Research On 10?12 Micron Long-wave Infrared Cadmium Selenide Optical Parametric Oscillator With Seed-injection

10?12?m long-wave infrared lasers have important applications in many fields such as infrared countermeasures,strong field physics,and atmospheric environment monitoring.Nonlinear frequency conversion technology is one of the technologies for realizing 10?12?m long-wave infrared laser.Its characteristics are compact structure,small volume,all-solid-state,wavelength tunable,and stable operation.As a place for nonlinear frequency conversion,nonlinear crystals have great influence on the optical conversion efficiency,beam quality and other indicators.At present,due to the lack of high-performance nonlinear crystals in the 10-12?m waveband,the output power of the nonlinear frequency conversion technology is only a few hundred milliwatts in this band,which needs to be further improved.Currently,the high-performance nonlinear crystals have not been successfully synthesized,so how to use existing low-performance(for example,lower nonlinear coefficient,low thermal conductivity,large absorption coefficient,or small size)nonlinear crystals to achieve10?12?m long-wave infrared laser with higher power and high beam quality is one of the urgent scientific problems in this field.In this context,this paper selects the domestically mature CdSe as a nonlinear crystal,and improves its output power by improving the optical conversion efficiency of the CdSe optical parametric oscillator(OPO).Through numerical calculation and experimental verification,this paper partially solves the problems of low output power and low optical-to-optical conversion efficiency of CdSe OPO,and achieves the highest average power and best beam quality of CdSe OPO in the 10-12?m waveband currently.According to the logical framework,this article can be summarized into three main parts:(1)In order to improve the optical-to-opticcal conversion efficiency of CdSe OPO,trying to start with lowering the lasing threshold.For this reason,the numerical calculation model of CdSe OPO is established through the three-wave coupling equation.Through numerical calculation,it is found that the output laser is established in quantum noise.If the substrate initially established for the output laser is increased to make the substrate and the output laser have the same wavelength,the lasing threshold can be significantly reduced.In response to this finding,it is proposed that on the basis of pulsed pump laser,an additional continuous-wave or pulsed seed laser with the same wavelength as the output signal or idler is injected,which is expected to significantly reduce the lasing pump threshold and therefore increase the optical-to-optical conversion efficiency.(2)The CdSe signal-resonant OPO(SROPO)is experimental constructed.The2.1?m pulsed laser generated by Ho:YAG Q-switched laser is used as the pump source,2.6?m continuous-wave laser output from Cr²⁺:Zn Se laser is used as seed laser.The 4-mirror ring cavity structure and intracavity beam expansion scheme are used to make the pump laser and the cavity resonant-laser have good mode-matching.It is found that at 22.4 W(22.4 m J)pump average power,the 160 m W continuous-wave seed injection reduces the pump threshold by about 14%and increases the output power by about 30%.The experimental results are in good agreement with the theoretical expectation.Finally,the output wavelength of 10.1?m,average power of1.05 W,pulse energy of 1.05 m J,optical conversion efficiency of 4.69%,and beam quality of M²?2.25 are obtained.The average power is 3.3 times of the existing best results of CdSe OPO in the 10?12?m waveband.(3)In order to improve the beam quality of idler of CdSe SROPO,a scheme of CdSe idler-resonant OPO(IROPO)is proposed.In the experiment,under incident power of 18.2 W pump and 90 m W continuous-wave seed,the output of idler with wavelength of 11.0?m,average power of 802 m W,pulse energy of 0.8 m J,optical-to-optical conversion efficiency of 4.41%,and beam quality of M²?1.23 are achieved.Compared with no seed injection,the threshold of the pump is reduced by 24%and the output power is increased by 102%.Compared with SROPO,IROPO achieves excellent beam quality(M²?1.23)with comparable output power and optical conversion efficiency.Therefore,IROPO is more competent for optical parametric amplifiers and other applications with higher requirements for better beam quality.In addition,compared with SROPO,IROPO has lower energy density in the cavity,so it is more suitable for high-power,long-term operation of laser.In conclusion,the findings and methods in this paper can be applied to nonlinear crystals with small or large nonlinear coefficients in this or other wavebands,which can help to improve the optical conversion efficiency of conventional OPOs while maintaining good beam quality.