Thermal Effects In High Average Power Solid-state Lasers

Solid state lasers have a bright future foreseen. As one of the most important potentials for laser weapons in the future, solid state lasers have several merits such like lightness in weight, smallness of size, compact structure and so on. American scientists are leading the research in the high power solid state laser weapons based on different platforms - planes, ships, vehicles and satellites, for instance. The first laser in the world is a solid state laser. However, with several decades of technical development, the average power of the most powerful solid state lasers is far below the most powerful chemical lasers now. The nub of the problem is the poor performance of thermal management in solid-state lasers.The topic of the dissertation is the thermal effect in solid state lasers and their frequency conversion. There are mainly three important parts in this paper.In the first part of this paper, we analyze the thermal effects in the traditional lasers with an active medium rod strictly. There are severe thermal lens and thermal birefringence in the rod when it is side-pumped and side-cooled. With strict derivation, we have found that the focal length formula of the thermal lens which is demonstrated by Koechner is wrong. What's more, we have proposed the correct formula of the thermal lens in a rod and the revision has been proved by experiments.The length of a real rod is finite and the equations of the thermal stress are too complex to solve directly in such a rod. With the software ANSYS based on finite element analysis, the stress and optical path difference can be calculated numerically. The results reveal that thermal birefringence formula can't calculate the optical path difference of the tangential polarization, while the optical path difference of the tangential polarization is almost as same as the axial polarization.In the second part of this dissertation, we analyze the thermal effects in heat capacity lasers. In order to improve the thermal managements in the solid state lasers, heat capacity lasers have developed rapidly in recent years. The active medium is not cooled during the period the system is lasing. Heat capacity mode is very easy to implement and can improve the thermal effects in the solid state lasers obviously with the result that the average power is increased greatly. A heat capacity Nd:glass laser with an active medium rod is built in our laboratory and it can be operated with about 2000 Watt of the output average power. In addition, we analyze the thermal effect of the heat capacity and derive the temperature profile formula.A Shack-Hartmann sensor nonintrusive measurement for the temperature profile in a heat-capacity neodymium-doped glass rod is proposed. This technique is possible because the optical path length of the rod changes with temperature linearly over a wide range. The temperature change of the solid-state laser rod is often recorded by using a thermocouple, thermal camera, or phase-shifting interferometer. Based on an analysis of temperature-induced changes in length and index of refraction, we can get the temperature profiles from the wavefront reconstructions in real time.Furthermore, a wavefront curvature sensor is used to measure the thermal lens in the rod of the heat capacity laser. This technique is possible because the curvature of the probe wavefront changes with temperature regularly. Based on the relation of the curvature and thermal lens, the focal length of the thermal lens in the rod of the heat capacity laser has been calculated.Considering the heat capacity can only erase the impact of the cooling gradient slope but a rod is still influenced by uneven pump absorption seriously, a disk shaped medium is more potential than rod shaped one when the laser is operated in heat capacity mode. The advantage of a disk is the improvement of the thermal effects of the nonuniform pump absorption. The temperature profile in a disk laser operating in heat capacity mode is analyzed in the paper.With the increase of the temperature of the laser medium in a heat capacity laser, the stimulated emission cross section decrease sharply. What's more, the pump threshold increase and the average power output decrease obviously. The increasing temperature also influences the absorption of the laser medium. With the decline of the absorption cross section, less pump energy is deposited.At last, we analyze the thermal effects in a nonlinear crystal. The thermal effects in a nonlinear crystal also influence the efficiency of frequency conversion when the average power is very high. The upper limit of average power in a single crystal with circle or square cross section is determined by the frequency of the laser, laser damage threshold of coating, the thermal conductivity, the refraction index, the efficient nonlinear index, and the temperature acceptance of the nonlinear crystal when it is side cooled. And what's more, the aspect of width and depth of a slab shaped crystal with rectangle cross section influences the upper limit of average power of the efficient frequency conversion when it is face cooled. It means that the average power of the efficient frequency conversions can be improved greatly by using a slab shaped nonlinear crystal.