR-MAWRM Department, P.O. Box MG-6, Bucharest RO-76900, Romania Received 3 1 May 1995 Abstract For thermal lensing compensation of a Nd:YAG rod, placed in different positive-branch unstable resonators with super- Gaussian reflectivity profile of the output mirror, a deformable thin glass plate was used as the rear mirror. Unstable resonators with magnifications of M= 1.5 and M= 1.8 were designed for 0.8 mm. mrad value of the beam quality. For unloaded resonators the output energies of a laser working at 10 Hz repetition rate and pump energies up to 50 J per pulse were close to those obtained for 1 Hz repetition rate. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 1. Introduction Unstable resonators that use radial variable-reflec- tivity mirrors as output couplers have been demon- strated to be particularly successful for generation of high-energy diffraction limited beams with a smooth transverse profile [ l-71. By a suitable technology based on vacuum deposition of dielectric thin films on a transparent substrate, different reflectivity profiles such as: parabolic, super-parabolic and super-Gaussian were obtained [ 8-121. The mirror reflectivity profile can be designed to provide an optimum balance between the energy and the beam quality of the laser output. Due to the sensitivity of resonator properties to the thermal lensing of the active media, unstable resonators were initially used in single shot operation or in Q- switched systems with low average output power. However, recently it has been demonstrated that the unstable resonators can be used in high power solid- state lasers if the resonator is properly designed. These new unstable resonator configurations are: (i) the “rod imaging unstable resonator” [ 131 and (ii) the “near- concentric unstable resonator” [ 141. From a pulsed Nd:YAG laser, output beams with divergence about twenty times lower than that obtained with a multimode stable resonator at an average power of 200 W have been obtained in first configuration. The second config- uration is characterised by a much lower sensitivity to thermal lensing as compared to other unstable resonator schemes: a maximum output power of 420 W and beam parameter products below 3 mm. mrad have been reported on a pulsed Nd:YAG laser [ 141. In this paper the results on output beam character- istics obtained from Nd:YAG positive-branch unstable resonators with super-Gaussian reflectivity profile of the output mirrors and thermal lens compensated by a deformable rear mirror are presented. This method of thermal lens compensation was used before in CO, stable resonator [ 151 and in Nd:YAlO, stable and unstable configuration [ 161. Here we report the results on this thermal lens compensation method in unstable resonators with output mirror of super-Gaussianreflec- tivity profile. This technique proves suitable for the 0030-4018/96/$12.00 0 1996 Elsevier Science B.V. All rights reserved SSD10030-4018(95)00468-8