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具有高光束质量的激光器以其空间相干性好、聚焦功率密度高、光束发散小、易于长程传输等优点,在遥感、工业加工以及定向能武器等领域发挥着重要作用[1-3]。经过长达60余年的发展,人们已经利用固体、光纤等激光工作物质在1 μm波段实现千瓦级的高光束质量连续波激光输出[3-5]。但是直接获得可见光、人眼安全等特殊波段的高功率运转却仍面临很大的挑战,究其原因是受到掺杂离子固有发射光谱、工作物质的热物性以及可用泵浦源等因素的限制。例如,虽然掺Nd3+离子YAG晶体的发射光谱除了常用的1 064 nm之外也覆盖了946 、1 122 、1319 nm等多个波长,但由于其他波长的荧光强度较弱,目前只有1 064 nm辐射波长最为常用且易于实现高功率运转[6];掺Er3+离子(或Er3+/Yb3+共掺)的光纤激光器是产生高功率1.5 μm波段人眼安全激光的最常用手段,但是高功率运转下光纤激光器固有的横模模式不稳定(TMI)及光谱展宽现象限制了其亮度的进一步提升及在诸多领域的应用[7-8];掺Pr3+、Dy3+等离子的晶体是目前常用的可直接获得可见光辐射的增益介质,但受到掺杂离子的固有增益特性和基质热物性的限制、并且所需的高功率蓝光泵浦源本身就难以获得,因此其输出功率往往仅有瓦量级[9]。
除了获得单一的特殊波长输出,双波长激光也因其在精密激光光谱、共振激光干涉、分子多光子分解及激光雷达等方面的应用而备受关注[10-12]。1.2 μm和1.5 μm波段激光位于重要的大气透过窗口,自然界中广泛存在的CO2和水分子对这两个波段的吸收率也远小于目前最常见的1 μm波段激光(见图1),因此1.2 μm和1.5 μm波段激光在遥感监测、雷达、通信等领域有具有重要的应用前景 [13-15]。为实现激光的长程传输,开展高功率高光束质量的1.2 μm和1.5 μm波段激光的研究具有重要的实际意义,但是受限于现有激光工作物质的可用发射光谱以及不同发射谱的增益差别较大,通过传统的粒子数反转激光器直接获得高功率1.2 μm和1.5 μm波段激光的同时输出仍存在较大挑战,甚至针对1.2 μm单一波长激光的高效产生和放大至今也并没有十分成熟的解决途径。
图 1 大气吸收带和主要吸收粒子
Figure 1. Atmospheric absorption bands and the particles responsible for the absorption
基于三阶非线性光学效应受激拉曼散射(SRS)的拉曼激光器是一种实现激光波长变换的有效手段,其原理是通过强光激发增益介质内部的分子或原子振动产生具有较大频移的Stokes光并进行放大输出 [16]。拉曼激光器相对于传统粒子数反转激光器,其特点在于只要相互作用的波长在材料的光谱透射区且达到激发阈值,理论上就可以通过选择泵浦波长和控制级联SRS实现任何波段的激光输出;结合SRS固有的光束净化特性,拉曼激光器已成为获得多波长、高功率且高光束质量激光输出的重要方式[17-21]。目前,光纤和晶体拉曼激光器是实现拉曼转换的主要手段,但是光纤拉曼激光器在功率提升中难以抑制的光谱展宽和TMI在一定程度上限制了其功率提升;晶体材料虽然和光纤相比能够负载高峰值功率的脉冲激光泵浦,但是受到传统拉曼晶体固有热物性的制约,输出功率很难突破百瓦量级。
随着化学气相沉积法(CVD)等晶体制备工艺的提升,具有优异物理和化学稳定性、高导热率(>2 000 W·m−1·K−1)和极宽光谱透过范围(>0.23 μm)的人造金刚石晶体逐渐走入人们的视野,光学级单晶金刚石晶体也因其高的拉曼增益系数(10 cm/GW@1 μm),成为高功率拉曼激光器的理想选择[2,22-24]。表1列举了常用拉曼晶体以及石英光纤的关键物理及拉曼参数特性。从表1可以看出,包括金刚石在内的晶体材料的拉曼增益线宽相较于光纤拉曼激光器中常用的石英光纤低了2~3个数量级,这意味着光纤拉曼激光器中难以抑制的光谱展宽现象在晶体拉曼激光器中可以得到有效控制;此外,金刚石的热导率是其他常用拉曼晶体的百倍以上,拉曼增益系数、拉曼频移和光谱透过范围也明显优于其他晶体。以上特性使得金刚石晶体在实现高功率、高效率、无光谱展宽的拉曼转换中具有巨大的优势。经过十余年的快速发展,金刚石拉曼激光器的波长覆盖范围也越来越广(短至紫外、长至中红外)、功率也达到了千瓦量级[25-30]。
表 1 常见拉曼晶体及石英光纤特性对比
Table 1. Comparison of properties of common Raman crystals and silica fiber
Raman gain media Thermal conductivity/
W·m−1·K−1Raman gain coefficient
@1 μm
/cm·GW−1Raman shift/
cm−1Raman linewidth/
cm−1Spectral transmission/
