Mid Infrared Optical Parametric Oscillator Based On BaGa₄Se₇ And KTiOAsO₄

The mid-infrared（IR）coherent sources have always been intensively desired for a wide range of scientific and technological applications in atmospheric environment sensing,electro-optical countermeasures,medicine,spectrum analysis etc.An optical parametric oscillator（OPO）is a typical and efficient approach to obtain MWIR laser sources due to advantages such as compact structure,full curing,wide band continuous adjustable and so on.It can be used in various platforms such as airborne and shipborne.In OPO systems,the nonlinear crystal elements are always the key devices.BaGa₄Se₇（BGSe）is a new IR nonlinear material with excellent overall properties.KTi OAs O₄（KTA）is a mature commercial crystal.Focusing on these two nonlinear crystals,we have studied the IR laser generation technology.The main research works of this dissertation are sumarized as follows:1.The phase matching curves and effective nonlinear coefficients(d_eff)of BGSe and KTA were calculated.The optimal phase matching angles,matching type and the maximum d_effwere obtained.BGSe（56.3,0,type I）and KTA（90,0,type II-A）can generated 3.5μm laser pumped by 1.06μm laser,and the cutting angles were the optimal phase matching angles.The effective nonlinear coefficients of BGSe and KTA are 11.90 pm/V and 3.64 pm/V,respectively.2.The performances of BGSe OPO and KTA OPO were studied when outputting idler light of 3.5μm.The effects of pump spot radius,OPO cavity length and signal light reflectivity of OPO cavity mirror on the pump threshold were calculated.The output laser performances with different cavity length and cavity mirror reflectivity were calculated by OPO numerical simulation.When the pump energy was fixed,we calculated the optimal reflectivity of OPO cavity mirror at the maximum idler output.Under the guidance of the simulation results,the BGSe OPO and KTA OPO with different cavities were demonstrated.The maximum optical-to-optical conversion efficiency of 8.4%was obtained with double-pass pumped singly-resonant OPO when the cavity length and the reflectivity of cavity mirror were 30 mm and 77%.To the best of our knowledge,it is the highest reported conversion efficiency for the BGSe OPO driven by the 1μm laser.3.The idler wavelength tuning of BGSe OPO and KTA OPO,including angle tuning and temperature tuning,were studied.The temperature tuning of BGse was demonstrated for the first time,to the best of our knowledge.Using Sellmeier equation of BGSe and KTA,we calculated and verified the relationship between output wavelength and（θ,φ）.Using the Sellmeier equation of BGSe and KTA with temperature,we obtained the relationship between output wavelength and temperature.The experimental results showed that BGSe（56.3,0,type I）OPO had a large change rate of output wavelength with temperature of 3.2 nm/℃,while KTA（90,0,type II-A）OPO had a small change rate of output wavelength with temperature of 0.073 nm/℃.Our results indicate that BGSe possesses advantages for application in wide band temperature tuning.4.The gain bandwidth and output spectral width of BGSe OPO and KTA OPO were studied,and the spectral width of BGSe OPO was narrowed for the first time,to the best of our knowledge.The gain bandwidth of KTA and BGSe were calculated by using the Sellmeier equation of BGSe and KTA.The gain bandwidth of KTA is slightly smaller than that of BGSe.The output spectral width of BGSe（56.3,0,type I）OPO was about 4.71 nm,while that of KTA（90°,0°,type II-A）OPO was about 2.45 nm.The experimental results were in good agreement with the theory.In order to obtain high energy and narrow spectral width mid infrared laser,the output spectral width of BGSe OPO was narrowed by inserting a FP etalon into the L-shaped cavity,and the spectral width of less than 2 nm was obtained.In addition,the idler light from KTA OPO was OPA（optical parametric amplification）in BGSe,and the spectral width of 3nm was obtained.We provided a calculation method for the selection of crystal cutting angle and OPO cavity parameters.The temperature tuning and output spectral width narrowing of the crystal were studied theoretically and experimentally.This provides a reference and basis for the development of optical parametric oscillation technology of BGSe,and also lays a theoretical and technical foundation for its practical application.