-
采用改进型Ramp-Fire种子注入技术实现满足设计要求的频率稳定性指标。种子注入是调Q激光器输出单纵模的标准技术,其基本原理是将一束频率特性优良的种子光注入到高增益的振荡级内,由于种子光强度远远高于其他自发辐射,会优先在腔内形成振荡,率先建立起脉冲,耗尽反转粒子数,因而可以抑制其他模式起振,实现激光以窄线宽、单纵模输出。根据引言中的描述,从短时间的测试结果来看,列举的主动腔控技术,在实验室无振动环境下,均有能力将1064 nm单纵模Nd: YAG激光器的频率稳定性控制在1 MHz以内。然而由于1116 nm波长的增益较低,造成了脉冲建立时间的延长,这对主动腔控技术的抗干扰能力以及腔长出现抖动后的反馈修复能力提出了更高的要求,因此,笔者尝试采用改进型Ramp-Fire种子注入技术,即带偏压反馈的Ramp-Fire种子注入技术来实现振荡级输出1116 nm脉冲光频率的稳定。
该技术的工作原理示意图如图2所示。输入的种子光经二分之一波片、四分之一波片和后腔镜(HR2)进入振荡级,以布儒斯特角入射到偏振片(pol),竖直分量被反射出腔外,水平分量在腔内经过一个往返再次到达偏振片;由于四次经过四分之一波片,偏振态旋转一周仍保持水平分量不变,因此可透过偏振片,随后两次经过四分之一波片,使偏振态变为竖直方向,因而再次到达偏振片时会被反射出腔外。这两部分反射光在光电探测器(PD)靶面上发生干涉,Hender-son等人给出了出现干涉极大值的条件[18],即
图 2 带偏压反馈的主动腔控技术原理示意图
Figure 2. Schematic diagram of the active cavity control technique with bias feedback
$$\begin{split} & k\left(2nd+2{L}_{1}+2{L}_{2}+2{L}_{3}\right)+3\pi /2+{\phi }_{R}-{\phi }_{L}=2m\pi \text{,}\\ & m=0,1,\cdots \end{split} $$ (1) 式中:n和d为RTP晶体的折射率和厚度;L1、L2和L3分别为HR2到RTP晶体的光程、RTP晶体到pol的光程以及pol到HR1的光程;$ {\phi _R} $和$ {\phi _L} $是种子光分解的右旋和左旋圆偏振光偏振态。种子光与腔内振荡模式实现腔模匹配的条件是:
$$ k\left(2nd+2{L}_{1}+2{L}_{2}+2{L}_{3}\right)=2m\pi \text{,}m=0,1,\cdots $$ (2) 如需在干涉波形出现极大值时,实现种子光频率与振荡级纵模频率的匹配,可以通过联立上述公式(1)、(2)得出理论关系式,即:
$$ {\phi }_{R}-{\phi }_{L}=\pi /2\pm 2{m}^{\prime }\pi \text{,}{m}^{\prime }=0,1,\cdots $$ (3) 因此,可以通过旋转二分之一波片,使种子光在经过四分之一波片之前的线偏振方向与水平方向夹角在±45°以内;随后旋转四分之一波片,实现快轴与水平方向一致,可使种子光的偏振态满足公式(3)中的条件。
当入射光通过上述方法被调整到合适的偏振态后,在每个泵浦周期内,利用Ramp-Fire控制盒在压电陶瓷PZT2上施加斜坡扫描电压,对腔长进行微调,当PD探测到干涉波形的极大值时,种子光频率与振荡级纵模频率完成精准匹配,此时通过Q驱动(Q-driver)触发电光调Q开关(RTP),随即输出与种子光频率一致的单纵模调Q脉冲。
在实际过程中,由于PD响应速度有限以及信号处理过程需要时间,使得从检测出干涉极大值到Q开关被触发的过程存在一个固定的延时,因此每次打开Q开关时刻对应的腔长并不是腔模匹配的最佳位置。为了补偿这个延时,可以再次调整入射光的偏振态,使干涉极大值的位置相对于腔模匹配点有一个固定的相位提前量。由于振荡级的有效腔长约为0.4 m,纵模间隔约375 MHz,理论上能够实现种子注入的纵模间隔相移区间约10 MHz,因此补偿后剩余的偏差即使不进行处理,激光也能够实现稳定的单纵模输出。
利用压电陶瓷对腔长进行扫描,压电陶瓷的非线性效应将会导致固定延时内腔长的伸缩量不会完全一样,从而造成频率的抖动。1116 nm基频光较长的脉冲建立时间将使频率抖动的问题凸显。针对这个问题,增加了一个用于反馈控制的压电陶瓷PZT1,在每一次输出激光脉冲后对上一周期中的腔长漂移进行补偿。将输出镜(HR1)固定在PZT1上,根据出光时间的变化,调节加载在PZT1上的直流偏压,通过实时的反馈控制实现腔长的微调,使每一周期内扫描到干涉极大值的时间基本上是位于PZT2上加载斜坡电压的同一点。
-
图3(a)是搭建的振荡级光路,其中包含的主要硬件设备参数如表1所示。图3(b)是随后利用带偏压反馈Ramp-Fire种子注入技术扫描腔长时获得的相关波形,其中红色波形是激光二极管泵浦信号;黄色波形是加载扫描电压之后的干涉信号,考虑到PZT2在启动时的不稳定性,实验时通过门控电路选取干涉波形的第2个极大值作为调Q触发信号;蓝色波形是调Q脉冲信号。
图 3 (a)振荡级光路;(b)带偏压反馈Ramp-Fire种子注入扫描波形实拍结果
Figure 3. (a) Optical path of oscillator; (b) Photograph of scanning waveform for Ramp-Fire seed injection with bias feedback
表 1 主要硬件设备参数
Table 1. Parameters of main hardware devices
Device Parameter Value Seeder Laser Wavelength/nm 1116-1116.5 (tunable) Power/W >2 Linewidth/kHz <10 Frequency stability/kHz <200@10 h (rms) Power stability <1%@10 h (rms) Oscillator LD laser power/W 2500 Nd:YAG rod size/mm φ4×100 Cavity control box RF-m Oscilloscope Sample rate/GS·s-1 40 Bandwidth/GHz 16 Sample precision/bit 8 Detector Bandwidth/GHz 5 Rise time/ps 70 采用5 GHz带宽的铟镓砷自由空间探测器对激光脉冲波形进行监测。在种子注入之前或入射光偏振态未调整到最佳状态时,振荡级会以多纵模运转,不同纵模间的拍频干涉峰会叠加在脉冲波形上,因此,脉冲曲线就显得非常杂乱,毛刺较多,如图4(a)所示;当种子注入成功以后,入射的种子光频率与振荡级纵模频率完成了较好的匹配,脉冲波形十分光滑,可以证明激光是以单纵模输出,如图4(b)所示。
