平行平面腔光学参量振荡器失谐特性实验研究 下载: 724次
Optical parametric oscillators(OPOs)have been proven to be effective,coherent light sources that can expand the wavelengths of commercial lasers(typically limited to narrow emission lines and bands)to a broad range from visible to far-infraredbands. Q-switched lasers with high peak powers have significantly promoted the development and applications of OPOs with the following advantageous characteristics:system compactness(for example,two cavity mirrors and a nonlinear crystal),relatively high conversion efficiency,singly resonant operation,and frequency-agile tunability(for example,angle tuning).A plane-parallel cavity with a large mode volume is well-suited for Q-switched laser pumps.This type of OPO is widely adopted,for example,as a pump/seed source in nonlinear terahertz or mid-infrared(MIR)generation or directly as an MIR source,owing to the wide tuning range and ease of construction.As the earliest configuration in a laser resonator,a plane-parallel cavity is critically stable and sensitive to mirror misalignment.The misalignment of laser cavities,including those of argon ion,CO2,and Nd∶YAG lasers,has been analyzed previously;however,studies on OPO cavities have rarely been reported.In this study,we performed an experimental investigation on the misalignment characteristics of a plane-parallel cavity-based OPO.
In this study,an OPO based on a plane-parallel cavity structure was developed.A potassium titanyl phosphate(KTP)crystal was utilized as the nonlinear medium(cut at θ=60°,φ=0°,and 10 mm×7 mm×20 mm,anti-reflection(AR)-coated at 532 nm/800-900 nm/1300 1600 nm).A frequency-doubled Nd∶YAG laser(532 nm,10 ns,and 10 Hz)was employed as the pump source.Two flat mirrors(AR-coated at 532 nm/1300-1600 nm and highly reflection-coated at 800 900 nm)formed a singly resonant cavity.The ns-pulsed OPO was operated at a wavelength of 1514 nm via o→e(signal)+o(idler)critical phase matching.The cavity mirrors were precisely controlled using piezoelectric optical mounts for alignment.Each mount was equipped with two piezo actuators,which could provide a two-dimensional(2D)adjustment(axes 1 and 2 for output mirror M1 and axes 3 and 4 for input mirror M2)with an angular resolution of ≤0.7 μrad.
Typical output results(pulse envelopes and beam profile)of the KTP-OPO are presented in Fig.2.The piezoelectric optical mounts with a motion controller module facilitate quantitative analysis of the influence of mirror misalignment on the OPO output.The variation in the output pulse energy with angular tilt δxis measured while scanning each cavity mirror along two directions around the well-aligned position(δx=0),as presented by the 2D graphs in Fig.3.The subscripts x=1–4 correspond to the four actuators,axes 1–4,respectively.The four curves presented in Fig.3 present envelopes along the principal axis(with the other three δ=0).The full widths at half maximum of the curves(called alignment tolerance)from axes 1 to 4 are 0.171,1.861,0.177,and 1.933 mrad,respectively,which are determined at a pump beamdiameter Φ=4 mm,cavity length L=65 mm,and output pulse energy=6.6 mJ.The discrepancy between the two mirrors along the same direction(axes 1 and 3 and axes 2 and 4)is minimal,which is verified by alternating the two mounts.The tolerances along the horizontal direction(axes 2 and 4, y-principal dielectric axis)are approximately 10 times those along the vertical direction(axes 1 and 3, x-z-principal plane).This can be attributed to the critical phase matching configuration(Fig.4).As presented in Fig.5(a),the alignment tolerance increases with the beam size at a specific pump intensity and cavity length because a larger interaction region(cross-section)can provide more effective round trips for misaligned signal beams.The relationship between the tolerance andoutput energy,shown in Fig.5(b),demonstrates an increasing trend because a higheroutput energycorresponds to a higher single-pass gain(easier to build up).The alignment tolerances of different cavity lengths are compared at fixed input[Fig.6(a)]and output pulse energies[Fig.6(b)].In addition,the other output characteristics vary with the OPO cavity length.A longer cavity length results in a higher threshold and lower energy conversion efficiency(Fig.7).The divergent angles decrease with the cavity length at approximately equal output energies and beam sizes(left y-axis of Fig.8).Better beam quality and worse stability can be obtained with a more extended cavity(right y-axis of Fig.8),and the root-mean-square(RMS)of pulse energy fluctuation increases(3.09%→3.61%→3.96%).
