下载:
-
砷酸钛氧钾晶体（KTA）属于非线性光学晶体KTP的同构体，不仅具有较大的非线性系数和良好的物理化学和机械性能，而且拥有更宽的透光波段、更高的损伤阈值和导热性能[1-2]。因此，KTA晶体在光参量振荡获得高效的人眼安全激光和中红外激光研究中被广泛应用[3-6]。1991年，Watson[7]首次报道了KTA晶体的拉曼光谱，并表明234 cm−1频移具有较强的拉曼增益。近年来，KTA晶体在受激拉曼激光器中优异的表现受到人们广泛的关注[8-10]。
受激拉曼散射(SRS)是一种高强度的三阶非线性光学现象，具有波长转换灵活、无需相位匹配等优势。至今，金刚石、钒酸盐、钨酸盐等许多晶体被证明具有较强的拉曼增益[11-14]，被广泛应用于高效的拉曼激光器设计。而KTA晶体同时是二阶非线性光学晶体，相对以上常见拉曼晶体可同时实现受激拉曼散射和二阶非线性变频 [15-17]，而且其增益较大的拉曼频移量较小，更有利于通过级联拉曼输出高阶斯托克斯激光。2008年，山东大学Liu等[18]报道了基于KTA晶体234 cm−1拉曼频移的1120 nm二阶斯托克斯激光。2013年，Lan等[19]采用端面抽运Nd:YAG/KTA被动调Q 获得了250 mW输出功率、3.3%转换效率的1091 nm一阶斯托克斯激光。2016年，笔者课题组[20]利用声光调Q基频偏振特性结合KTA晶体不同拉曼构型增益的差异，通过旋转KTA晶体分别实现了1.15 μm波段的234 cm−1拉曼频移的三阶斯托克斯激光和671 cm−1拉曼频移的一阶斯托克斯激光。结果表明KTA晶体234 cm−1和671 cm−1拉曼频移都可以实现高效的拉曼频移激光。最近，团队基于Nd:YAG/Cr4+:YAG/KTA被动调Q 拉曼自倍频，成功实现了671 cm−1拉曼频移的一阶斯托克斯倍频输出[21]。
Cr4+:YAG被动调Q的固态激光器具有体积小、成本低等优点，可用于紧凑型高重复频率脉冲激光器。特别在级联拉曼激光设计中，被动调Q压缩谐振腔，有利于增加腔内功率密度，提升拉曼转换，从而实现级联拉曼转换，获得普通固体激光难以实现的1178 nm和1212 nm等新波长的斯托克斯激光。相关波长倍频橙黄光在在钠导星、激光雷达和激光医疗等领域有重要应用[22-23]，特别1212 nm对富含脂质组织具有吸收亲和性，可用于刺激皮下组织脂肪细胞，是激光辅助皮肤愈合和预防疤痕形成的理想来源[24-25]。文中利用Nd:YAG/Cr4+:YAG被动调Q激光驱动的KTA晶体，结合腔镜对腔内各阶斯托克斯波长激光损耗控制，成功实现了671 cm−1和234 cm−1频移级联的1178 nm和1212 nm双波长激光输出。
-
KTA晶体拥有234 cm−1和671 cm−1两个较强的拉曼频移峰。由于其属于正交晶系的各向异性晶体，对不同拉曼构型对应频移的拉曼增益差异非常大[26]。文中采用如图1所示的激光系统，即利用Nd:YAG/Cr4+:YAG被动调Q激光驱动KTA晶体。由于可饱和吸收晶体和激光晶体都是YAG基质，将可饱和吸收晶体Cr4+:YAG与激光晶体Nd:YAG键合，可使系统更紧凑、更易于散热。激光晶体选用尺寸为3 mm×3 mm×7 mm的掺Nd3+浓度1.0 at.%的Nd:YAG晶体，可饱和吸收体选用尺寸为3 mm×3 mm×3 mm的Cr 4+:YAG晶体，初始透射率约为85%。KTA晶体沿X轴晶向切割，尺寸为4 mm×4 mm×25 mm。被动调Q的Nd:YAG基频激光腔内是无偏振的，而无偏振激光沿KTA晶体X轴晶向激发拉曼光谱中234 cm−1和671 cm−1两个频移峰具有相当强度[21]，所以比较容易实现两个不同频移级联拉曼转换。
图 1 被动调Q KTA级联拉曼激光实验装置图
Figure 1. Experimental setup of passively Q-switched KTA cascade Raman laser
实验的抽运源采用中心波长为808 nm的光纤耦合输出的半导体激光器，其光纤芯径为200 μm，数值孔径为0.22。用一对消色差耦合透镜将抽运光聚焦到激光晶体内，该耦合系统聚焦后的束腰直径约为320 μm。两块晶体用铟箔包裹放置在紫铜块中，通过半导体制冷模组（TEC）控温，使其温度均保持在300 K左右。由抽运输入镜(IM)和激光耦合输出镜(OC)组成的平凹腔保证了基频光和拉曼光有效振荡，腔内各光学元件紧凑布置，总腔长约为50 mm。系统中，腔镜镀膜设计对级联拉曼实验至关重要。腔镜IM镀对808 nm抽运光高透（T>95%)，并且对1.05~1.25 μm高反的膜系。腔镜OC为曲率半径为500 mm的平凹镜，并镀对1.05~1.16 μm波段高反，从1.16~1.25 μm波段部分透过率的膜系。具体的镀膜参数如图2所示，图中还给出了基于234 cm−1和671 cm−1频移的斯托克斯波长的透过率。基于实验谐振腔参数，可以计算得到当热透镜焦距为25 cm时，对应泵浦模式比1∶1，晶体处的基模半径刚好为160 μm；基模半径随透镜焦距减小而缩小。
图 2 腔镜OC在不同斯托克斯波长的透过率
Figure 2. Transmittance of cavity mirror OC at different Stokes wavelengths
-
在激光调节过程中，随着激光输出功率变强，发现有几十毫瓦较强的黄光激光产生。用型号AvaSpec-3648光纤光谱仪测量了该黄光光谱，如图3所示，其中心波长位于573.4 nm。为保证测量近红外波段拉曼激光功率的纯度，采用对可见光波段增透，对1.05~1.25 μm高反的分光镜将可见光分离测量。通过对激光系统的调节优化，实现了高效的级联拉曼激光输出。图4给出了用Thorlabs公司型号PM310D的功率计测量得到激光平均输出功率随入射抽运功率的变化值。图5给出了使用卓立汉光公司型号Omni-λ500光栅单色仪测量的不同功率下的激光波长。
图 3 伴随拉曼激光输出的可见光光谱
Figure 3. Visible light spectra associated with Raman laser output
