Beam propagation in an inhomogeneous medium of a static gas cesium diode pumped alkali laser: three-dimensional wave optics and fluid dynamics simulation

Abstract

Analysis of beam propagation and kinetic and fluid dynamic processes in static Cs diode pumped alkali lasers (DPALs), using the wave optics model and gasdynamic code, is reported. The analysis is based on a three-dimensional, time-dependent computational fluid dynamics (3D CFD) model. The gas flow conservation equations in the DPAL cell are coupled to a fast-Fourier-transform algorithm for the laser beam transverse modes’ propagation to obtain a solution of the scalar paraxial wave equation where the gain and refractive index in the DPAL medium affect the wave amplitude and phase. Using the CFD and beam propagation models, the gas flow pattern and spatial distributions of the pump and laser intensities and the laser beam phase in a plano-concave resonator were calculated for end-pumped Cs DPAL. The DPAL medium temperature and refractive index, along with the laser power and laser beam quality factor M2, were calculated as a function of pump power. The results of the theoretical model for laser power were compared to experimental results of Cs DPAL. In addition, the pump and laser induced thermal effects in the DPAL cell on the laser beam quality were studied for Cs DPAL with hydrocarbon only as a buffer gas. For methane it was found that the temperature and the resulting refractive index gradients in the DPAL cell are larger than for He rich buffer gas. The large radial gradient of the refractive index in the heated gain medium, achieved for the present resonator and wide-aperture pump beams, along with the fact that in gases the thermo-optic coefficient is negative, results in improvement of M2 as compared to a gain medium with a uniform refractive index. This counterintuitive conclusion is contrary to the behavior of the beam quality of solid-state lasers, which deteriorates when the gain medium is heated by the pump beams.