基于闲频光谐振的中红外高能量、高光束质量KTiOAsO_(4)光学参量振荡器

High Energy,High Beam Quality Idler-resonant mid-infrared Optical Parametric Oscillator Based on KTiOAsO_(4)

基于砷酸钛氧钾(KTA)晶体优良的非线性光学特性,研究了由1μm Nd:YAG纳秒脉冲调Q激光器泵浦的高能量、高光束质量闲频光单谐振光参量振荡器。选取合适镀膜参数的腔镜、优化腔型设计,建立了一个稳定紧凑的半球形对称闲频光单谐振腔,实现了高能量、高光束质量的近-中红外激光输出。在输入泵浦光的(1.064μm)最大能量为20.2 mJ时,输出信号光(1.535μm)和闲频光(3.468μm)的最大能量分别为2.91 mJ和1.13 mJ,对应信号光和闲频光的斜效率分别为20.9%和8.1%。闲频光单谐振的光参量振荡器具有更大的衍射损耗和光束发散角,可以极大的限制输出光束的光谱带宽、提高谐振闲频光的光束质量等优势,测量了输出中红外闲频光在两个正交方向上的光束质量因子分别为M_(x)^(2)≈1.1,M_(y)^(2)≈1.1。

The high energy,high beam quality mid-infrared lasers(3~5μm),located in the most transparent atmospheric window band,including super-resolution molecular absorption microscopy,molecular spectroscopy,remote sensing,infrared countermeasures,military,and medical treatments,have been widely investigated for a wide variety of applications.In this research,we report on a high energy,high beam quality idler-resonant mid-infrared Optical Parametric Oscillator(OPO)based on a KTiOAsO_(4) and pumped by a conventional Q-switched Nd:YAG laser.The all solid state Q-switched Nd:YAG laser wavelength at 1.064μm(25 ns,50 Hz,and 20 mJ)was used as a pump source,its nearly Gaussian spatial formed output was loosely focused a waist radius ofω0=500μm to the center of the crystal.A Half-Wave Plate(HWP)and a Thin-Film Polarizer(TFP)were used to control the pump energy injected into the OPO.The idler-resonant oscillator compromised of a concave input mirror(R=500 mm)with antireflection for pump and high-reflection for signal and idler outputs,and a plane output coupler with partial reflection(R=80%)for idler output,and high transmission to the pump and signal outputs.The physical length of this nearly half-symmetric OPO cavity was fixed~35 mm.The compact half symmetric-stable cavity with appropriate radius curvature of the cavity mirrors provided high efficiency near-and mid-infrared generation.The KTiOAsO_(4)crystal as a nonlinear parametric gain medium has high transmission in the 3~5μm region,it was cut along its X-axis(θ=90°,φ=0°)to realize typeⅡNoncritical Phase Matching(NCPM)among pump,signal and idler outputs.A Ge filter is used to filter out the signal and undepleted pump beam from the idler output.We recorded the spatial form of pump beam using a conventional CCD camera,a Spiricon pyroelectric camera was also used to observe spatial forms of the signal and idler outputs.With this idler-resonant OPO,we have obtained 2.91 mJ of 1.535μm nearinfrared and 1.13 mJ of 3.468μm mid-infrared energy outputs at the maximum pump energy of 20.2 mJ,corresponding to the slope efficiencies of 20.9%and 8.1%,respectively.In this system,although the mid infrared idler beam has a large beam divergence,when it is reflected back to the crystal by the input concave mirror with a radius of curvature of 500 mm,it is size reduced to the same size as the pump beam.This greatly increases the spatial overlapping efficiency of the pump and idler fields in a nonlinear crystal,resulting high energy,high beam quality mid-infrared output being achieved in this idler-resonant half symmetric-stable cavity.It is also worth noting that the compared to the signal-resonant OPO with the same pumping and cavity condition,in the idler-resonant OPO,idler beam with large angular dispersion allows to generate narrower spectral bandwidth and higher quality beams,it is an effective approach to improve the beam quality of the mid-infrared output radiation from OPO.We also measured the beam quality factor(M2)of the mid-infrared idler output by using the knife-edge method,resulting beam quality factors of the idler outputs were M_(x)^(2)≈1.1,M_(y)^(2)≈1.1 along the horizontal and vertical directions,respectively.These values significantly indicated that the idler-resonant OPO can generate excellent beam quality mid-infrared outputs.We believe that a high energy,high beam quality mid-infrared beam will play an important role in the variety of applications.Further selecting an appropriate nonlinear crystal and optimizing the cavity design are expected to yield improved OPO performance.