Efficient compensation of thermal birefringence of a flash-lamp pumped Nd:YAG laser by a simple but novel method S Mondal, S Datta, S Dey, S Bera, S P Singh, P K Datta Department of Physics & Meteorology, IIT Kharagpur-721 302, India ABSTRACT A simple but novel technique for eliminating depolarization loss resulting from thermally induced stress birefringence of a flash lamp-pumped free running Nd:YAG laser is reported here. The output characteristics of the beam with minimum depolarization loss have been systematically investigated by testing various resonator configuration, pump repetition rate and pump power for a Nd:YAG rod of 4mm diameter and 65mm length. It is shown that, by a tilted Glan-Taylor polarizer inside a stable cavity optimally reduces depolarization loss upto 8%. Keywords: Nd:YAG laser, Thermal birefringence, Depolarization loss 1. INTRODUCTION In recent years, one of the major aims of laser research is to develop a high-power laser with highly polarized beam, but thermally induced stress birefringence of solid state laser is one of the common problems that create depolarization losses in polarization dependent cavities. The birefringence induced in the cylindrical laser rod, are dependent on (r,φ) for incident beam along [111] direction of Nd:YAG crystal. The depolarization loss also depends on direction of crystal plane through which the beam propagates. A well-known method of thermally induced birefringence compensation is to use two identical cylindrical rod spaced by a 90° quartz rotator 1 . Although, this configuration does not avoid the bipolar focusing effect, because the different polarizations propagate along different optical paths in the two rods, and r-φ mode-volumes are non-equivalent in the two rods and phase equalization reconstruction is not complete. Moreover, in this scheme, the birefringence compensation gets incomplete and less effective with increased pump powers. Some other groups have tried using 45° faraday rotator and variable radius of curvature mirror (VRM) 2 and also using quarter wave-plate 3 in between two laser rods but they cannot compensate the depolarization loss significantly. But in all these case the Nd:YAG rod is side-pumped by diode lasers. Diode lasers can emit a selective wavelength that mostly absorbed by the laser gain material causing less heating effect, thermal birefringence and thermal lensing to the material. In contrast, for flash-lamp pumped rod, the spectrum of the flash-lamp is very wide and maximum power of the flash-lamp causes thermal stress in the rod. The temperature at the centre of the rod is much higher than the surface of the rod as it is cooled by the coolant flowing outside. Therefore, radial and tangential thermal stress gradient increases from zero at the centre to the maximum to the surface of the rod. Here we introduce a different approach to compensate depolarization loss. Without using any conventional compensating element in the cavity, just by introducing a tilted Glan-Taylor polarizer inside a stable cavity of a Xe flash lamp pumped Nd:YAG laser, we reduce depolarization loss from 50% to a maximum of 8%, which is very efficient in comparison to other established methods. The expression for depolarization loss is reviewed in next section. Different schemes for compensation of depolarization loss are reviewed in section 3. Our scheme of compensation is described in section 4. The result and its explanation is presented in section 5. 2. THEORETICAL CONSIDERATIONS The optical behavior of the laser rod is like that of an optical lens when a refractive-index gradient is produced due to pumping. An electromagnetic vector is assumed to pass through such a laser rod independently in the radial and tangential polarized directions. In a represented orthogonal cylindrical (r,φ) coordinate system, a laser rod is also assumed to be cylindrically symmetrical. If the gain saturation and laser diffraction are ignored, we Solid State Lasers XX: Technology and Devices, edited by W. Andrew Clarkson, Norman Hodgson, Ramesh Shori, Proc. of SPIE Vol. 7912, 79122F · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.877084 Proc. of SPIE Vol. 7912 79122F-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 03/27/2014 Terms of Use: http://spiedl.org/terms