-
该稳频系统结构图如图2所示。该系统主要由DFB-LD、氰化氢气室(HCN Cell)、光电探测器(PD 1和PD 2)、除法器(÷)、温控模块(TEC)、比例-积分(PI)模块等模拟电路组成。黑线为电路,红线为光路。DFB-LD出射的连续激光经90/10的耦合器后,90%的激光用于外部应用,或进入波长计(WM,WA-1100)实现波长实时检测。该波长计显示分辨率为1 pm。剩余10%的激光经过50/50的耦合器后分为两路,一路经过HCN气体吸收池被吸收谱线吸收后由光电探测器(PD 1)转化为电信号,另一路直接被另一个光电探测器(PD 2)接收并转化为电信号,两路电信号相除后得到的比值电压即为H13C14N分子吸收谱线的透过率,在一定范围内透过率与激光频率成正比。高精度参考电压(Vref)值为预先设定的H13C14N分子吸收谱线的透过率值,实验中设定为1548.958 nm处的透过率值。将该比值电压与高精度参考电压经比较器(COMP)进行比较后,由比例-积分电路处理并产生需要调整的反馈电压以驱动半导体激光器在波长1548.958 nm处工作。
实验证明,该系统可以将激光频率锁定到H13C14N分子吸收谱中的不同谱线的右侧,实现不同激光波长的锁定。
-
选用高功率窄线宽蝶形封装的DFB-LD(四川腾光科技),通过改变激光器内部温度和驱动电流来改变激光输出波长。其主要参数如表1所示。
表 1 DFB-LD主要参数
Table 1. main parameters of DFB laser diodes
Parameter Value Power/mW 40-60 Linewidth/ kHz 474 Central wavelength/nm 1549.100 Temperature tuning coefficient/nm·℃−1 0.100 Current tuning coefficient/nm·mA−1 0.010 Operation temperature/℃ −20-70 实验中保持激光器工作温度在24 ℃,控温精度为±0.03 ℃,通过闭环负反馈驱动电流使其输出波长锁定在1548.958 nm附近。
-
氰化氢分子的吸收谱线覆盖范围为1530~1565 nm,如图3所示[15]。
实验使用了气压约2.4 Torr 的H13C14N气室(Wavelength References),低气压是为了减少分子间的碰撞,从而压缩吸收谱线的线宽,增大吸收谱线两侧包络线的陡峭程度,提高稳频系统中频率误差信号的分辨率和灵敏度。经厂商测定,实验中锁定的P9线的半高全宽为5 pm,比标准气室(25 Torr, 1 Torr ≈133.322 Pa)的线宽(~15 pm)要窄很多,所以使得误差信号分辨率大大提高,灵敏度也大大增加,有利于提高稳频系统的精度。
-
为了尽量减少DFB-LD光源系统内部噪声对频率锁定效果的影响,实验中将负反馈电路与驱动电路集成在一块PCB电路板上,其部分原理图如图4所示。图2中经过吸收池的光入射到光电探测模块U7,未经过吸收池的光入射到光电探测模块U1。两者电压经过除法器U3相除后送入电压比较器U4的同相输入端。U12电压基准模块产生10 V、精度为0.1%的基准电压。通过调节电位器P1来改变电压比较器U4的反相输入电压,即参考电压,将该参考电压调到使激光器输出波长在1548.958 nm处附近,输出波长可通过波长计观测得到。U4的比较输出电压经PI电路U5的比例与积分作用后生成一直流误差信号,该误差信号经限流保护模块U6反向输出到激光二极管中作为驱动与补偿电流使用。其中限流保护模块U6的限制电流设为373.43 mA。
Frequency stabilization technology of HCN absorption in 1.5 μm DFB semiconductor laser
-
摘要: 在远距离相干测量系统中,分布反馈式半导体激光器(DFB-LD)以其直接高速调制特性、低成本、可批量生产等优势成为精密遥测系统的核心光源,因此对DFB-LD的窄线宽和短时频率稳定性提出了更高的要求。为了实现DFB-LD的频率稳定,通过边频锁定与光电反馈回路的方法将激光频率锁定在H13C14N气体吸收谱线1548.956 nm的一侧。将光电探测模块、后续误差信号生成与处理模块和激光器驱动模块集成在一块模拟电路板上,从而有效地降低了系统的噪声;使用除法器代替减法器来产生鉴频信号,大大提高了系统灵敏度和稳频精度;通过这两项技术的改进,将DFB-LD的秒级频率稳定度提高了两个数量级,从稳频前的秒级频率稳定度3.67 × 10−8提高到稳频后的秒级频率稳定度2.34×10−10。实验结果表明,该DFB-LD稳频方案具有高的稳频精度,且系统结构简单、体积小、可批量生产,适合于无人机机载应用场景,是远距离相干测量系统的理想光源。Abstract: A distributed feedback laser diode (DFB-LD) has become the key light source, in which a narrow linewidth and short-term frequency stability are highly demanded, in the far-distance coherent measurement systems due to the characteristics of high-speed direct modulation, low cost, and mass production. To improve frequency stability of a DFB-LD, a new method of frequency locking was proposed. The frequency of the DFB-LD was locked to an absorption line of H13C14N gas at the wavelength of 1548.956 nm by a photoelectric feedback loop based on the principle