Study On All Solid-state High Beam Quality Mid-infrared ZnGeP₂ Optical Parametric Oscillator

3-5?m mid-infrared coherent light source is one of the infrared atmosphere transmission windows,which includes absorption peaks of many atoms and molecules.So it is called the"fingerprint spectrum"of molecular.Its unique characteristics make it widely used in military and civilian fields such as spectrum detection,environmental monitoring,medical diagnosis and optoelectronic countermeasures.The ZnGeP₂ OPO pumped by 2?m Tm-Ho laser is the main means to achieve high performance 3-5?m laser output.The research in this area has important guiding significance for the development of mid-infrared laser.In this thesis,based on the optimization design of Tm:YLF and Ho:YAG lasers and the theoretical analysis and design of ZnGeP₂ OPO,we have studied the mid-infrared ZnGeP₂ OPO with high beam quality.The primary research work and achievements of the paper is as follows:Theoretically,the overall operation mechanism of the three-wave coupling process of OPO is modeled and solved,and the OPO output characteristics and conversion efficiency are analyzed.Furthermore,doubly resonant ring oscillator?DRRO?structure is used to suppress reverse conversion effect of the doubly resonant plane parallel resonator?DRO?.Finally,based on the practical operating conditions of the ZnGeP₂ OPO laser system,a transient temperature distribution model of ZnGeP₂ crystal pumped by high repetition rate and narrow pulse width laser is established and simulated.And the photothermal dispersion characteristics of ZnGeP₂ crystal are analyzed.These results provide a theoretical basis for the realization of the ZnGeP₂ OPO laser system.In aspect of experimental design,the pump source of ZnGeP₂ and the high beam quality ZnGeP₂ OPO laser system were optimized and analyzed.Based on the establishment and analysis of the thermal effect of the Tm:YLF laser system,the optimal design of the crystal heat sink structure and aberration elimination pump coupling system is proposed to alleviate the thermal effect of the crystal.The influence of the double guiding effect in the Ho:YAG laser on the formation of the output laser transverse mode is analyzed.And the laser output transverse mode of the 2?m Ho:YAG laser is optimized by changing the effective absorption coefficient.In order to improve the beam quality of mid-infrared ZnGeP₂ OPO laser,the optimal design of non-planar annular cavity structure is proposed.By establishing the equivalent sphere representation and calculating the image rotation angle,a special four-mirror non-planar ring resonator structure with 90°image rotation angle is designed.In addition,the self-reproduction of the transverse mode in the asymmetric axial resonator is further established.Through the simulation analysis of the mode distribution of the optical field in the cavity,it is verified that the resonator design can improve the beam quality of the OPO laser output.Experimentally,firstly,the pump source Tm:YLF and Ho:YAG laser of ZnGeP₂ OPO are studied experimentally.The output power exceeding 26W of Tm:YLF laser is achieved,with the central wavelength of 1908.56nm and the linewidth of 0.15nm.Secondly,the Ho:YAG laser is dual-end pumped by two orthogonally polarized Tm:YLF lasers.When the acousto-optic Q-switch repetition rate is 6kHz,the 22.8W average power output of2090.73nm is achieved,with the pulse width of 21.02ns,and the beam quality ofM_x²=1.53,M_y²=.155,the 2?m Ho:YAG laser with high power and high beam quality is obtained.Finally,the ZnGeP₂ OPO is pumped by the above Tm:YLF-Ho:YAG laser system.The effects of different pumping structures and different cavity lengths on the beam quality under parallel planar resonator are analyzed,and the optimization of the structure from two-pass pumping short cavity?M²?8?to single-pass pumping long cavity?M²?.35?.Under the same experimental conditions,the planar ring resonator is used for experimental study.The output beam quality isM²?.28,which proves that the structure can improve the beam quality.In order to further improve the beam quality,the non-planar ring resonator based on the previous theory is studied experimentally.The parametric light output with a beam quality ofM²?.18is obtained in a 90°image-rotating four-mirror non-planar ring resonator structure.The results show that the beam quality of ZnGeP₂ OPO can be further improved by this resonator structure.When the repetition rate is 6kHz,the transmittance of output coupler is 40%,and the pump power is 21.5W,the output of 5.97W mid-infrared laser is obtained.The corresponding beam quality in the X and Y directions isM_x²=.181and M_y²=.161.Finally,the mid-infrared ZnGeP₂ OPO with high beam quality is realized in this thesis.