| The 3-5?m mid-infrared laser is located in the main transmission window of the atmosphere,which has a wide application prospect in the fields of spectral detection,environmental monitoring,medical diagnosis and treatment,optoelectronic countermeasures and so on.For optical parametric oscillator?QPM-OPO,Quasi-Phase Matching Optical Parametric Oscillator?based on quasi-phase matching technology,there are outstanding advantages such as high conversion efficiency and flexible tuning mode,so it is one of the effective methods to obtain highly tunable mid-infrared laser.With the development of mid-infrared OPO technology,there are some drawbacks of limits the application range of OPO such as large size and complex structure.Moreover compared with the traditional mid-infrared OPO technology,a new technical method for obtaining mid-infrared laser is formed.The mid-infrared self-optical parametric oscillation technology based on Nd-doped MgO:PPLN crystal can realize the organic fusion of gain unit and mid-infrared frequency conversion unit.In the paper,starting with the laser characteristics and quasi-phase matching theory of Nd3+-doped MgO:LN crystals,the influence of high power pump thermal effect on self-frequency conversion process is analyzed.After that,the intra-cavity self-OPO composite resonance model is established,following by its dynamic process is studied.The technical feasibility is verified by experiments.Furthermore,the 3.8?m middle infrared laser output is obtained for the first time by self-OPO based on Nd:MgO:PPLN crystal,while the highest average output power reaches the watt level.The specific research work carried out in the paper and the main research results are as follows:Firstly,starting with the laser characteristics of Nd:MgO:PPLN crystal,the dispersion characteristics of Nd:MgO:PPLN crystal are analyzed.Moreover,combined with the theory of laser oscillation energy level,the formation reason and control method of dual-wavelength laser with fundamental frequency orthogonal polarization of 1084 nm and1093nm in Nd:MgO:PPLN crystal are studied.Furthermore,based on the quasi-phase matching theory of Nd:MgO:PPLN matrix crystal,the structure design of mid-infrared frequency conversion polarization is completed.Secondly,the thermal field model of high power pumped Nd:MgO:PPLN crystal is established.After that the relations of crystal temperature rise,thermal stress and thermal focal length under different pump structure,different pump power and different pump spot combination variables are simulated numerically.Moreover,the influence of thermal effect of Nd3+doping at high pump power on frequency conversion parameters such as parametric optical wavelength and down-conversion efficiency is analyzed.Furthermore,based on this,the physical model of intra-cavity compound resonance self-optical parametric oscillation is established.The structural parameters such as cavity length,mirror curvature and spot matching overlap position are optimized.Thirdly,the dynamic model of self-OPO composite resonance based on Nd:MgO:PPLN crystal is established,while the variation of photon flow in the process of self-OPO three-wave coupling is simulated and analyzed.Furthermore,the key optimization parameters to improve the frequency conversion efficiency.For example,the down conversion efficiency and the optimal threshold multiple of the self-OPO,are further studied.Moreover,on the basis of which the parameters such as initial perturbation term,pumping term and Q-switching dynamic loss are introduced.At last,the active and passive Q-switching rate equations of intra-cavity composite resonance from OPO are established,and the pulse output shape is numerically evolved under the operation of Q-switching in OPO.Finally,in the experiment,the high beam quality and polarization matching fundamental frequency light in OPO cavity is realized.It is achieved by optimizing the wavelength and polarization state of fundamental frequency light controlled in the thermal stability region of resonant cavity,while the operation of single wavelength.Moreover,the1084 nm fundamental frequency laser with the highest average power of 13 W and the beam quality factor of Mx2=1.53,My2=1.55 is obtained under two-ended 40 W pump power.Furthermore,on this basis,the experiments of passive and acousto-optic Q-switched mid-infrared laser from OPO are carried out respectively.Never the less,according to the theoretical derivation,the down conversion efficiency of mid infrared parametric light is improved and the inverse energy conversion is effectively suppressed by optimizing the threshold and the optical transmittance of resonant parameters.Moreover,when the acousto-optic Q-switching repetition frequency is 60 kHz and the double-ended 40 W pump,the output of 3.825?m mid-infrared laser with maximum average power 1.12 W and pulse width 4.73 ns is achieved,and the optical-to-optical conversion efficiency is 2.8%. |
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