High Beam Quality Nanosecond Mid-Infrared Optical Parametric Oscillator

The absorption characteristics of 2.6?m mid-infrared tunable laser in atmospheric molecules make lasers have very important application value in military,medical,remote sensing,communication,industrial processing and other fields.The tunable mid-infrared solid-state lasers based on optical parametric technology that has the advantages of high conversion efficiency,wide tuning range,good beam quality,simple structure etc.,which makes them become the research hotspot of mid-infrared lasers with decades of development Among many nonlinear crystals,KTP crystals have the characteristics of large nonlinear coefficient,high thermal conductivity,small mismatch,small walk-off angle,non-deliquescence,stable chemical and mechanical properties etc.Based on the excellent optical and physical characteristics of KTP crystals,which are often used as parametric medium in optical parameter oscillators(OPOs).At present,the working wavelength of KTP-OPO is mostly in the 2.1?m.But there are few reports about near the 2.6?m band.In this thesis,we focus on 2.6?m tunable mid-infrared OPO.1.In this paper,based on the nonlinear optical three-wave coupling equation,the gain,threshold and conversion efficiency of the OPO were obtained,and the relations of OPO threshold with the crystal length,the cavity length,the pump pulse width were obtained.The conversion efficiency with the multiples of pump energy over-threshold was simulated.Under the condition of II phase matching,the relations of output wavelength,effective nonlinear coefficient and parameter gain with crystal cutting angle were obtained,so that the size of the crystal was determined to be 10×10×20 mm3 and the cutting angle in the xz plane is 59.8°.In the end,the effect of KTP-OPO operation mode and cavity structure design on the output of OPO was analyzed.2.Based on the requirement of high-energy laser,a single resonant short flat cavity structure was designed.Under a single-pass pumping condition,a 22 mJ,2.6?m mid-infrared laser output can be obtained at a pump energy of 300 mJ,and its energy stability was 2.0%RMS.When the pump laser passed through the crystal twice,the maximum single pulse energy was 12.6 mJ,corresponding to the conversion efficiency of 8%,and the wavelength tuning range was 2.4-2.8?m.The near diffraction limit of the beam quality was measured to be 5 times.The relationship of laser output energy with the cavity length,temperature and beam quality were measured,which provided a basis for further optimization.3.In order to obtain the laser with high beam quality,an OPO with ring cavity was designed to suppress the laser multimode oscillation,the energy of output was 6.1 mJ when the pump energy was 160 mJ,and its energy stability was 6.9%RMS.The near diffraction limit of the beam quality was measured to be 1.7 times,which met the experimental requirements of high beam quality,and provided a basic guarantee for the long-distance transmission of mid infrared laser.In order to improve the stability of the laser,based on the three mirrors ring cavity structure,a prototype of ring cavity laser was developed,the stability of the prototype was measured to be 2.9%RMS.