Theoretical research on cavity mode characteristics in a resonator with two rods in tandem

To obtain a laser with high power and high beam quality, it is important to study the dependence of the laser beam quality on the distances between the optical elements or on the aperture size. Considering the thermal expansion of the end planes of the rods, the effect of distance between two rods of a double laser-head resonator on the stability and the beam quality were investigated using ABCD ray transfer matrix. The beam quality factor M², TEM₀₀ mode volume and mode profile with respect to the temperature difference and the distance between the rods in a plane-parallel two-rod resonator without or with thermal birefringence compensation are numerically analyzed. The results show that, in a thermal-birefringence compensation scheme, low M² value can be achieved when the rods are placed near to each other or to the laser mirror. And it is found that the distance between two inner principal planes of two rods should be close to the sum of the thermal focal lengths in order to compensate a thermal birefringence and get high power without optical damage.