Study On High-peak-power Mid-infrared Solid-state Laser Based On PPMgLN Crystal

The laser of nonlinear frequency conversion based on PPMgLN(periodically poled lithium niobate crystal doped Mg O)can meet the requirements of military optoelectronic countermeasures for the light-weight,compact and high-peak-power3?5?m coherent light source.However,due to the limitations of the crystal size,damage threshold and long-wavelength mid-infrared laser absorption,the volume and peak power of the PPMgLN mid-infrared laser need to be reduced and improved respectively.For this purpose,based on the theoretical analysis and design of PPMgLN optical parametric oscillator(OPO)and optical parametric amplifier(OPA),the compact and high-peak-power PPMgLN mid-infrared solid-state laser and its pump sources are studied in this paper.In theory,the steady-state equations of three waves mixing and the principle of quasi-phase matching are studied in the processes of optical parametric oscillation and optical parametric amplification.On this basis,the oscillation threshold,conversion efficiency,line width,seed optical gain and axial power distribution of PPMgLN OPA are discussed.Based on the theories above,in order to obtain high-peak-power mid-infrared laser,the PPMgLN mid-infrared laser pumped by a 1?m laser is designed.According to Sellmeier equation and the target parametric light wavelength,the poling period of PPMgLN crystal is designed,and the variation laws of the parametric light wavelength with the crystal poling period,temperature and pump wavelength are analyzed.The thermal effect of 1?m laser pumped PPMgLN was studied and the relationships between the output power and the pump radius,output mirror transmittance,crystal length,OPA seed light intensity are also analyzed.In order to explore the 2?m laser pumped PPMgLN OPO,the poling period of the crystal is designed,the wavelength tuning characteristics of the parametric light is discussed,and the thermal effect of the crystal is also studied.In the experiment,the pump sources of the PPMgLN mid-infrared solid-state laser are studied first.When the pump wavelength is 1?m,a 885 nm LD(laser diode)end resonant pumped Nd:YAG laser is designed and realized.In quasi-continuous operation,the output power is 14.44 W with the absorption pump power of 23.31 W,and the slope efficiency is 77.2%,approaching the quantum limit;In the electro-optical Q-switched operation and pulse repetition frequency of 2 k Hz,the output power is 15.6 W with the absorption pump power of 79.3 W,the peak power,center wavelength,spectral linewidth are 1.1 MW,1064.4 nm and 0.48 nm,respectively.The beam quality M² factors in x and y directions are 1.8 and 2.3.The research shows that the scheme based on resonance pulse end-pumped combined with active Q-switched can realize the output of MW peak power pulse with good beam quality without amplification process,which solves the problem that the existing PPMgLN OPO+OPA needs two independent pump sources to pump separately,which leads to the large system volume.When the pump wavelength is 2?m,a Tm fiber pumped Ho:Lu VO₄ laser is designed and realized.The mode selection of the laser is realized by using F-P etalon method.The output power is 163 m W and the central wavelength is 2074.91 nm in the single longitudinal mode.Then,the experimental research on PPMgLN mid-infrared solid-state laser with high-peak-power is carried out.PPMgLN OPO and OPA are pumped by a splitted Q-switched Nd:YAG laser with the pulse repetition frequency rate of 2 k Hz.The OPO adopts a signal resonant ring cavity with four mirrors structure.When the injection pump power is 1.53 W,the output power of the idler light is 0.145 W,and the pulse width is 4.9 ns.The beam quality M² factors in x and y directions are 2.0 and 2.6,respectively.When the injection pump power is 7.36 W,the output power of OPA idler light is 0.97 W,and the pulse width is 5.3 ns,corresponding the peak power of91.1 k W.The beam quality M² factors in x and y directions are 3.4 and 3.7,respectively.The idler wavelengths of OPO and OPA are both 3.731?m,and the corresponding line widths are 7.9 nm and 8.8 nm respectively.When the crystal temperature increases from 20°C to 45°C,the idler light wavelength decreases from3.749?m to 3.704?m.The result shows that the miniaturized high-peak-power mid-infrared laser output can be realized by using PPMgLN OPO+OPA combined with pump beam splitting structure.The laser can be used in airborne directional infrared countermeasure system.