图 4 激光脉冲波形。(a)种子注入前;(b)种子注入成功后
Figure 4. Waveform of laser pulse. (a) Before seed injection; (b) After seed successful injection
为了验证输出脉冲激光频率的稳定性,利用拍频对脉冲激光频率与种子光频率的差进行监测。由于1116 nm种子光具备极高的频率稳定性(如表1所示),将其作为频率参考源,通过振荡级输出的脉冲光与该种子光进行拍频,所得频率差可间接反映出脉冲光的频率稳定性[11,19]。为了能够将拍频信号准确地记录下来,采用5 GHz带宽的铟镓砷探测器进行光电转换,并通过采样速率为40 GS/s的高速示波器对干涉波形进行采集。该示波器自带快速傅里叶变换功能,选取hamming窗作为窗函数后,可直接展示出拍频后的频谱信息,见图5(a),其中蓝色线表示拍频后的干涉波形,黄色线尖峰位置为两束光间的频率差,其抖动即为振荡级输出脉冲光的频率稳定性。图5(b)是10 min的频率稳定性的测量结果,平均值为1.164 GHz,表征了脉冲光与连续光间的频率差;频率峰峰值(pp)为3.84 MHz,频率rms值为543.24 kHz。因此,从振荡级输出1116 nm脉冲光的频率稳定性满足小于1 MHz的指标要求,验证了改进型Ramp-Fire种子注入技术能够满足文中频率稳定性的要求。根据蒙特卡洛方法,在图5所示的频率稳定性条件下,可仿真得到频率抖动和频率漂移引起的系统误差为0.51 K和0.61 m/s。
图 5 实验测试结果。(a)脉冲光与连续光拍频得到的干涉波形实拍图;(b)振荡级输出脉冲激光频率稳定性测量结果
Figure 5. The results of experimental test. (a) Pphotograph of the interference waveform obtained by beating frequency between pulsed laser and continuous laser; (b) Frequency stability measurement result of the pulsed laser from the oscillator
Frequency stability study of the laser source for iron resonance fluorescence Doppler lidar
-
摘要: 种子注入的372 nm稳频Nd: YAG激光器作为铁共振荧光多普勒激光雷达的激光光源,其性能将直接影响大气温度和径向风速的测量精度,属于研制难度较大但极其重要的关键技术。文中对激光光源的频率稳定性进行了仿真分析和实验研究。利用蒙特卡洛方法,仿真了振荡级输出1116 nm脉冲光的频率稳定性(均方根)应小于1 MHz;对改进型Ramp-Fire种子注入技术进行了详细介绍,并在振荡级光路中采用了该技术;通过激光拍频实验,测量得出1116 nm脉冲光在10 min内的频率稳定性的均方根为543.24 kHz,其结果满足指标要求,可将频率抖动和频率漂移引起的系统误差减少至0.51 K和0.61 m/s。文中所做工作为铁共振荧光多普勒激光雷达实现大气温度和径向风速的高精度测量提供了必要保障。
-
关键词:
- 铁共振荧光多普勒激光雷达 /
- 频率稳定性 /
- 蒙特卡洛方法 /
- 种子注入技术 /
- 激光拍频
Abstract:Objective Temperature and wind, as important environmental parameters, characterize the state of the atmosphere. In the region of upper mesosphere and lower thermosphere (UMLT, 75-115 km), due to a lack of effective tools, there is a relative lack of observation data. The resonance fluorescence lidar uses metal atoms in the UMLT region as a neutral tracer, and it is possible to measure temperature and wind by stimulating resonance fluorescence signals of the tracer. Among many in-situ and remote sensing measurement methods, the resonance fluorescence lidar, with its high spatial and temporal resolution, high accuracy and continuous observation, has become a powerful tool. Sodium resonance fluorescence lidar is widely used in the world, while iron resonance fluorescence Doppler lidar (Fe lidar) has the advantage of whole day measurement and is also an effective means to measure temperature and wind profile. The narrow-band, frequency-stabilized laser operating at 372 nm wavelength is one of core technology in the development of Fe lidar, especially for wind measurement. To yield pulsed laser with outstanding characteristic of frequency stabilization, a theoretical and experimental study of the frequency stability of laser sources are presented. Methods