下载:
-
650~660 nm波段处于光电探测设备的高增益响应波段，作为作用光源可以在相对较低功率密度下对目标可见光探测设备进行削弱和破坏，对己方设备进行保护，在可见光光电对抗领域中具有重要地位。同时，该波段光源在激光医疗和激光打印等领域前景广阔。目前，固体激光器由于其功率输出高、光束质量好的特点是该波段主要光源，已被应用在光电对抗中。但固体激光器需半导体激光器泵浦获得1.3 μm波段激光后晶体倍频方能得到660 nm波段激光，能量利用率低[1-3]。目前报道的研究集中在提高功率和能量转化率方面。2007年，南京微结构国家实验室使用808 nm半导体激光器侧面泵浦Nd:YAG激光器得到1319 nm基波，使用PPSLT晶体倍频后获得659.5 nm输出光，输出功率2.4 W，泵浦光到输出光的总光光转换效率为0.5%[4]。2015年，中国科学院理化技术研究所使用LD侧面泵浦Nd:YLF激光器获得1321 nm激光，使用LBO晶体倍频后获得660.5 nm输出光，输出功率23 W，泵浦光到输出光的总光光转换效率为6.1%[5]。
半导体激光器直接电驱动输出650~660 nm波段激光，具有转换率高、体积小、质量轻、机动性高等优势，目前德国FBH研制了650 nm锥形半导体激光器，激光单元的快轴光束质量M2≈1.3，输出功率1 W[6]，其后又研制了660 nm锥形半导体激光器，激光单元快轴光束质量M2=1.5，输出功率1.5 W[7]；山东华光公司研制的660 nm半导体激光器激光单元输出功率1.2 W时，水平发散角为6°(95%)，垂直发散角为39°（95%）[8]；芬兰Modulight的商用650 nm半导体激光线阵水平发散角为4°(FWHM)，垂直发散角35° (FWHM)，输出功率10 W；德国Dilas的650 nm半导体激光线阵输出功率5 W，可耦合进入芯径为400 μm，NA为0.22的光纤[9]。
半导体激光单元的输出功率低，仅为瓦级，激光线阵输出功率高，可达十瓦级，但是由于在空间中的发光尺寸变大，光束质量变差，因此需要通过激光合束的方式提升光束质量。目前常规激光合束方法有偏振合束、空间合束、相干合束与光谱合束。偏振合束通过改变激光偏振态将两束偏振光合为一束，激光功率提高，光束质量不变；空间合束通过提高激光束的填充因子从而增加输出功率，中国科学院长春光学精密机械与物理研究所使用4个输出功率为5 W的半导体激光单元，利用空间合束和偏振合束获得了输出功率为15.22 W的激光光源，可耦合进入芯径为105 μm，NA为0.2的光纤[10-11]。相干合束通过控制激光单元的频率、振动方向和相位差，使激光单元发生相干干涉从而提升光束质量[12-13]，美国ACM和ORNL使用47个输出功率为1.8 W的半导体激光单元，利用相干合束技术获得了输出功率为12.8 W，发散角为1.5 mrad的激光光源[14]；光谱合束通过调谐波长将多个激光单元光束合为一束，光束质量与输出功率同时提高[15-24]，美国MIT使用板条耦合光波导激光器获得了输出功率为29 W，光束质量为M2=1.2的输出激光[15]。文中使用外腔光谱合束技术，基于内置10个90 μm条宽激光单元的单个红光半导体激光线阵，成功实现连续功率7.3 W，光谱合束效率为54.6%，电光转换效率23.09%的650 nm激光输出，光束质量接近固体激光器，为M2X=1.95，M2Y=11.11。
-
光谱合束结构原理图如图1所示，光谱合束方向为X方向。半导体激光线阵（Laser Diode Array, LDA）的前腔面镀减反膜，反射率R<1%，后腔面镀高反膜，反射率R>95%，提供了激光增益；LDA放置在变换透镜(Transform lens)的前焦点上；衍射光栅(Diffractive grating)在变换透镜的后焦点上；外腔镜（Output coupler）镀部分反射膜，放置在光栅衍射方向上，并与衍射输出激光的传输方向垂直。
图 1 光谱合束结构原理图
Figure 1. Schematic diagram of spectral beam combining structure
激光单元的输出激光经快轴准直镜（FAC）、光束变换器（BTS）和慢轴准直镜(SAC)整形后，以不同角度入射到衍射光栅上，并在光栅上重叠，将激光单元的位置信息转换为角度信息，经光栅衍射后垂直入射到外腔镜上，部分激光反馈回激光单元形成谐振，激光器后腔面与外腔镜构成外腔谐振。由于所有单元光束均满足相同光栅方程，且具有相同的衍射角，根据激光单元入射到光栅的不同角度使激光单元在不同波长产生谐振。激光单元的输出激光经外腔镜反馈后同轴出射，因此合束后的光束质量与激光单元的光束质量一致，合束功率为所有单元功率之和。
根据光栅公式：
$$ m\lambda_ i{\text{ = }}\varLambda \left( {\sin \theta_ i + \sin \theta_ d} \right) $$ (1) 式中：$ m $为光栅衍射级次，实验使用的透射光栅$ m $=−1；$\varLambda$为光栅周期；$ \theta_ i $为第i个激光单元的入射角；$ \theta_ d $为经光栅衍射后共同的衍射角；$ \lambda_ i $为合束后第i个激光单元被调谐的中心波长。当$ \theta_ i $=$ \theta _d $=$\theta _{\rm Littrow}$时，衍射效率最大。对光栅公式两边求导，色散公式为：
$$ \frac{{{\rm{d}}\theta }}{{{\rm{d}}\lambda }} = \frac{1}{{\varLambda \cdot \cos \theta _{\rm Littrow}}} $$ (2) 由几何关系可得光谱展宽$ \Delta \lambda $为：