图 4 拉曼激光输出功率与入射抽运功率的关系图
Figure 4. Raman laser output power as a function of incident pump power
由图4可知KTA拉曼激光阈值约为5.5 W，平均输出功率以6.2%斜率效率随着入射抽运功率增加而增加。在10.05 W的入射抽运功率下，获得了280 mW的平均输出功率，转化效率2.8%。结合图5的激光波长可知，在阈值附近到8 W抽运功率下的输出激光波长以1 178 nm为主，同时伴随微弱的1 146 nm波长。其中1 146 nm激光是基频1 064 nm激光通过KTA晶体671 cm−1频移获得的一阶斯托克斯激光，而1 178 nm激光是分别经过234 cm−1和671 cm−1频移各一次得到的斯托克斯激光。随着抽运功率的进一步提升，出现了1 212 nm波长激光。该波长激光为经过671 cm−1一次频移和234 cm−1两次频移级联的斯托克斯激光。随着入射抽运功率的逐渐增强，1 212 nm这条谱线在总输出强度所占的比重也有所增大。在10.05 W的抽运下，获得了1 178 nm和1 212 nm同时输出的双波长激光。从最高抽运功率下的激光光谱图还可以看到非常弱的1 182 nm的激光谱线，该谱线为1 064 nm基频激光经过四次234 cm−1频移得到四阶斯托克斯波长。由于腔镜在1.05~1.16 nm的高反射，抑制了234 cm−1频移的一~三阶斯托克斯激光输出，到四阶斯托克斯波长的光强已较弱。其中234 cm−1频移的一阶和二阶斯托克斯激光还是会经过671 cm−1频移获得1 178 nm和1 212 nm的激光，但234 cm−1频移的三阶和四阶斯托克斯激光以及1 146 nm倍频的黄绿激光产生会对1 178 nm和1 212 nm的激光产生增益竞争，一定程度上影响了最终的激光效率。
图 5 不同入射抽运功率下KTA级联拉曼的输出激光光谱
Figure 5. Output laser spectra of KTA cascade Raman at different incident pump powers
进一步采用型号DET08 C/M的InGaAs光电探测器（带宽5 GHz，上升沿70 ps）和示波器对被动调Q的激光脉冲进行测量。结果表明脉冲重复频率随着入射抽运功率的增加而增加，而脉冲宽度在1.5~1.1 ns范围波动。图6给出了脉冲重复频率和计算得到的单脉冲能量随着抽运功率的变换关系图。脉冲重复频率在5.9 W抽运功率下为1.9 kHz上升到10.05 W抽运功率下为10.3 kHz。单脉冲能量随着抽运功率快速增加，然后趋于稳定，这符合被动调Q的脉冲特性。根据被动调Q脉冲形成机理可知，随着功率的提高，可饱和吸收体会更快地被“漂白”，从而使形成脉冲的周期变短，重复频率快速增加，而保持单脉冲能量相对稳定。最高抽运功率下实测脉冲序列和单脉冲波形如图7所示。脉冲序列较为稳定，重复频率为10.3 kHz，脉冲宽度在1.2 ns左右。
图 6 输出重复频率和脉冲能量与入射抽运功率的关系曲线
Figure 6. Pulse repetition frequency and pulse energy as a function of incident pump power
图 7 在10.05 W的入射抽运功率下测得的脉冲波形和序列
Figure 7. Temporal pulse profile and pulse train of the yellow laser emission as measured under an incident pump power of 10.05 W
-
文中对端面抽运Nd:YAG/Cr4+:YAG被动调Q激光驱动的KTA晶体级联拉曼激光进行实验，研究了不同入射抽运功率下级联拉曼激光的输出功率、光谱和脉冲特性。通过选择腔镜控制腔内各阶斯托克斯波长的激光损耗，成功实现了671 cm−1和234 cm−1频移级联的1.2 μm波段斯托克斯激光输出。随着抽运功率的增加，输出激光波长从以671 cm−1和234 cm−1频移级联拉曼的1 178 nm单波长过渡到与1 212 nm同时输出的双波长。实验中也发现了高阶斯托克斯光和拉曼自倍频对腔内激光增益的竞争。最终入射泵抽运功率为10.05 W时，测得的最大平均输出功率为280 mW。测量的脉冲宽度约为1.2 ns，脉冲重复频率约为10.3 kHz。对应单脉冲能量和峰值功率分别为27.2 μJ和22.7 kW。考虑1 178 nm和1 212 nm波长的特殊应用，进一步实验可通过严格控制两个波长的损耗，实现单波长的激光输出。因此，基于KTA两个相当增益强度的频移，结合腔镜镀膜控制可以获得丰富KTA晶体高阶斯托克斯激光波长拓展固体激光应用。
End-pumped Nd:YAG/Cr4+:YAG/KTA passive Q-switched cascade Raman laser
-
摘要: 报道了基于KTA晶体671 cm−1和234 cm−1频移的LD端面抽运被动调Q级联拉曼激光器。采用Nd:YAG/Cr 4+:YAG复合晶体产生被动调Q的脉冲基频激光来驱动KTA晶体，研究了不同入射抽运功率下级联拉曼激光的输出功率、光谱和脉冲特性。随着抽运功率的增加，输出激光波长从以671 cm−1和234 cm−1频移级联拉曼的1178 nm单波长过渡到与1212 nm同时输出的双波长。在10.05 W的入射抽运功率下，获得了280 mW平均输出功率，6.2%转化效率的双波长激光。对应的脉冲宽度和重复频率分别为1.2 ns和10.3 kHz，单脉冲能量和峰值功率分别为27.2 μJ和22.7 kW。结果表明：基于KTA两个相当增益强度的频移，结合腔镜镀膜控制可以获得丰富的斯托克斯激光波长。Abstract:
