高效率连续波Nd∶YVO4/KGW内腔拉曼激光器
Stimulated Raman scattering (SRS) in the crystalline Raman gain media is a well-established technique for extending the spectral coverage of lasers. However, as a third-order nonlinear process, the SRS suffers a relatively low nonlinear gain and consequently has a high threshold, specifically when operating in a continuous-wave (CW) scheme. The intra-cavity pump scheme, in which the Raman crystal is located within the fundamental laser cavity, is an effective alternative to achieve efficient CW Raman output with moderate primary pump power because the high circulating fundamental laser power in the cavity generates sufficient Raman gain. To date, the highest CW Stokes output power of end-pumped intra-cavity Raman lasers has been realized with the self-Raman scheme, in which the processes of lasing and SRS take place in one crystal to minimize insertion losses. However, intra-cavity Raman lasers with separate lasers and Raman gain media have the advantages of a more flexible output wavelength and distributed thermal load, which are helpful for power scaling. This study presents an efficient CW Nd∶YVO4/KGW intra-cavity Raman laser. The output power of the CW Stokes wave at 1177 nm reaches 6.63 W under an incident laser diode (LD) pump power of 36.6 W, with the corresponding optical efficiency being 18.1%.
The experimental setup of the CW intra-cavity Raman laser is shown in Fig. 1. A 15 mm long a-cut Nd∶YVO4 crystal and a 20 mm long Np-cut KGW crystal serve as the fundamental laser and Raman gain media, respectively. The LD pump wavelength is 878.6 nm, and the pump beam radius at the laser crystal is 280 μm. The Nd∶YVO4 crystal has a low doping atomic fraction of 0.2% to alleviate the thermal effect. The 1064 nm fundamental laser cavity is defined by a flat highly reflective (HR) mirror (M1) and a curved HR mirror (M2) with a radius of curvature of 100 mm. The M2 also has a transmissivity of 0.4% at a Stokes wavelength of 1177 nm. A flat dichroic mirror (M3) with HR coating at 1.15-1.18 μm and highly transmissive at 1064 nm is inserted into the cavity to make the Raman Stokes cavity with M2. The lengths of the fundamental and Stokes cavities are 50 mm and 22 mm, respectively.
First, the polarization direction of the linearly polarized fundamental frequency light generated by Nd∶YVO4 is parallel to the Nm axis of the KGW crystal (E∥Nm). With this polarization, the Raman gain coefficient of the 901 cm-1 Raman line is over two times larger than that of the 768 cm-1 Raman line. The Stokes output power as a function of incident LD pump power is shown in Fig. 2. The SRS threshold is 7.5 W LD power, and the maximum Stokes output power reaches 6.63 W under the maximum pump power of 36.6 W. Only the first Stokes field at 1177.3 nm is observed during the entire process. The spectral linewidths of the fundamental laser and Stokes wave are 0.08 nm and 0.02 nm at the SRS threshold and are broadened to 0.3 nm and 0.2 nm, respectively, at the maximum power, as shown in Fig. 3. Because of the astigmatic thermal lens in the KGW crystal, the Stokes output beam profile becomes the Hermite-Gaussian (HG) mode at the maximum power, as shown in Fig. 4. We also attempt fundamental polarization parallel to the Ng axis of the KGW crystal. In this case, the laser output power and conversion efficiency are lower than those for E∥Ng. The Stokes output power under the same maximum pump power of 36.6 W is only 4.86 W. We find that the output wavelength contains both 1159 nm and 1177 nm components, which correspond to the 768 cm-1 and 901 cm-1 Raman shifts, respectively, when the pump power exceeds the SRS threshold of 7.5 W. The cascaded Raman Stokes light at 1171 nm and 1189 nm corresponded to the 89 cm-1 Raman shift also occurs at higher pump power, as shown in Fig. 5. The multiline Stokes field decreases the effective Raman gain, whereas the cascaded Raman conversion decreases the interaction between the fundamental Stokes fields. Therefore, the E∥Nm arrangement, in which the 901 cm-1 Raman shift dominates, is more suitable for efficiently generating high-power Stokes outputs with high spectral purity.
In conclusion, we present an efficient CW Nd∶YVO4/KGW intra-cavity Raman laser. The effects of the fundamental laser polarization direction on the power, spectral mode, and transverse mode of the KGW Raman laser are investigated experimentally. When the fundamental polarization distribution is parallel to the Nm axis of the Np-cut KGW crystal, the laser benefits from a higher Raman gain at 901 cm-1 Raman shift. The 6.63 W CW Stokes output at 1177.3 nm is obtained under an incident LD pump power of 36.6 W, with corresponding optical and slope efficiencies of 18.1% and 24.7%, respectively.