$$ \Delta \lambda = \frac{l}{f} \cdot \frac{{{\rm{d}}\lambda }}{{{\rm{d}}\theta }} = \frac{l}{f} \cdot \varLambda \cdot \cos \theta _{\rm Littrow} $$ (3) 式中：$ l $为半导体激光线阵的光斑尺寸；$ f $为变换透镜的焦距。
实验使用的半导体激光器线阵由10个条宽为90 μm，间距为500 μm的激光单元组成，快轴方向发散角为60°，慢轴方向发散角为5.5°，对应激光单元快轴的光束质量为M2f-E=1.26，慢轴的光束质量为M2s-E=10.43，不考虑“smile”的影响，激光线阵快轴的光束质量为M2f-A=1.26，慢轴方向的光束质量为M2s-A=532；TE偏振，偏振度为86.4%；变换透镜焦距为300 mm，镀增透膜。实验使用的衍射光栅为透射光栅，光栅线数为1 908 线/mm，在650 nm处的TE偏振光−1级衍射效率约为90%，$\theta_{\rm Littrow}$为38.32°，理论光谱展宽为6.33 nm。
-
激光芯片采用微通道热沉（MCC）封装，在水冷温度为20 ℃、驱动电流为15 A时，波长锁定前的光谱和光场分布如图2所示，此时激光单元为自激射波长，各个激光单元在相近波长谐振，中心波长为649.3 nm，如图2（a）所示，经变换透镜传输后，激光单元所发出的自激射光束以不同角度入射到光栅，经光栅衍射后以不同的角度出射，因此，远场在X方向呈现出相互分离的光场分布，如图2（b）所示，每个光斑对应一个激光单元。
图 2 光谱合束前的激光特性。（a）光谱合束前的自由激射光谱；（b）光谱合束前的光场分布
Figure 2. Laser characteristics before spectrum combining. (a) Free lasing spectrum before wavelength locking; (b) Optical field distribution before wavelength locking
光谱合束后的光谱和光场分布如图3所示，与图2存在明显区别。受外腔反馈和光栅衍射作用，每个激光单元被锁定至不同波长，如图3（a）所示，整体谱宽为6.45 nm，输出光谱由10个明显的尖峰组成，分别对应10个激光单元被选择的波长，未发现有其他的杂峰存在，说明每个激光单元均被完全锁定。另外，考虑输出光谱呈现中间低两边高的现象，中间几个合束单元的谐振峰值明显低于两侧合束单元，说明中间几个单元的输出功率低于两侧，这可能是由于激光线阵封装存在“smile”效应，激光单元所发出的激光光束经微透镜后指向不一致，使得部分激光单元不能与外腔镜形成高效谐振，输出功率偏低。实测光谱宽度为6.45 nm，比理论值大0.12 nm，主要是由于光栅入射角度偏离理论Littrow角所致。激光单元经外腔反馈光谱锁定的光场分布如图3（b）所示，此时，激光单元发出的光束在X方向重叠，不能区分光斑，说明成功实现了光谱合束。
图 3 光谱合束后的激光特性。（a）光谱合束后的激射光谱；（b）光谱合束后的光场分布
Figure 3. Laser characteristics after spectral combination. (a) Laser spectrum after spectral combination; (b) Light field distribution after spectrum combining
图4为功率曲线和电光转换效率（E-O转化率）曲线，实验水冷温度为20 ℃，驱动电流在15 A时，自由运转时所测得的输出功率为13.88 W，电光转化效率为42.25%，光谱合束后所测得的输出功率为7.3 W，电光转换效率为23.09%，光谱合束相对于自由运转的转换效率为54.6%。引起损耗的因素有：1）激光线阵的线偏振度低，仅86.4%可以实现高效率衍射，另外13.6%由于偏振不匹配被损耗，直接影响输出功率；2）光栅的衍射效率低，在650 nm波长处仅有90%，导致约10%的功率损失；3）激光线阵的“smile”效应造成部分发光单元不能高效谐振，输出功率下降；4）为实现有效光谱锁定，使用反射率为18%的高反馈外腔镜，对光谱合束效率产生较大的影响；5） FAC、BTS、SAC、半波片等光学元件的增透膜不是100%增透，存在一定损耗。为进一步提升效率，可更换高性能的激光器和衍射光栅，减少功率损耗；优化封装工艺，减少“smile”效应带来的影响；更换散热更好的水冷剂，减少热效应带来的影响。
图 4 自由运转和光谱合束的功率曲线和E-O转换效率曲线
Figure 4. Power curve and E-O conversion efficiency curve of free running and spectral combining
测量驱动电流为15 A时输出光束的光束质量，使用Thorlabs型号为M2M的光束质量分析仪。测量开始前，使用衰减片将光束衰减至毫瓦级，过滤背景光，将合束激光垂直入射到光束质量分析仪中，软件通过控制移动平台测量束腰前后的光斑尺寸，拟合曲线如图5所示。光谱合束后的光束质量分别为M2X=1.95，M2Y=11.11，接近单个激光单元的光束质量，比激光线阵的光束质量提升了近47倍，说明实现了高光束质量激光输出。
图 5 光束直径的线性拟合
Figure 5. Linear fitting of beam diameter
-