μmDiamond 2 000 10-12 1 332.5 2 > 0.23 YVO4 5.2 4.5 890 3.0 0.4-5 KGd(WO4)2 2.6(a),3.8(b),3.4(c) 3.5 767, 901.5 7.8, 5.9 0.34-5.5 Ba(NO3)2 1.17 11 1 047.6 0.4 0.35-1.8 CaWO4 16 3.0 908 4.8 0.2-5.3 GdVO4 10.5 >4.5 885 3.0 0.35-5 BaWO4 3.0 8.5 926 1.6 0.26-3.7 Silica fiber 1.38 9.4×10−3 440 1 333 0.38-2.1 文中报告了一台1.2 μm和1.5 μm双波长输出的金刚石拉曼激光器。利用1 μm准连续激光作为泵浦源,通过线性结构的外腔拉曼振荡器,在1 μm泵浦光稳态功率为414 W时分别获得了1.2 μm一阶拉曼72 W和1.5 μm二阶拉曼110 W的输出,总输出功率182 W(转换效率44.0%),并在实验中观察到双波长激光同时实现光束净化的现象。该研究结果为实现高功率的双波长激光输出提供了新的途径。
Hundred-watt dual-wavelength diamond Raman laser at 1.2 /1.5 μm (Invited)
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摘要: 具有高功率、高光束质量的双波长激光器在精密光谱、共振干涉测量和激光雷达等领域有着重要的应用。但是受到激光工作物质固有的光谱和增益特性制约,通过传统的粒子数反转激光器难以直接获得高功率的双波长激光输出,因此通常需要结合非线性光学频率变换技术将常规的单一波长高功率激光拓展至一个或若干个特殊波段。受激拉曼散射作为一种三阶非线性效应,具有频移大、自相位匹配和光束净化等优点,是实现高效率、高光束质量波长转换有效手段。利用具有宽光谱透过范围(>0.23 μm)、超高热导率(>2 000 W·m−1·K−1)和大拉曼频移(1 332 cm−1)等优异特性的金刚石晶体作为拉曼增益介质,通过外腔振荡结构实现了1 μm泵浦光直接向1.2 μm和1.5 μm双波长激光的高效转换,在最高稳态泵浦功率414 W的条件下获得了1.2 μm和1.5 μm功率分别为72 W和110 W的输出。该研究为实现高功率的双波长激光输出开辟了新的途径。Abstract: Dual-wavelength lasers with high power and high beam quality are critical to the applications such as precision spectroscopy, resonant interferometry, lidar, etc. However, limited by the intrinsic spectral and gain characteristics of currently available laser gain materials, it is difficult to realize high-power dual wavelength lasing directly from inversion lasers. To overcome this problem, nonlinear optical frequency conversion has been applied to convert the high-power laser in a conventional band to another or several hard-to-reach bands. As a third-order nonlinear effect, stimulated Raman scattering has advantages including large frequency shift, self-phase matching, and beam clean-up effect that lead to Raman laser an effective means to achieve high efficiency and high beam quality wavelength conversion. In this paper, diamond crystal that with a wide spectral transmission range (>0.23 μm), ultra-high thermal conductivity (>2 000 W·m−1·K−1) and large Raman frequency shift (1 332 cm−1) was used as the Raman gain medium. By using a 1 μm laser as pump source, dual-wavelength lasing at 1.2 and 1.5 μm was achieved based on an external cavity Raman oscillator. With a maximum steady-state pump power of 414 W, output powers up to 72 W at 1.2 μm and 110 W at 1.5 μm were obtained. This study has opened a new way to realize high-power dual-wavelength laser output.
