Study On The Stability Of Mirrors Of High Power TEA CO₂ Laser

Because TEA CO₂ laser has the advantages of high single-pulse energy, high peak power, high average power and its wavelength in the atmospheric window, it is promising in the application of optoelectronic countermeasure, laser propulsion and laser weapon broadly. These applications required kilometers or even thousands of kilometers laser transmission. Therefore, the good output beam quality is generally required.In high power laser system, the resonator mirrors absorb laser energy during laser works, which may result in temperature rising and nonuniform thermal deformations, even destruction of the mirrors. The deformation of the resonator mirrors changes the design shape of resonator, which deteriorates the output laser beam quality.In addition to laser itself, the thermal distortion of mirrors and windows of laser is closely related to the output beam quality. This uneven thermal distortion would lead to decline of beam quality, and restricted the long-distance laser transmission seriously. So, it is very important to study on the thermal stability of mirrors and windows in high power laser system. This dissertation is devoted to reduce the thermal distortion of mirrors and windows.At first, according to general thermal conduction equations, the analytical expressions of the temperature distributions and the thermal deformations of laser mirrors subject to irradiation of high power lasers are obtained by making use of Bessel's function methods. In addition, the temperature distributions and thermal deformations of Si mirror for high power lasers are given.Secondly, A Vortex-Tube-Cooling system was put forward, and used in the cooling the windows and convex mirrors in coaxial output unstable resonator. The vortex tube is a device with no moving parts and convert an incoming compressed fluid stream (such as air) with homogeneous temperature into two streams with different temperatures, one wanner than the inlet and one cooler than the inlet. The Vortex-Tube-Cooling system is consisted of swirling flow gas module, vortex tube and intake pipe. From the numerical simulation and experimental studies, it can be drawn that the developed Vortex-Tube-Cooling system can effectively reduce the thermal deformation of the convex mirrors and windows, and improve the laser beam quality.Next, the thermal deformation distributions of the water-cooled mirror with micro-channel were analyzed in detail. The structure of the mirror has significant effects on the thermal deformation, with the thickness of the mirror, channel size and the cooling area being the main factors. Among these factors, the cooling area shows the most significant role. The whole cooling mirror is not the optimal structure, even small deformation can be achieved by using partial cooling area. The channel parameters of water-cooled mirror have significant effects on the thermal deformation. For a given rate of flow of water, the temperature rise and thermal deformation decrease with the decreasing of channel width. With a constant rate of flow and channel width, the thermal deformation decreases with the channel depth. For the water-cooled mirror, the channel size smaller effect is better. The linear cannel and spiral cannel of water-cooled mirror were discussed at last. If the cannel form was linear the mirror would rapidly reach temperature and deformation balance. If the cannel form was spiral the mirror would have lower temperature and deformation.Lastly, two kinds of new methods to reducing the thermal deformation were put forward. First, a heat pipe cooling system was developed and investigated for mirrors of high power TEA CO₂ laser system. Heat pipes were embedded into the body of mirrors which were worked as to balance the body temperature of the mirror and accelerate its heat conduction at the same time. In the back of the mirror there was a water cooling system. In the same structure and conditions, solid mirror, micro channel water cooled mirror and heat pipe cooling mirror are simulated by ANSYS program. The results indicate that the temperature rise and Peak-valley difference of surface deformation of heatpipe cooling mirror were both the best among the three mirrors. Secondly, Optical compensation method was put forward too. Preliminary simulation of ANSYS showed that it is effective to decrease the PV value of mirror distortion.Multidimensional multiform researches and analyses of the thermal behaviors of high power laser mirrors and windows provided systematic theoretical bases for further understanding the laws of thermal deformations of laser windows and mirrors, optimizing and designing its structures, and decreasing the thermal deformations of its surfaces.