为了提升650 nm波段半导体激光源的光束质量，采用光谱合束技术，实现输出功率为7.3 W，光束质量为M2X=1.95，M2Y=11.11的650 nm波段激光，电光转换效率为23.4%，光谱合束转化率为54.6%，相对于激光线阵光束质量提升了近47倍，与单个激光单元的光束质量相当，下一步通过提高激光单元数量并结合偏振合束可进一步提升合束功率，为实现高功率、高光束质量、高转换效率的650 nm波段激光提供有效途径。
650 nm semiconductor laser based on external cavity spectral combination
-
摘要: 高功率650~660 nm波段激光器在可见光光电对抗领域具有重要作用，目前该波段光源由固体激光器通过半导体激光器泵浦并倍频输出，输出功率高、光束质量近衍射极限，但转换效率低。半导体激光器的转换效率高，但输出功率低，需要通过增加激光单元的方法提升功率，并通过激光合束的方式提升光束质量。文中提出外腔光谱合束的650 nm波段半导体激光器结构，通过实验验证可实现连续功率为7.3 W、光谱线宽为6.45 nm、电光转换效率为23.4%的650 nm波段激光输出，光束质量为M2X=1.95，M2Y=11.11，接近固体激光器，未来通过增加合束的激光单元数量并结合偏振合束可以获得更高功率的650 nm波段激光。Abstract:
Objective High-power 650-660 nm laser can weaken and destroy the target visible light detection equipment under relatively low-power density, and protect its own equipment. It has an important position in the field of visible optoelectronics confrontation. At present, due to its high-power output and near diffraction limited beam quality, solid-state laser is the main light source of this band and has been applied to photoelectric confrontation. However, pumping solid-state laser by semiconductor-laser and then frequency doubling to output 650-660 nm laser, the conversion efficiency is low. Methods This article builds an outer cavity spectrum beam system. The output end of the laser resonator is plated with anti-membrane, the reflectance is <1%. Output laser is plasticized by the fast axis collimator, beam transformation system and slow axis collimator. Various angles of the output laser are integrated into this part of their diffraction grating, and overlap on the refracting ring. After diffraction through the grille, the laser is perpendicular to the external cavity feedback through the outlet. Some laser beam feedback backs to the laser unit to form resonance. Because all light beams conform to the same grating equation and have the same diffraction angle, the diffraction grating selects different wavelengths for laser units based on the angle of incident into the grating. The laser unit's output laser is located in the same axis after the laser passes through the feedback mirror of the external cavity. Results and Discussions The beams overlap in the x direction, which cannot be distinguished by light spots, indicating that the spectral beam is successfully achieved. Each laser unit is locked to different wavelengths. The spectral line width is 6.45 nm. Corresponding to the wavelength of the 10 laser units, no other peaks are found, indicating that each laser unit is completely