Objective KTA crystal as Raman gain medium has attracted increasing attention. Its larger Raman gain and smaller Raman shift make it more advantageous for cascade Raman conversion to obtain new wavelength Stokes laser of 1 178 nm and 1 212 nm. The frequency doubling of both two Stokes laser could realize yellow and orange emissions which have important applications in remote sensing, laser medicine and so on. In particular, 1 212 nm laser has an absorption affinity for lipid-rich tissues and can be used to stimulate adipose cells and mesymal fine tissue in subcutaneous tissues, which is an ideal source for laser assisted skin healing and prevention of excessive scar formation. In view of the important applications of 1 178 nm and 1 212 nm laser, KTA cascade Raman operation driven by passive Q-switched laser for these two wavelengths generation was further investigated. Methods The laser system for KTA cascade Raman operation is shown (Fig.1). Nd:YAG/Cr4+:YAG composite was used for passive Q-switched laser generation. Cr4+:YAG crystal, with an initial transmittance of about 85%, was used as saturable absorber crystal and diffusion bonded to Nd:YAG crystal to make the laser system more compact and easy to dissipate heat. An x-axis cut KTA crystal with 25 mm in length was used as the Raman crystal. The plano-concave cavity with the total cavity length of about 50 mm guarantees the effective oscillation of the fundamental laser and Raman laser. The specific coating parameters are shown (Fig.2). The transmittance of Stokes wavelengths based on the Raman shift of 234 cm−1 and 671 cm−1 is also given (Fig.2). Results and Discussions With the increase of Raman laser output power, we found the Stokes laser was accompanied by a small amount of yellow laser output, and the corresponding yellow light spectrum was shown (Fig.3). Under an incident pumping power of 10.05 W, an average output power of 280 mW was obtained, and the conversion efficiency is 2.8%. The output power and laser wavelength are shown (Fig.4-5). From the threshold to 8 W incident pump power, the main intensity laser wavelength was 1 178 nm, accompanied by a weak 1 146 nm wavelength. With the further increase of incident pump power, 1 212 nm wavelength laser appeared. As the incident pump power gradually further increased, the proportion of 1 212 nm line in the total output intensity also increased. A dual-wavelength laser with 1 178 nm and 1 212 nm output was obtained under the incident pump power of 10.05 W. The results show that the third- and fourth- Stokes laser generation with a Raman shift of 234 cm−1, as well as the yellow laser produced by frequency doubling of 1 146 nm