1 引言
基于非线性晶体受激拉曼散射(SRS)效应的固体拉曼激光器是拓宽激光辐射波长范围的重要技术途径。SRS过程不受相位匹配条件的限制,在晶体通光波段均可实现拉曼激光输出,目前固体拉曼激光输出波长覆盖了紫外到中红外波段,SRS独特的光束净化效应也有助于产生高光束质量的激光输出[1-7]。
作为三阶非线性效应,SRS过程的非线性增益较低,在连续波运转时这一短板体现得尤为明显。即便使用金刚石等拉曼增益很高的新型非线性晶体,连续波外腔拉曼激光器的阈值仍多在10~20 W水平[8-9],因此只有在数十瓦或更高的泵浦功率下才能实现较高的光光效率。为实现拉曼激光器低阈值连续波运转,研究人员经常采用内腔泵浦方式:将拉曼晶体置于基频激光的谐振腔内,利用腔内振荡的高功率基频光产生较高的拉曼增益,能够在瓦级的半导体激光器(LD)泵浦功率下实现拉曼激光器的连续波运转,并高效产生数瓦功率水平的输出[10-12]。2012年,Lin等[13]通过双端偏振泵浦Nd∶GdVO4自拉曼激光器获得了4.1 W连续波1173 nm斯托克斯光输出和3.46 W倍频黄光输出。端面泵浦连续波内腔拉曼激光器的最高斯托克斯输出由Fan等[10]于2016年报道:基于YVO4-Nd∶YVO4-YVO4键合晶体的自拉曼频率变换获得了5.3 W的1176 nm斯托克斯光输出。与自拉曼激光器相比,基于分立激光晶体和拉曼晶体的内腔拉曼激光器尽管插入损耗较大,但不同激光晶体和拉曼晶体的组合使其输出波长选择更为灵活;其热负载分散在两块晶体中,也有助于缓解热效应,提高功率上限。此外,分立晶体允许分离的基频光与斯托克斯光谐振腔设计,方便使用选频器件实现激光波长调谐和线宽控制[1,14-16]。2012年,Savitski等[17]报道了基于高功率侧泵Nd∶YLF基频激光的钨酸钆钾(KGW)内腔拉曼激光器,获得了6.1 W斯托克斯光输出功率,这是连续波内腔拉曼激光器的最高斯托克斯光输出。2019年,Sheng等[14]在Nd∶GdVO4/BaWO4内腔拉曼激光器的基频光谐振腔内插入标准具控制其光谱线宽,利用SRS过程不受空间烧孔效应影响的特性,实现了3.42 W的1178 nm连续波单频斯托克斯光输出,并将其二次谐波波长调谐至589.16 nm钠信标波长。近期Chen等[18-20]采用Nd∶YVO4/Nd∶GdVO4等激光晶体和KGW拉曼晶体的组合,实现了578、579、588 nm等多个波长的斯托克斯光倍频输出,连续波倍频黄光和近红外斯托克斯光的输出功率最高分别为6.8 W和3.2 W。
本文采用Nd∶YVO4和KGW分别作为激光和拉曼增益介质,设计搭建了复合腔结构的连续波内腔拉曼激光器,在36.6 W泵浦功率下获得了6.63 W的1177.3 nm一阶斯托克斯光输出。实验详细对比了基频光偏振方向对KGW拉曼激光器光谱、功率和模式特性的影响,发现基频光沿KGW晶体的Nm轴偏振更易于实现高效、单一波长的拉曼输出。
2 实验装置
3 实验结果与讨论
KGW为双轴晶体,不同方向的拉曼谱区别很大。实验中首先令Nd∶YVO4产生的线偏振基频光偏振方向平行于KGW晶体的Nm轴(E∥Nm),此时频移901 cm-1的拉曼谱线相比其他谱线更强,其拉曼增益系数gR为~6 cm/GW。
图 3. E∥Nm时激光输出光谱。(a)7.5 W泵浦功率;(b)36.6 W泵浦功率
Fig. 3. Laser spectra when E∥Nm. (a) Pump power of 7.5 W; (b) pump power of 36.6 W
图 4. 不同泵浦功率下的典型基频光和斯托克斯光光斑。(a)~(d)1064.4 nm基频光光斑;(e)~(g)1177.3 nm斯托克斯光光斑
Fig. 4. Typical fundamental and Stokes beam profiles under different pump powers. (a)‒(d) 1064.4 nm fundamental beam profiles; (e)‒(g) 1177.3 nm Stokes beam profiles
作为对比,实验中也测试了基频光偏振方向平行于KGW晶体Ng轴(E∥Ng)时激光器的输出特性。如
图 6. E∥Ng时的激光输出光谱(泵浦功率为36.6 W,斯托克斯光输出功率为4.86 W)
Fig. 6. Laser spectrum when E∥Ng with pump power of 36.6 W and Stokes output power of 4.86 W
4 结论
基于复合腔结构的Nd∶YVO4/KGW内腔拉曼激光器实现了高效的斯托克斯光输出,实验研究了基频光偏振方向对拉曼激光器功率和光谱特性的影响。当基频光偏振方向平行于沿Np轴切割的KGW晶体的Nm轴时,在36.6 W泵浦功率下获得了6.63 W的1177.3 nm一阶斯托克斯光输出;而当基频光沿Ng轴偏振时,由于768 cm-1和901 cm-1拉曼谱线的竞争以及对应89 cm-1拉曼谱线的级联斯托克斯光的产生,激光器的输出功率偏低。基频光沿KGW晶体的Nm轴偏振更易于实现高效、单一波长的拉曼输出。实验中还研究了拉曼激光器的模式随功率的变化规律,观察到高功率下KGW晶体像散的热透镜效应使斯托克斯光以HG0,1模式运转的现象。
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盛泉, 耿婧旎, 李锦辉, 付士杰, 史伟, 姚建铨. 高效率连续波Nd∶YVO4/KGW内腔拉曼激光器[J]. 中国激光, 2024, 51(5): 0501003. Quan Sheng, Jingni Geng, Jinhui Li, Shijie Fu, Wei Shi, Jianquan Yao. Efficient Continuous-Wave Nd∶YVO4/KGW Intra-cavity Raman laser[J]. Chinese Journal of Lasers, 2024, 51(5): 0501003.