Study On External Cavity MgO:PPLN Mid-infrared Optical Parametric Oscillator Pumped By Multichannel Fiber Laser

The mid-infrared band of 3-5 μm is located in the region with high atmospheric transmittance,which is widely used in material processing,surgery operation,telecommunications,spectroscopy,national defense applications and basic science.Optical parametric oscillation based on nonlinear crystal MgO:PPLN is an effective technical method in obtaining 3-5 μm mid-infrared laser.Multichannel fiber pumping sources pumped MgO:PPLN-OPO can effectively reduce the thermal effect and improve the injection power of parametric optical oscillation system.When the pumping injection power is increased,the multichannel fiber pump sources pumping mode can disperse the thermal effect accumulation of nonlinear crystals in OPO,thereby avoiding some problems such as beam quality deterioration and crystal damage,which is an effective measure to improve laser output power.In this paper,the design of three-stage amplification system for1065 nm ytterbium-doped fiber laser(YDFL)and the theoretical analysis and structural optimization of MgO:PPLN-OPO pumped by multichannel fiber pump sources are discussed.The 3.4 μm mid-infrared laser output performance of MgO:PPLN-OPO pumped by multiple fiber pump sources is studied to achieve high-efficiency 3.4 μm mid-infrared laser output based on multichannel fiber laser pumping MgO:PPLN-OPO.The specific work is as follows:In theory,the tuning curve of nonlinear crystal MgO:PPLN based on quasi-phase matching is simulated by MATLAB numerical analysis software,and the influence factors of OPO gain and threshold under multi-channel pumping are analyzed.It is pointed out that compared with the single-channel pump structure,the three-pass gain produced by the multi-channel pump structure can increase the laser output energy,and the multi-channel pump structure can effectively reduce the laser threshold.The influence of temperature distribution field of nonlinear crystal on the transformation process of optical parameters under the condition of multiple pump sources is simulated by Comsol finite element analysis software.It is found that the accumulation of thermal effect in the crystal can be reduced by multi-channel pumping structure.The external cavity resonant optical parametric oscillation structure models of multichannel pump sources are established,and the structural parameters such as cavity length,cavity mirror curvature,beam focusing depth and spot matching are optimized.In the aspect of experiment,firstly,the output performance of three-channel 1065 nm YDFL optical fiber pump sources is tested to determine the output characteristics of each pump source,and then the experimental study of external cavity optical parametric oscillator pumped by single,double and three optical fiber pump sources is carried out.Using different forms of optical fiber pump sources to pump MgO:PPLN-OPO,the output characteristics of optical parametric oscillation under different repetition rate and pulse width are studied,and multichannel fiber laser focusing experiments are carried out.The experimental results show that by changing the focusing depth,the multichannel beam focusing control can be realized,and the expected focusing and spot overlap can be achieved.Using three-channel fiber laser sources pumped MgO:PPLN-OPO,the maximum average power of 5.73 W is obtained at 37 W pumping power,repetition rate of 100 k Hz and pulse width of 200 ns.The output wavelength is 3.43 μm,the optical-optical conversion efficiency is 15.4 %,the beam quality factor M2 is 1.83,and the output pulse width is 147.7 ns.