of side frequency locking. The photodetection module, subsequent error signal generation and processing module and the laser drive module were integrated on the same analog circuit board to minimize the noise of the system. The frequency discrimination signal was generated by using a divider instead of a subtractor to increase the system sensibility and precision of frequency stability significantly. The second-level frequency stability of the DFB-LD was improved by more than two orders of magnitude from 3.67×10−8 to 2.34×10−10 by using two techniques. The experimental results show that the frequency stability scheme of DFB-LD has high precision frequency stability, in addition to the features of simple structure, low cost, mass production and suitable for UAV applications. The DFB-LD is an ideal light source for far-distance coherent measurement.
-
Key words:
- frequency stabilization technology /
- DFB laser /
- analog circuit /
- photoelectric feedback
-
表 1 DFB-LD主要参数
Table 1. main parameters of DFB laser diodes
Parameter Value Power/mW 40-60 Linewidth/ kHz 474 Central wavelength/nm 1549.100 Temperature tuning coefficient/nm·℃−1 0.100 Current tuning coefficient/nm·mA−1 0.010 Operation temperature/℃ −20-70 -
[1] 陈良惠, 杨国文, 刘育衔. 半导体激光器研 究进展[J]. 中国激光, 2020, 47(05): 13-31. Chen L H, Yang G W, Liu Y X. Development of semiconductor lasers [J]. Chinese Journal of Lasers, 2020, 47(5): 0500001. (in Chinese) [2] 杨成奥, 张一, 尚金铭等. 2~4 μm中红外锑化物半导体激光器研究进展(特邀)[J]. 红外与激光工程, 2020, 49(12): 20201075. doi: 10.3788/IRLA20201075 Yang C A, Zhang Y, Shang J M, et.al. Research progress of 2-4 μm mid-infrared antimonide semiconductor lasers (Invited) [J]. Infrared and Laser Engineering, 2020, 49(12): 20201075. (in Chinese) doi: 10.3788/IRLA20201075 [3] 孙胜明, 范杰, 徐莉等. 锥形半导体激光器研究进展[J]. 中国光学, 2019, 12(1): 48-58. doi: 10.3788/co.20191201.0048 Sun S M, Fan J, Xu L, et al. Progress of tapered semiconductor diode lasers [J]. Chinese Optics, 2019, 12(1): 48-58. (in Chinese) doi: 10.3788/co.20191201.0048 [4] 韩顺利, 仵欣, 林强. 半导体激光器稳频技术[J]. 红外与激光工程, 2013, 42(5): 1189-1193. doi: 10.3969/j.issn.1007-2276.2013.05.015 Han Shunli, Wu Xin, Lin Qiang. Frequency stabilization technologies of semiconductor laser [J]. Infrared and Laser Engineering, 2013, 42(5): 1189-1193. (in Chinese) doi: 10.3969/j.issn.1007-2276.2013.05.015 [5] 花金平, 江毅. 可调谐外腔半导体激光器研究进展[J]. 半导体光电, 2021, 42(01): 11-19+56. Hua J P, Jiang Y. Recent progresses of tunable external cavity diode laser [J]. Semiconductor Optoelectronics, 2021, 42(1): 11-19, 56. (in Chinese) [6] 王杰, 高静, 杨保东等. 铷原子饱和吸收光谱与偏振光谱对 780 nm半导体激光器稳频的比较[J]. 