The general heat conduction equation is solved for the case of variable thermal conductivity and heat desposition. The solution is applied to some cases of interest for understanding the thermal gradients which exist in high average power solid state lasers. The effect of the temperature dependence of the thermal conductivity and the spatial inhomogeneity of the excitation energy on the accuracy of calculating the magnitude of the temperature gradient in a laser rod is considered. The analytical solutions for some cases of interest are presented, as are some specific results.
Laser heads based on two and three dimensional nonimaging concentrators were built and tested. The ray tracing and experimental results for concentrators illuminated by imaging primary concentrators are presented. These concentrators are currently used for pumping of solid state lasers of five hundred watts of laser power.
A compact, parallel array of three Nd:Cr:GSGG laser rods is used to construct a quasi-cw laser. The array is pumped by concentrated solar light and is mounted in a single solar concentrator. The three laser rods use a common pair of laser mirrors to define the optical resonator. The three laser beams are not coherently coupled in these experiments. The simplicity of the design, and its reasonable stability in terms of vibration and optical misalignment, suggest that the design may be scalable for higher power.
The temperature dependence of solar-pumped solid state lasers of Nd:YAG and two types of Nd:Cr:GSGG was studied over the temperature range of +30 to -60°C in a quasi-cw mode. All lasers had higher output powers at -40°C. The Nd:Cr:GSGG laser with a chromium concentration of 2.5 at. % produced 70 W of power at -40°C, quasi-cw. If extrapolated to true cw operation this is equivalent to 350 W. The temperature dependence of the laser performance is attributed to changes in both the stimulated emission cross section and the resonator configuration.
The relative performance of solar-pumped Nd:YAG and Nd:Cr:GSGG lasers was evaluated at both 300 and 80 K. Measurements of the slope efficiency and the lasing threshold were made on several lasers containing these crystals. The stress-induced birefringence and the divergence were also studied. The measurements were used to calculate the values of the intrinsic efficiencies and the losses at both temperatures. The possible mechanisms for the observed temperature dependence are discussed. Due to the improved thermal conductivity of the laser crystals at low temperature, all lasers showed significantly improved performance at low temperature. Both the slope efficiencies and the thresholds improved by a factor of 2 to 3 on cooling. The absolute value of the beam quality, and its sensitivity to changes in the resonator configuration or pump power were significantly better at low temperature.
Laser performance degradation occurs in solar pumped solid state lasers due to thermal
effects in the active medium. We demonstrated improvement of the beam quality by reduction
of the operating temperatures, as well as the use of phase conjugation to eliminate the induced