Studies On Cat's Eye Cavity Of TEA CO₂ Laser For Phase Conjugation

With the improvement of its performance, TEA CO2 laser has been widely applied in many areas such as processing, defense, environment protection and scientific research. It is considered as the most favorable laser for the application of laser propulsion which is increasingly studied recently. Beam quality is a key factor that has significant influence on this kind of energy transportation application, and it is recognized that phase conjugation cavity would improve the beam quality of TEA CO2 laser effectively. For the purpose of high quality high power TEA CO2 laser, the principle of phase conjugation cavity and the method to obtain high quality pump beams which are required by this kind of cavity are both theoretically and experimentally studied.Firstly, the principles and key technologies of phase conjugation are introduced. Then, the principles of four wave mixing technology and the characteristics of phase conjugation cavity are discussed. The prerequisite of realizing phase conjugation beam is to obtain pump beams with high beam quality and appropriate energy. It is recognized that cat's-eye cavity could be one of the most effective methods to obtain appropriate pump beams due to its excellent retro-reflectivity. Thus, the characteristics of cat's-eye cavity are discussed, and theoretical simulations are also made with the specific conditions of our experiments.Several methods of beam quality valuation are compared, and the times of diffraction limitβis chosen as the criterion applied in the experiments. Theβfactor of the cat's-eye cavity of TEA CO2 laser is measured by infrared CCD using focused beam measurement method. The b factors are much better than those of plano-conic cavity and plano-concave cavity. The reasons for low pulse energy are also discussed. It is concluded that the influence of atmosphere attenuation could be ignored in the experiment and the diffraction loss would not be the reason of low energy also. However, the influence of maladjustments of the plano-plano output coupler should be minimized as small as possible. At last, the temporal waveform of laser pulse is measured by a photon drag detector. The interval between the end of discharge and the beginning of the laser pulse of cat's-eye cavity is longer than that of the plano-concave cavity, which may be the results of high loss of the cat's-eye cavity. Besides, the phenomenon of"beat frequency"is also confirmed by the experiments.