Herein,we quantitatively characterize the mirror misalignment of a plane-parallel cavity-based OPO,which has been widely utilized as a convenient coherent light source with a desired wavelength.A green laser-pumped KTP-OPO equipped with piezoelectric optical mounts is constructed.An almost circular Gaussian beam with a wavelength of 1514 nm is delivered with a slope efficiency of ≥25% and a pulse energy fluctuation(RMS)of ≤4%. The output shrinkage is measured by scanning the cavity mirrors around a well-aligned position.The alignment appears to be significantly more sensitive in the critical direction than in the noncritical direction,which can be explained based on the phase-matching configuration.The alignment tolerance increases with the beam size and input intensity.In addition,the cavity length dependence is analyzed at specific input and output pulse energies.This paper presents a type of ns-pulsed,singly resonant,and critical phase-matched OPO with a wide-angle tuning capability.
1 引言
光学参量振荡器(OPO)极大地扩展了相干光源的输出波长范围,将有限的商用激光波长(通常为固定的发射线或较窄的发射带)扩展为从可见光到远红外波段的不间断覆盖[1]。高峰值功率的调Q激光器进一步推动了OPO的发展,使其具有紧凑的结构(如只需两个腔镜和一个非线性晶体)、较高的转换效率和单谐振运转等优点,成为实现指定波长相干光产生的一种有效手段。平行平面腔是最早提出的激光谐振腔型,在OPO(特别是脉冲运转OPO)中被广泛采用。基于这类光腔的OPO具有大的模体积,易于准直,而且能够实现大范围角度调谐,因此被大量用作非线性太赫兹和中红外光源的泵浦/种子源[2-4],或直接作为中红外光源[5-7]。
平行平面腔属于临界腔,对腔镜准直角度的扰动(失谐)较为敏感[8]。针对激光器的腔失谐研究主要集中在20世纪60—80年代,实验工作主要涉及氩离子激光器[9]、CO2激光器[10]和Nd∶YAG激光器[11-13]等,计算分析最初建立在微扰理论框架下[11-12],后来发展出了考虑空间变化的速率方程模型[13]。1979年,原杭州大学王绍民[14]完整地阐述了非共轴(失谐)系统光束传输的矩阵和图论处理方法;1984年,中国科学院上海光学精密机械研究所方洪烈等[15]利用多尺度微扰方法推导了失谐平行平面腔积分方程的解析解;1986年,四川大学吕百达[16]综合分析了失谐腔的几种理论方法,包括几何光学法、高斯光束法、衍射积分方程法和矩阵光学法;2001年,中国科学院西安光学精密机械研究所胡亚红等[17]提出了一种基于光斑边缘检测的谐振腔失谐自动校正方法,并采用He-Ne激光器进行了验证。所查资料鲜有对OPO腔的相关研究。
本团队研究了平行平面腔OPO的失谐特性,搭建了532 nm绿光激光泵浦KTiOPO4(KTP)晶体的OPO装置。该装置代表了纳秒脉冲泵浦临界相位匹配OPO这类结构。本团队观察并解释了腔镜对准容限在不同方向上的显著差异,分析了对准容限随光束尺寸、泵浦强度和腔长等参数的变化。
2 实验装置
KTP晶体平行平面腔OPO(KTP-OPO)的结构如
3 实验结果分析与讨论
该KTP-OPO的输出结果如
图 2. KTP-OPO输入泵浦光(虚线)、输出闲频光(实线)和剩余泵浦光(点线)的脉冲包络,插图为OPO输出光束的轮廓
Fig. 2. Pulse envelops of KTP-OPO input pump(dashed),output idler(solid)and depleted pump(dotted),where the inset exhibits OPO output beam profile
