Thermally Induced Effects in Solid State Laser Rods

摘要

An experiment was performed to measure changes in optical length of ruby and neodymium in glass laser rods both during and after excitation by the pump flash. A Twyman-Creen interferometer was used to produce fringes, the fringe movement being correlated with changes in optical length. Movement and distortion of fringes was observed using a multiframe image converter camera.nThe results show that all rods expand linearly with time during the time the pump is on and that all rods develop a distortion which is dependent on the radial energy density distribution within the rod. After the pump goes off, the rods begin a relaxation back to the unexcited state. The relaxation is interrupted when hot air propagates from the flash tube to the rod causing the rod to reheat and causing an extremely concave appearance. The rods then relax slowly to an equilibrium state with ruby reaching this state in much shorter times (0.4 sec) than glass due to its higher thermal conductivity. These results stress the importance of uniform pumping if low beam divergence is to be achieved. Immersion of the rod in an index-matching infrared-absorbing fluid would be desirable not only to pump the laser uniformly but also to shield it from the infrared radiation which causes concave distortions after 20 msec.