The thermal behavior of a Nd:YAG rod under flashlamp pumping has been investigated experimentally using a simple interferometric technique. The degree of flashlamp pumping uniformity and the heat generation rate inside the rod were measured for the single pulse pumping. The temperature increase at the center of the rod under a high-repetition rate pulsed operation was also measured, and the suface haet transfer coefficient was deduced from it. And parameters of an empirical formula for the thermal focal length of the rod were determined in this experiment.
With these results, we have designed and fabricated the optical resonators which could compensate the thermal lensing effect of the rod. In case of a plane-parallel (P-P) resonator, the average output power and the beam divergence of multimode laser were 140 W and 12 mrad, respectively. For a plano-convex (P-C) resonator, the slightly reduced average output power of 110 W was obtained at the same input power as P-P resonator. In order to overcome this lower efficienc
y in P-C resonator, a new three mirror resonator (modified plano-convex resonator : M/P-C resonator) was invented. In case of lower input power in this new resonator, the output power of M/P-C resonator was increased by 15% comparing to the P-C resonator and in the higher input power, the output power was almost the same as P-P resonator. The M/P-C resonator output beam has almost the same divergence in sagittal and tangential plane.
The misalignment (tilt) effect on the output power was also measured for these three kinds of resonator configurations. In higher input power operation, the rear mirror alignment was found to be not so critical for all kinds of resonators. In case of lower input power, the alignment is more critical for the P-C and the M/P-C resonator than for the P-P resonator.
In conclusion, the thermal behavior of a Nd:YAG rod in thermally equilibrium state was investigated using a simple interferometric technique in order to design the more practical laser resonators. A new M/P-C resonator and other conventional resonators (P-P and P-C) were investigated in detail for the output power and the beam divergence viewpoints.
