Research On High Repetition Rate Mid And Near Infrared Solid State Optical Parametric Oscillator

1.4～1.6μm near-infrared and 3～5μm mid-infrared are located in the atmospheric transmission window,and the loss in the atmosphere is small.Also,there are many absorption lines of molecules and atoms in these bands.Therefore,these bands of the light source are widely used in remote sensing,laser radar,laser imaging,spectral detection,and other fields.Optical parametric oscillator（OPO）is an important technical way to realize mid-and near-infrared laser,which has the advantages of various tuning methods,compact structure,high conversion efficiency,etc.In this paper,the high repetition rate mid-and near-infrared Mg O:PPLN pumped by an end pumped Nd:YVO₄acousto-optic Q-switched is researched theoretically and experimentally.In the theory,the quasi phase matching theory is analyzed based on the three wave coupling theory of optical parametric oscillator,and the characteristics of quasi phase matching are obtained.On this basis,the expressions of the threshold and conversion efficiency of OPO are derived,and the factors affecting these parameters are analyzed.Then the thermal effect of Mg O:PPLN crystal in the pumping process is studied.The temperature field distribution under different pump beam sizes is simulated when the crystal was pumped at one end.Also,the thermal lens focal length is calculated.The beam size and the stability of the cavity under the influence of thermal effect in the plano-plano cavity and the palno-convex cavity are analyzed and calculated.It is found that the plano-convex cavity can maintain good thermal stability and large mode volume.Therefore,the plano-convex cavity is used to compensate for the thermal effect of the crystal.The temperature tuning curve and periodic tuning curve of Mg O:PPLN crystal were calculated respectively,and the polarization period was determined to be 29.5μm single period crystal.In the aspect of the experiment,firstly,the high repetition rate pump source of the optical parametric oscillator is designed and researched.The pumping mode,laser crystal,and Q-switching mode of the pump source are compared and analyzed.Finally,the Nd:YVO4 acousto-optic Q-switched laser is used as the pump source,and a 1064 nm laser with stable output and good beam quality is obtained.When the repetition frequency is30 k Hz,the maximum output power is 5.7 W,and the pulse width is 64.4 ns.The temperature tuning characteristic of Mg O:PPLN crystal is studied experimentally.When the polarization period is 29.5μm,1471.4～1510.4 nm signal and3599.6～3842.6 nm idler are obtained by temperature tuning.The output characteristics of mid-and near-infrared optical parametric oscillators in the signal optical single resonant plano-plano cavity are studied.When the temperature is30℃,the polarization period of the crystal is 29.5μm,the input power is 5.7 W,the highest output power of 0.45 W is obtained at 3842.6 nm,and the corresponding pulse width of signal is 11.9 ns.When the input power is 5.4 W,the maximum output power is1.06 W,and the pulse width is 19.0 ns at 1471.4 nm.When the input power is 3.1 W,the maximum optical-to-optical conversion efficiency of 27.42%is obtained.In order to solve the problem of poor beam quality of the plano-plano cavity,the OPO cavity is optimized.The plano-plano cavity is replaced by the plane-convex cavity.When the input power is 5.7 W,the maximum output power of 1.09 W is obtained,and the corresponding pulse width is 17.1ns.When the input power is 3.1 W,the maximum optical-to-optical conversion efficiency is 28.71%.At the same time,the beam quality is also improved.