利用压电光学调整架精准控制腔镜的偏角,实现对OPO腔失谐特性的定量研究。对每个腔镜在其理想准直位置附近各进行两个方向的角度扫描,得到输出能量随失谐角度δx的变化如
图 3. OPO输出能量随腔镜不同方向失谐角度的变化
Fig. 3. OPO output versus cavity mirror misalignment angle in different directions
上述不同方向上对准容限的显著差异是由该OPO的临界相位匹配形式导致的。KTP晶体的3个介电主轴和通光方向如
图 4. KTP晶体介电主轴及晶向示意图
Fig. 4. SchematicofKTPprincipaldielectricaxesand crystallographic orientation
根据
利用不同大小的光阑改变泵浦光束的尺寸,重复
图 5. 对准容限与泵浦光束直径和输出能量的关系。(a)对准容限与光束直径的关系;(b)对准容限与输出能量的关系
Fig. 5. Dependenceof alignment toleranceonpumpbeam diameter and output energy.(a)Dependence of alignment tolerance on pump beam diameter;(b)dependence of alignment tolerance on output energy
改变OPO腔长,观察输入能量不变和输出能量不变两种情况下对准容限的变化,结果如
图 6. 在输入和输出能量不变的情况下对准容限与腔长的关系。(a)在输入能量不变的情况下;(b)在输出能量不变的条件下
Fig. 6. Alignment tolerance as functions of cavity length at given input and output energies.(a)At given input energy;(b)at given output energy
腔长的改变也会引起OPO其他输出特性的变化。腔长增大导致OPO起振阈值升高(10.7 mJ→13.3 mJ→18.3 mJ)、斜率效率降低(28.7%→26.2%→25.3%),这可用此前的理论模型解释[19]。利用热释电相机测量输出光束的模场分布和发散角,结果显示,三种腔长下的OPO均产生了接近圆形的高斯光束,水平和垂直方向的发散角近似相等。由
图 7. 不同腔长下KTP-OPO的输入-输出关系
Fig. 7. Input-output relationship of KTP-OPO with different cavity lengths
图 8. 输出光束水平和垂直方向发散角以及脉冲能量波动的方均根随腔长的变化
Fig. 8. Variation of output beam divergent angles in horizontal andverticaldirectionsandroot-mean-square(RMS)pulse energy fluctuation with cavity length
4 结论
本文研究了平行平面腔OPO的腔镜失谐特性。基于这种腔型结构的OPO作为宽调谐相干光源已被广泛采用。利用532 nm绿光脉冲泵浦的KTP晶体,通过临界相位匹配产生1514 nm波长,获得了近圆形高斯光斑,斜率效率不低于25%,脉冲能量波动的方均根值不大于4%。利用压电光学调整架精准控制腔镜的偏角,实现了对OPO腔失谐特性的定量研究。实验中观察到该OPO输出对临界方向(极角)失谐的敏感程度远高于对非临界方向(方位角)失谐的敏感程度,通过分析相位失配解释了这一现象。扩宽泵浦光束和提高泵浦光强可以增大谐振腔的对准容限。在输入和输出脉冲能量一定的条件下,分析了腔长对腔镜对准容限及其他输出特性的影响。上述结论适用于脉冲运转、平行平面腔、临界相位匹配的OPO。
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Article Outline
付俏俏, 刘鹏翔, 祁峰, 李惟帆, 牛春草, 李伟, 郭丽媛, 汪业龙, 刘朝阳. 平行平面腔光学参量振荡器失谐特性实验研究[J]. 中国激光, 2022, 49(24): 2408002. Qiaoqiao Fu, Pengxiang Liu, Feng Qi, Weifan Li, Chuncao Niu, Wei Li, Liyuan Guo, Yelong Wang, Zhaoyang Liu. Characteristics of Mirror Misalignment of Plane-Parallel Cavity-Based Optical Parametric Oscillators[J]. Chinese Journal of Lasers, 2022, 49(24): 2408002.