locked (Fig.3). The output power measured after the spectral combining is 7.3 W, the electro-to-optical conversion efficiency is 23.09% (Fig.4). The beam quality is M2X= 1.95, M2Y= 11.11. The beam quality of a single laser unit is nearly 47 times compared to the laser beam quality (Fig.5). Conclusions The external cavity spectral combining technology was used to improve the beam quality of the 650 nm semiconductor laser sources, the output of 650 nm laser with CW power of 7.3 W. The spectral line width is 6.45 nm, and the electro-to-optical conversion efficiency is 23.4%. The beam quality was M2X = 1.95, and M2Y = 11.11. Compared with the laser beam quality, the quality of the beam is nearly 47 times, similar to the beam quality of a single laser unit. In the future, higher-power 650 nm laser can be obtained by increasing the number of combined laser units and polarization-combination, which provides effective ways to achieve high-power, high-beam quality, and high-conversion efficiency. -
Key words:
- semiconductor laser /
- laser light source /
- spectral beam combination /
- high-beam quality
-
图 2 光谱合束前的激光特性。（a）光谱合束前的自由激射光谱；（b）光谱合束前的光场分布
Figure 2. Laser characteristics before spectrum combining. (a) Free lasing spectrum before wavelength locking; (b) Optical field distribution before wavelength locking
图 3 光谱合束后的激光特性。（a）光谱合束后的激射光谱；（b）光谱合束后的光场分布
Figure 3. Laser characteristics after spectral combination. (a) Laser spectrum after spectral combination; (b) Light field distribution after spectrum combining
图 4 自由运转和光谱合束的功率曲线和E-O转换效率曲线
Figure 4. Power curve and E-O conversion efficiency curve of free running and spectral combining
-
[1] Dolotov S M, Koldunov M F, Kravchenko Y V, et al. An efficient solid-state laser based on a nanoporous glass-polymer composite doped with phenalemine dyes emitting in the 600-660-nm region [J]. Quantum Electronics, 2002, 32(8): 669-674. doi: 10.1070/QE2002v032n08ABEH002268 [2] Zhu P F, Li B, Liu W Q, et al. All-solid-state continuous-wave frequency doubling Nd: YAG/LBO laser with 8.2 W output power at 660 nm [J]. Optics and Spectroscopy, 2012, 113: 560-564. doi: 10.1134/S0030400X12110082 [3] Bian Q, Bo Y, Zuo J W, et al. 1 338 nm single wavelength operation of acousto-optic Q-switched Nd: YAG laser [J]. IEEE Photonics Technology Letters, 2022, 34(11): 567-570. doi: 10.1109/LPT.2022.3173169 [4] Hu X P, Wang X, Yan Z, et al. Generation of red light at 660 nm by frequency doubling