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Key words:
- laser /
- diamond /
- Raman /
- dual wavelength /
- high power
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表 1 常见拉曼晶体及石英光纤特性对比
Table 1. Comparison of properties of common Raman crystals and silica fiber
Raman gain media Thermal conductivity/
W·m−1·K−1Raman gain coefficient
@1 μm
/cm·GW−1Raman shift/
cm−1Raman linewidth/
cm−1Spectral transmission/
μmDiamond 2 000 10-12 1 332.5 2 > 0.23 YVO4 5.2 4.5 890 3.0 0.4-5 KGd(WO4)2 2.6(a),3.8(b),3.4(c) 3.5 767, 901.5 7.8, 5.9 0.34-5.5 Ba(NO3)2 1.17 11 1 047.6 0.4 0.35-1.8 CaWO4 16 3.0 908 4.8 0.2-5.3 GdVO4 10.5 >4.5 885 3.0 0.35-5 BaWO4 3.0 8.5 926 1.6 0.26-3.7 Silica fiber 1.38 9.4×10−3 440 1 333 0.38-2.1 -
[1] Extance A. Military technology: Laser weapons get real [J]. Nature News, 2015, 521(7553): 408. doi: 10.1038/521408a [2] Williams R J, Kitzler O, Bai Z, et al. High power diamond Raman lasers [J]. IEEE Journal of Selected Topics in Quantum Electronics, 2018, 24(5): 1602214. [3] Zervas M N, Codemard C A. High power fiber lasers: A review [J]. IEEE Journal of Selected Topics in Quantum Electronics, 2014, 20(5): 219-241. doi: 10.1109/JSTQE.2014.2321279 [4] Comaskey B J, Beach R, Albrecht G, et al. High average powers diode pumped slab laser [J]. IEEE Journal of Quantum Electronics, 1992, 28(4): 992-996. doi: 10.1109/3.135218 [5] Wang H, Lin L, Ye X. Status and development trend of high power slab laser technology [J]. Infrared and Laser Engineering, 2020, 49(7): 20190456. (in Chinese) doi: 10.3788/IRLA20190456 [6] Koechner W. Solid-state Laser Engineering [M]. US: Springer, 2006. [7] Supradeepa V R, Nicholson J W. Power scaling of high-efficiency 1.5 μm cascaded Raman fiber lasers [J]. Optics Letters, 2013, 38(14): 2538-2541. doi: 10.1364/OL.38.002538 [8] Jauregui C, Stihler C, Limpert J. Transverse mode instability [J]. Advances in Optics and Photonics, 2020, 12(2): 429-484. doi: 10.1364/AOP.385184 [9] Huo Xiaowei, Qi Yaiyao, Li Yuqi, et al. Research progress of LD-pumped Pr3+-doped solid-state laser in visible wavelength [J]. Electro-optic Technology & Application, 2019, 34(5): 7-15. (in Chinese) doi: 10.3969/j.issn.1673-1255.2019.05.002 [10] Sharma U, Kim C S, Kang J U. Highly stable tunable dual-wavelength Q-switched fiber laser for DIAL applications [J]. IEEE Photonics Technology Letters, 2004, 16(5): 1277-1279. doi: 10.1109/LPT.2004.825991 [11] Akbari R, Zhao H, Major A. High-power continuous-wave dual-wavelength operation of a diode-pumped Yb: KGW laser [J]. Optics Letters, 2016, 41(7): 1601-1604. doi: 10.1364/OL.41.001601 [12] Deng Q, Wu D, Kuang Z, et al. 532 nm/660 nm dual wavelength lidar for self-calibration of water vapor mixing ratio [J]. Infrared and Laser Engineering, 2018, 47(12): 1230004. (in Chinese) doi: 10.3788/IRLA201847.1230004 [13] Alavipanah S K, Matinfar H R, Rafiei Emam A, et al. Criteria of selecting satellite data for studying land resources [J]. Desert, 2010, 15(2): 83-102. [14] Vatnik I D, Churkin D V, Babin S A, et al. Cascaded random distributed feedback Raman fiber laser operating at 1.2 μm [J]. Optics Express, 2011, 19(19): 18486-18494. doi: 10.1364/OE.19.018486 [15] Bai Z, Williams R J, Kitzler Ondrej, et al. 302 W quasi-continuous cascaded diamond Raman laser at 1.5 microns with large brightness enhancement [J]. Optics Express, 2018, 26(16): 19797-19803. doi: 10.1364/OE.26.019797 [16] Boyd R W. Nonlinear Optics[M]. 