中国光学, 2011, 4(3): 305-312. doi: 10.3969/j.issn.2095-1531.2011.03.014 Wang J, Gao J, Yang B D, et al. Comparison of frequency locking of 780 nm diode laser via rubidium saturated absorption and polarization spectroscopies [J]. Chinese Optics, 2011, 4(3): 305-312. (in Chinese) doi: 10.3969/j.issn.2095-1531.2011.03.014 [7] 丁振名. 基于飞秒光梳的激光稳频方法研究[D]. 中国计量大学, 2019. Ding Z M. Research on laser frequency stabilization method based on femtosecond optical comb[D]. Hangzhou: China Jiliang University, 2019. (in Chinese) [8] 吉经纬, 程鹤楠, 张镇, 刘亢亢, 项静峰, 任伟, 李琳, 吕德胜. 可搬运铷喷泉原子钟全自动激光稳频系统[J]. 光学学报, 2020, 40(22): 163-169. Ji J W, Cheng H N, Zhang Z, et al. Automatic laser frequency stabilization system for transportable 87Rb fountain clock [J]. Acta Optica Sinica, 2020, 40(22): 2214002. (in Chinese) [9] 陆丹, 杨秋露, 王皓, 贺一鸣, 齐合飞, 王欢, 赵玲娟, 王圩. 通信波段半导体分布反馈激光器[J]. 中国激光, 2020, 47(07): 11-29. Lu D, Yang Q L, Wang H, et al. Review of semiconductor distributed feedback lasers in the optical communication band [J]. Chinese Journal of Lasers, 2020, 47(7): 0701001. (in Chinese) [10] Jun Tsuboi, Takeshi Kuboki, Kazutoshi Kato. Wide-capture-range, high-precision wavelength stabilization within ±50 MHz for flexible-grid wavelength division multiplexing by photomixing technique [J]. Japanese Journal of Applied Physics, 2016, 55(8S3): 08RB10. doi: 10.7567/JJAP.55.08RB10 [11] Guo J J, Liu N H, Deng Y et al. Frequency stabilization of a semiconductor laser based on gas absorption cell[C]//2016 25th Wireless and Optical Communication Conference (WOCC), 2016: 1-3. [12] 梅教旭, 汪磊, 谈图, 刘锟, 王贵师, 高晓明. 基于二次谐波特性的DFB激光器稳频新方法研究[J]. 光谱学与光谱分析, 2019, 39(10): 2989-2992. Mei J X, Wang L, Tan T, et al. Research on new method of frequency stabilization of DFB laser based on second harmonic characteristics [J]. Spectroscopy and Spectral Analysis, 2019, 39(10): 2989-2992. (in Chinese) [13] 岱 钦, 宋文武, 王希军. 高频半导体激光器的驱动设计及稳定性分析[J]. 光学精密工程, 2006, 14(5): 745-748. doi: 10.3321/j.issn:1004-924X.2006.05.004 Dai Q, Song W W, Wang X J. Design and stability analysis of high frequency LD's driving circuit [J]. Optics and Precision Engineering, 2006, 14(5): 745-748. (in Chinese) doi: 10.3321/j.issn:1004-924X.2006.05.004 [14] 丛梦龙, 李 黎, 崔艳松, 等. 控制半导体激光器的高稳定度数字化驱动电源的设计[J]. 光学精密工程, 2010, 18(7): 1629-1636. Cong M L, Li L, Cui Y S, et al. Design of high stability digital control driving system for semiconductor laser [J]. Optics and Precision Engineering, 2010, 18(7): 1629-1636. (in Chinese) [15] Gilbert S, Swann W, Wang C. Hydrogen cyanide H13C14N absorption reference for 1530 nm to 1565 nm wavelength calibration—SRM 2519a[S/OL]. (2005-08-01)[2021-06-11]. https://doi.org/10.6028/NIST.SP.260-137.