3D Ray Tracing Model for Laser Beams Influenced by Thermal Lensing in Solid State Gain Media

Abstract: In the last few decades, research on new laser systems with high output power has strongly progressed. The required pump power for such high power lasing also induces a significant amount of heat load within the gain medium. This thermal influence leads to different effects, the most important are thermal lensing, beam degradation and depolarization. The purpose of this thesis is to develop a ray tracing model for the simulation of thermal lensing, beam degradation and depolarization in single pass solid state amplifiers. The therefore necessary theory is given in the first part of the thesis. The simulation model, which traces rays through the thermally influenced media, is described in detail, just as the evaluation of the data for determining the thermal focal length, beam degradation and depolarization. With this model, the influences of pump and seed beam size, quantum defect and heat distribution, crystal properties and amplifier dimensions are investigated. Those give a general idea how to prevent and compensate for the thermal effects in amplifiers. The thermally induced focal power was shown to rise linear with the pump power and decreases linear with the quantum defect. Also the dioptric power decreases exponentially with increasing pump beam radius. The most dominant influences on the beam quality degradation are the ratio between seed and pump radius, the pump beam shape and the pump power as well. Ratios of seed to pump close to unity lead to stronger beam degradation, while too small ratios may significantly decreases the efficiency of the amplifier. Since ray tracing is already an approximation of the actual ray trajectory, its limits are investigated carefully. Other effects, such as gain guiding or gain saturation, are not included in this simulation, which further limit the simulation results. However, if those limits are not exceeded, the simulation gives good results, as is shown by comparison with experimental data.