would lead to gain competition, and reduce the conversion efficiency of Stokes laser at 1 178 nm and 1 212 nm. The recorded pulse profile and pulse train at the highest output power are shown (Fig.7). The pulse repetition frequency was 10.3 kHz and the pulse width was about 1.2 ns. The compact passive Q-switched Raman laser cavity increased the power density, and resulted in the improvement of the Raman conversion, so as to realize the high-order Stokes waves. Conclusions In this study, a diode end-pumped passive Q-switched cascade Raman laser with the Raman shifts of 671 cm−1 and 234 cm−1 based on KTA crystal is reported. Nd:YAG/Cr4+:YAG composite crystal is used to generate pulsed fundamental laser and then to drive KTA crystal. The output power, spectrum and pulse characteristics of cascade Raman laser with different incident pump power are studied. With the increasing pump power, the output laser wavelength shifted from single wavelength of 1 178 nm based on 671 cm−1 and 234 cm−1 cascaded Raman shifts to dual-wavelength output of 1 178 nm and 1 212 nm. Under an incident pump power of 10.05 W, a dual-wavelength laser with an average output power of 280 mW and a conversion efficiency of 6.2% was obtained. The corresponding pulse width and pulse repetition frequency are 1.2 ns and 10.3 kHz, respectively. The single pulse energy and peak power are 27.2 μJ and 22.7 kW, respectively. The results show that rich Stokes laser wavelengths could be obtained based on two comparable gain Raman shifts of KTA crystal with the coating control of the cavity mirror. -
Key words:
- Raman laser /
- KTA crystal /
- passive Q-switch /
- YAG crystal
-
图 1 被动调Q KTA级联拉曼激光实验装置图
Figure 1. Experimental setup of passively Q-switched KTA cascade Raman laser
图 2 腔镜OC在不同斯托克斯波长的透过率
Figure 2. Transmittance of cavity mirror OC at different Stokes wavelengths
图 4 拉曼激光输出功率与入射抽运功率的关系图
Figure 4. Raman laser output power as a function of incident pump power
图 5 不同入射抽运功率下KTA级联拉曼的输出激光光谱
Figure 5. Output laser spectra of KTA cascade Raman at different incident pump powers
图 6 输出重复频率和脉冲能量与入射抽运功率的关系曲线
Figure 6. Pulse repetition frequency and pulse energy as a function of incident pump power
-
[1] Pack M V, Armstrong D J, Smith A V. Measurement of the χ(2) tensors of KTiOPO4, KTiOAsO4, RbTiOPO4, and RbTiOAsO4 crystals [J]. Applied Optics, 2004, 43(16): 3319-3323. doi: https://doi.org/10.1364/AO.43.003319 [2] Gao Z L, Sun Y X, Yin X, et al. Growth and electric-elastic properties of KTiOAsO4 single crystal [J]. Journal of Applied Physics, 2010, 108(2): 024103. doi: 10.1063/1.3446042 [3] Bai Fen, Wang Qingpu, Liu Zhaojun, et al. Comparison of signal-resonant and idler-resonant KTA-SROs [J]. Chinese Optics Letters, 2016, 14(7): 071402. doi: 10.3788/COL201614.071402 [4] Duan Y M, Zhu H Y, Xu C W, et al. Compact self-cascaded KTA-OPO for 2.6 μm laser generation [J]. Optics Express, 2016, 24(23): 26529-26535. doi: 10.1364/OE.24.026529 [5] 卞进田, 孔辉, 徐海萍, 等. 3.5 μm KTiOAsO4光参量振荡器温度调谐特性[J]. 中国激光, 2021, 48(4): 0401015. doi: 10.3788/CJL202148.0401015 Bian Jintian, Kong Hui, Xu Haiping, et al. Temperature tuning properties of 3.5-μm KTiOAsO4 optical parametric oscillator [J]. Chinese Journal of Lasers, 2021, 48(4): 0401015. (in Chinese) doi: 10.3788/CJL202148.0401015 [6] Meng Jun, Li Chen, Cong Zhenhua, et al. Investigations on beam quality improvement of an NCPM-KTA-based high energy optical parametric oscillator using an unstable resonator with a Gaussian reflectivity mirror [Invited] [J]. Chinese Optics Letters, 2022, 20(9): 091401. doi: 10.3788/COL202220.091401 [7] Watson G H. Polarized Raman spectra of KTiOAsO4 and isomorphic nonlinear-optical crystals [J]. Journal of Raman Spectroscopy, 1991, 22(11): 705-713. doi: 10.1002/jrs.1250221116 [8] Zhu H Y, Shao Z H, Wang H Y, et al. Multi-order Stokes output based on intra-cavity KTiOAsO4 Raman crystal [J]. Optics Express, 2014, 22(16): 19662-19667. doi: 10.1364/OE.22.019662 [9] Huang Y J, Chen Y F, Chen W D, et al. Dual-wavelength eye-safe Nd: YAP Raman laser [J]. Optics Letters, 2015, 40(15): 3560-3563. doi: 10.1364/OL.40.003560 [10] 张喜梅, 陈思梦, 施沈城, 周青青, 段延敏, 朱海永. 级联Nd: GdVO4自拉曼1309 nm激光性能研究[J]. 红外与激光工程, 2019, 48(11): 1105002 . doi: 10.3788/IRLA201948.1105002 Zhang Ximei, Chen Simeng, Shi Shencheng, et al. Study on the performance of cascaded Nd: GdVO4 self-Raman laser at 1 309 nm [J]. Infrared and Laser Engineering, 2019, 48(11): 1105002. (in Chinese) doi: 10.3788/IRLA201948.1105002 [11] 孙冰, 丁欣, 姜鹏波, 白云涛, 禹宣伊, 王靖博, 赵蕾, 刘阳, 李腾腾, 吴亮, 张贵忠, 姚建铨. 波长锁定878.6 nm LD调制泵浦Nd: YVO4自拉曼激光器[J]. 红外与激光工程, 2021, 50(12): 20200227. doi: 10.3788/IRLA20200227 Sun Bing, Ding Xin, Jiang Pengbo, et al. Efficient wavelength-locked 878.6 nm diode-modulated-pumped Nd: YVO4 self-Raman laser [J]. Infrared and Laser Engineering, 2021, 50(12): 20200227. (in Chinese) doi: 10.3788/IRLA20200227 [12] Duan Y M, Sun Y L, Zhu H Y, et al. YVO4 cascaded Raman laser for five-visible-wavelength switchable emission [J]. Optics Letters, 2020, 45(9): 2564-2567. [13] 白振旭, 陈晖, 张展鹏, 等. 百瓦级1.2/1.5 μm双波长金刚石拉曼激光器(特邀)[J]. 