The technical solution for the generation of pulsed laser is to use Nd: YAG laser to generate pulsed laser at 1 116 nm wavelength, and then convert to 372 nm wavelength through subsequent second and third harmonic generation. Since the performance of oscillator determines the characteristics of the entire laser system, a theoretical and experimental study of the frequency stability mainly focuses on the oscillator. Frequency stability of pulsed laser at 1 116 nm wavelength from the oscillator is simulated to be less than 1 MHz (RMS) by using the Monte Carlo method (Fig.1). A detailed description for the modified Ramp-Fire method is conducted (Fig.2), and this technology is used in the optical path of the oscillator (Fig.3(a)). In the beat frequency experiments, because of the high-frequency stability of seeder laser, it can be used as a frequency reference, and the frequency difference can indirectly reflect the frequency stability of the pulsed laser by beating with the continuous-wave laser output from seeder laser. To record the beat frequency signal accurately, an indium gallium arsenic (InGaAs) detector with 5 GHz bandwidth is used for photoelectric conversion, and the interference waveform is acquired by a high-speed oscilloscope with a sampling rate of 40 GS/s. A fast Fourier transform algorithm is applied to the digitized beat frequency signal to obtain the spectrum information. Results and Discussions Frequency stability of 543.24 kHz root mean square over 10 min is obtained by using beat frequency experiments (Fig.5). It is verified that injection-seeded technique combined with the modified Ramp-Fire method can meet the requirements of frequency stability. According to the Monte Carlo method (Fig.1), the systematic error of temperature and wind measurement are estimated to be 0.51 K and 0.61 m/s. Conclusions The long-term frequency stability is a prerequisite for the high-precision measurement of temperature and wind. The frequency stability of laser source of Fe lidar is studied in this paper. By Monte Carlo method, the simulation analysis shows that the frequency stability for temperature and wind measurement should be less than 3 MHz at 372 nm wavelength, and thus, it should be less than 1 MHz at 1 116 nm wavelength. In the oscillator, injection-seeded technique combined with modified Ramp-Fire method is applied to maintain resonance with the seeder laser. The frequency stability of pulsed laser output from the oscillator over 10 min was measured to be 543.24 kHz by the beat frequency experiment. This work promotes the practical application of Fe lidar, and it also provides ideas for the development of other lidar systems with frequency stability. -
图 1 频率稳定性仿真结果和风速反演统计柱状图。(a)加入10 MHz的频率抖动;(b) 10 MHz频率抖动造成的风速测量系统误差;(c)加入3 MHz的频率抖动;(d) 3 MHz频率抖动造成的风速测量系统误差;(e)加入3 MHz的频率抖动和10 MHz的频率漂移;(f) 3 MHz频率抖动和10 MHz频率漂移综合影响造成的风速测量系统误差