a Nd: YAG laser with periodically-poled stoichiometric LiTaO3 [J]. Applied Physics B, 2007, 86: 265-268. doi: 10.1007/s00340-006-2373-0 [5] Wang Z, Wang B, Chen M, et al. High-power 660.5 nm red laser from diode-side-pumped intracavity frequency-doubled Nd: YLF laser [J]. Laser Physics Letters, 2015, 12(12): 125801. doi: 10.1088/1612-2011/12/12/125801 [6] Adamiec P, Sumpf B, Rüdiger I, et al. Tapered lasers emitting at 650 nm with 1 W output power with nearly diffraction-limited beam quality [J]. Optics Letters, 2009, 34(16): 2456. doi: 10.1364/OL.34.002456 [7] Kaspari C, Blume G, Feise D, et al. Optimisation of 660 nm high-power tapered diode lasers [J]. Optoelectronics Iet, 2011, 5(3): 121-127. doi: 10.1049/iet-opt.2010.0034 [8] 朱振, 张新, 肖成峰, 等. 高可靠性瓦级660 nm半导体激光器研制[J]. 中国激光, 2018, 45(5): 0501002. . doi: 10.3788/CJL201845.0501002 Zhu Zhen, Zhang Xin, Xiao Chengfeng, et al. Fabrication of highly reliable watt-level 660 nm semiconductor lasers [J]. Chinese Journal of Lasers, 2018, 45(5): 0501002. (in Chinese) doi: 10.3788/CJL201845.0501002 [9] Köhler B, Kissel H, Flament M, et al. High-power diode laser modules from 410 nm to 2200 nm[C]//Proceedings of SPIE, 2010, 7583: 134-146. [10] 王立军, 彭航宇, 张俊, 等. 高功率高亮度半导体激光器合束进展[J]. 红外与激光工程, 2017, 46(4): 401001-0401001(10). doi: 10.3788/IRLA201746.0401001 Wang Lijun, Peng Hangyu, Zhang Jun, et al. Development of beam combining of high power high brightness diode lasers [J]. Infrared and Laser Engineering, 2017, 46(4): 0401001. (in Chinese) doi: 10.3788/IRLA201746.0401001 [11] 朱洪波, 郝明明, 刘云, 等. 808 nm高亮度半导体激光器光纤耦合器件[J]. 光学精密工程, 2012, 20(8): 1684-1690. doi: 10.3788/OPE.20122008.1684 Zhu Hongbo, Hao Mingming, Liu Yun, et al. 808 nm high brightness module of fiber coupled diode laser [J]. Optics and Precision Engineering, 2012, 20(8): 1684-1690. (in Chinese) doi: 10.3788/OPE.20122008.1684 [12] Fsaifes I, Daniault L, Bellanger S, et al. Coherent beam combining of 61 femtosecond fiber amplifiers [J]. Optics Express, 2020, 28(14): 20152-20161. doi: 10.1364/OE.394031 [13] Müller M, Klenke A, Steinkopff A, et al. 3.5 kW coherently combined ultrafast fiber laser [J]. Optics Letters, 2018, 43(24): 6037-6040. doi: 10.1364/OL.43.006037 [14] Liu B, Liu Y, Braiman Y. Coherent beam combining of high power broad-area laser diode array with a closed-V-shape external Talbot cavity [J]. Optics Express, 2010, 18(7): 7361-7368. doi: 10.1364/OE.18.007361 [15] Huang R K, Chann B, Missaggia