红外与激光工程, 2021, 50(12): 20210685. doi: 10.3788/IRLA20210685 Bai Zhenxu, Chen Hui, Zhang Zhanpeng, et al. Hundred-watt dual-wavelength diamond Raman laser at 1.2/1.5 μm (Invited) [J]. Infrared and Laser Engineering, 2021, 50(12): 20210685. (in Chinese) doi: 10.3788/IRLA20210685 [14] Yang Ce, Chen Meng, Ma Ning, et al. Picosecond multi-pulse burst pump KGW infrared multi-wavelength Raman laser [J]. Infrared and Laser Engineering, 2020, 49(11): 20200044. doi: 10.3788/IRLA20200044 [15] Liu Z J, Wang Q P, Zhang X Y, et al. Coexistent optical parametric oscillation and stimulated Raman scattering in KTiOAsO4 [J]. Optics Express, 2008, 16(21): 17092-17097. [16] Zhu H Y, Guo J H, Duan Y M, et al. Efficient 1.7 μm light source based on KTA-OPO derived by Nd: YVO4 self-Raman laser [J]. Optics Letters, 2018, 43(2): 345-348. [17] Huang H T, Wang H, Wang S Q, et al. Designable cascaded nonlinear optical frequency conversion integrating multiple nonlinear interactions in two KTiOAsO4 crystals [J]. Optics Express, 2018, 26(2): 642-650. [18] Liu Z J, Wang Q P, Zhang X Y, et al. 1120 nm second-Stokes generation in KTiOAsO4 [J]. Laser Physics Letters, 2008, 6(2): 121-124. [19] Lan W X, Wang Q P, Liu Z J, et al. A diode end-pumped passively Q-switched Nd: YAG/KTA Raman laser [J]. Optik, 2013, 124(24): 6866-6868. doi: 10.1016/j.ijleo.2013.05.154 [20] Duan Y M, Zhu H Y, Wang H Y, et al. Comparison of 1.15 μm Nd: YAG\KTA Raman lasers with 234 and 671 cm-1 shifts [J]. Optics Express, 2016, 24(5): 5565-5571. doi: 10.1364/OE.24.005565 [21] Mao W J, Zhang D, Lu H Q, et al. Compact passively Q-switched KTA self-frequency doubled Raman laser with 671 cm−1 shift [J]. Optics and Laser Technology, 2022, 156: 108619. doi: 10.1016/j.optlastec.2022.108619 [22] Sheng Q, Lee A, Spence D, et al. Wavelength tuning and power enhancement of an intracavity Nd: GdVO4-BaWO4 Raman laser using an etalon [J]. Optics Express, 2018, 26(24): 32145-32155. doi: 10.1364/OE.26.032145 [23] Sheng Q, Li R, Lee A, et al. A single-frequency intracavity Raman laser [J]. Optics Express, 2019, 27(6): 8540-8553. doi: 10.1364/OE.27.008540 [24] Centurion P, Noriega A. Fat preserving by laser 1210-nm [J]. Journal of Cosmetic and Laser Therapy, 2013, 15(1): 2-12. doi: 10.3109/14764172.2012.758376 [25] Philandrianos C, Bertrand B, Andrac-Meyer L, et al. Treatment of keloid scars with a 1210-nm diode laser in an animal model [J]. Lasers in Surgery and Medicine, 2015, 47: 798-806. [26] Liu Z J, Wang Q P, Zhang, X Y, et al. Self-frequency-doubled KTiOAsO4 Raman laser emitting at 573 nm [J]. Optics Letters, 2009, 34(14): 2183-2185. doi: 10.1364/OL.34.002183 -
点击查看大图
图(7)
计量
- 文章访问数: 129
- HTML全文浏览量: 35
- PDF下载量: 43
- 被引次数: 0