3 ed, US: Academic Press, 2008. [17] Pask H M. The design and operation of solid-state Raman lasers [J]. Progress in Quantum Electronics, 2003, 27(1): 3-56. doi: 10.1016/S0079-6727(02)00017-4 [18] Piper J A, Pask H M. Crystalline raman lasers [J]. IEEE Journal of Selected Topics in Quantum Electronics, 2007, 13(3): 692-704. doi: 10.1109/JSTQE.2007.897175 [19] Supradeepa V R, Feng Y, Nicholson J W. Raman fiber lasers [J]. Journal of Optics, 2017, 19(2): 023001. [20] Bai Z, Williams R J, Jasbeer H, et al. Large brightness enhancement for quasi-continuous beams by diamond Raman laser conversion [J]. Optics Letters, 2018, 43(3): 563-566. doi: 10.1364/OL.43.000563 [21] Bai Zhenxu, Chen Hui, Li Yuqi, et al. Development of beam brightness enhancement based on diamond Raman conversion [J]. Infrared and Laser Engineering, 2021, 50(1): 20200098. (in Chinese) doi: 10.3788/IRLA20200098 [22] Mildren R P, Rabeau J R. Optical Engineering of Diamond [M]. Berlin: Wiley‐VCH Verlag GmbH & Co. KGaA, 2013. [23] Li Y, Ding J, Bai Z, et al. Diamond Raman laser: a promising high-beam-quality and low-thermal-effect laser [J]. High Power Laser Science and Engineering, 2021, 9: e35. [24] Bai Zhenxu, Yang Xuezong, Chen Hui, et al. Research progress of high-power diamond laser technology (Invited) [J]. Infrared and Laser Engineering, 2020, 49(12): 20201076. (in Chinese) doi: 10.3788/IRLA20201076 [25] Granados E, Spence D J, Mildren R P. Deep ultraviolet diamond Raman laser [J]. Optics Express, 2011, 19(11): 10857-10863. doi: 10.1364/OE.19.010857 [26] Yang X, Kitzler O, Spence D J, et al. Diamond sodium guide star laser [J]. Optics Letters, 2020, 45(7): 1898-1901. doi: 10.1364/OL.387879 [27] Li Y, Bai Z, Chen H, et al. Eye-safe diamond Raman laser [J]. Results in Physics, 2020, 16: 102853. doi: 10.1016/j.rinp.2019.102853 [28] Sabella A, Piper J A, Mildren R P. Diamond Raman laser with continuously tunable output from 3.38 to 3.80 μm [J]. Optics Letters, 2014, 39(13): 4037-4040. doi: 10.1364/OL.39.004037 [29] Antipov S, Sabella A, Williams R J, et al. 1.2 kW quasi-steady-state diamond Raman laser pumped by an M2= 15 beam [J]. Optics Letters, 2019, 44(10): 2506-2509. doi: 10.1364/OL.44.002506 [30] Yang X, Bai Z, Chen D, et al. Widely-tunable single-frequency diamond Raman laser [J]. Optics Express, 2021, 29(18): 29449-29457. doi: 10.1364/OE.435023 [31] Williams R J, Kitzler O, McKay A, et al. Investigating diamond Raman lasers at the 100 W level using quasi-continuous-wave pumping [J]. Optics Letters, 2014, 39(14): 4152-4155. doi: 10.1364/OL.39.004152 [32] Bai Z, Zhang Z, Wang K, et al. Comprehensive thermal analysis of diamond in a high-power Raman cavity based on FVM-FEM coupled method [J]. Nanomaterials, 2021, 11(6): 1572. doi: 10.3390/nano11061572 [33] Antipov S, Williams R J, Sabella A, et al. Analysis of a thermal lens in a diamond Raman laser operating at 1.1 kW output power [J]. Optics Express, 2020, 28(10): 15232-15239. doi: 10.1364/OE.388794 [34] Kitzler O, McKay A, Spence D J, et al. Modelling and optimization of continuous-wave external cavity Raman lasers [J]. Optics Express, 2015, 23: 8590-8602. doi: 10.1364/OE.23.008590 [35] Williams R J, Spence D J, Lux O, et al. High-power continuous-wave Raman frequency conversion from 1.06 µm to 1.49 µm in diamond [J]. Optics Express, 2017, 25(2): 749-757. doi: 10.1364/OE.25.000749 [36] Li M, Kitzler O, Mildren R P, et al. Modelling and characterisation of continuous wave resonantly pumped diamond Raman lasers [J]. Optics Express, 2021, 29(12): 18427-18436. doi: 10.1364/OE.426067 [37] Lux O, Sarang S, Kitzler O, et al. Intrinsically stable high-power single longitudinal mode laser using spatial hole burning free gain [J]. Optica, 2016, 3(8): 876-881. doi: 10.1364/OPTICA.3.000876 [38] Sheng Q, Li R, Lee A J, et al. A single-frequency intracavity Raman laser [J]. Optics Express, 2019, 27(6): 8540-8553. doi: 10.1364/OE.27.008540 [39] Casula R, Penttinen J P, Guina M, et al. Cascaded crystalline Raman lasers for extended wavelength coverage: Continuous-wave, third-Stokes operation [J]. Optica, 2018, 5(11): 1406-1413. doi: 10.1364/OPTICA.5.001406