L J, et al. High-brightness wavelength beam combined semiconductor laser diode arrays [J]. IEEE Photon Technology Letters, 2007, 19(4): 209-211. doi: 10.1109/LPT.2006.890717 [16] Fan T Y, Sanchez A, Daneu V, et al. Laser beam combining for power and brightness scaling[C]//2000 IEEE Aerospace Con-ference. Proceedings (Cat. No. 00 TH8484). IEEE, 2000, 3: 49-54. [17] Zhu Z, Jiang M, Cheng S, et al. Narrow line width operation of a spectral beam combined diode laser bar [J]. Applied Optics, 2016, 55(12): 3294-3296. doi: 10.1364/AO.55.003294 [18] Vijayakumar D, Jensen O B, Ostendorf R, et al. Spectral beam combining of a 980 nm tapered diode laser bar [J]. Optics Express, 2010, 18(2): 893-898. doi: 10.1364/OE.18.000893 [19] Zink C, Werner N, Jechow A, et al. Multi- wavelength operation of a single broad area diode laser by spectral beam combining [J]. IEEE Photonics Technology Letters, 2014, 26(3): 253-256. doi: 10.1109/LPT.2013.2291963 [20] Zhang Jun, Peng Hangyu, Fu Xihong, et al. CW 50 W/M2=10.9 diode laser source by spectral beam combining based on a transmission grating [J]. Optics Express, 2013, 21(3): 3627-3632. doi: 10.1364/OE.21.003627 [21] 彭航宇, 张俊, 付喜宏, 等. 高效外腔光谱合束半导体激光器阵列[J]. 中国激光, 2013, 40(7): 0702015. doi: 10.3788/CJL201340.0702015 Peng Hangyu, Zhang Jun, Fu Xihong, et al. High-efficiency external cavity spectral-beam-combined diode laser array [J]. Chinese Journal of Lasers, 2013, 40(7): 0702015. (in Chinese) doi: 10.3788/CJL201340.0702015 [22] 张俊, 彭航宇, 付喜宏, 等. 基于光谱合束的800 nm高亮度半导体激光源[J]. 中国激光, 2020, 47(7): 0701021. doi: 10.3788/CJL202047.0701021 Zhang Jun, Peng Hangyu, Fu Xihong, et al. High-brightness 800-nm semiconductor laser source based on spectral beam combining [J]. Chinese Journal of Lasers, 2020, 47(7): 0701021. (in Chinese) doi: 10.3788/CJL202047.0701021 [23] Haas M, Rauch S, Nagel S, et al. Beam quality deterioration in dense wavelength beam-combined broad-area diode lasers [J]. IEEE Journal of Quantum Electronics, 2017, 53(3): 1-11. doi: 10.1109/JQE.2017.2686358 [24] 孙舒娟, 谭昊, 孟慧成, 等. 高亮度半导体激光器无输出耦合镜光栅外腔光谱合束[J]. 红外与激光工程, 2019, 48(3): 306006-0306006(8). doi: 10.3788/IRLA201948.0306006 Sun Shujuan, Tan Hao, Meng Huicheng, et al. High brightness diode laser by coupler free grating external cavity spectral beam combining [J]. Infrared and Laser Engineering, 2019, 48(3): 0306006. (in Chinese) doi: 10.3788/IRLA201948.0306006 -
点击查看大图
图(5)
计量
- 文章访问数: 140
- HTML全文浏览量: 31
- PDF下载量: 36
- 被引次数: 0