Figure 1. Simulation results of frequency stability and the inversion statistics histogram of wind velocity. (a) Adding 10 MHz frequency jitter; (b) System errors in wind velocity measurement due to 10 MHz frequency jitter; (c) Adding 3 MHz frequency jitter; (d) System errors in wind velocity measurement due to 3 MHz frequency jitter; (e) Adding 3 MHz frequency jitter and 10 MHz frequency shift; (f) System errors in wind velocity measurement due to combined effects with 3 MHz frequency jitter and 10 MHz frequency shift
图 5 实验测试结果。(a)脉冲光与连续光拍频得到的干涉波形实拍图;(b)振荡级输出脉冲激光频率稳定性测量结果
Figure 5. The results of experimental test. (a) Pphotograph of the interference waveform obtained by beating frequency between pulsed laser and continuous laser; (b) Frequency stability measurement result of the pulsed laser from the oscillator
表 1 主要硬件设备参数
Table 1. Parameters of main hardware devices
Device Parameter Value Seeder Laser Wavelength/nm 1116-1116.5 (tunable) Power/W >2 Linewidth/kHz <10 Frequency stability/kHz <200@10 h (rms) Power stability <1%@10 h (rms) Oscillator LD laser power/W 2500 Nd:YAG rod size/mm φ4×100 Cavity control box RF-m Oscilloscope Sample rate/GS·s-1 40 Bandwidth/GHz 16 Sample precision/bit 8 Detector Bandwidth/GHz 5 Rise time/ps 70 -
[1] She C Y, Friedman J S. Atmospheric Lidar Fundamentals[M]. London: Cambridge University Press, 2022. [2] Chu X Z, Papen G C. Resonance Fluorescence Lidar for Measurements of the Middle and Upper Atmosphere[M]//Fujii T, Fukuchi T. Laser Remote Sensing. Boca Raton: CRC Press, 2005: 197-450. [3] 闫召爱, 胡雄, 郭文杰等. 临近空间多普勒激光雷达技术及其应用(特邀)[J]. 红外与激光工程, 2021, 50(3): 202101001-2021010010. doi: 10.3788/IRLA20210100 Yan Z A, Hu X, Guo W J, et al. Near space Doppler lidar techniques and applications (Invited) [J]. Infrared and Laser Engineering, 2021, 50(3): 20210100. (in Chinese) doi: 10.3788/IRLA20210100 [4] Chu X, Gardner C S, Li X, et al. Vertical transport of sensible heat and meteoric Na by the complete temporal spectrum of gravity waves in the MLT above McMurdo (77.84°S, 166.67°E), Antarctica [J]. Journal of Geophysical Research: Atmospheres, 2022, 127(16): e2021JD035728. doi: https://doi.org/10.1029/2021JD035728 [5] Li T, Fang X, Liu W, et al. Narrowband sodium lidar for the measurements of mesopause region temperature and wind [J]. Applied Optics, 2012, 51(22): 5401-5411. doi: 10.1364/AO.51.005401 [6] Krueger D A, She C Y, Yuan T. Retrieving mesopause temperature and line-of-sight wind from full-diurnal-cycle Na lidar observations [J]. Applied Optics, 2015, 54(32): 9469-9489. doi: 10.1364/AO.54.009469 [7] Xia Y, Du L F, Cheng X W, et al. Development of a solid-state sodium Doppler lidar using an all-fiber-coupled injection seeding unit for simultaneous temperature and wind measurements in the mesopause region [J]. Optics Express, 2017, 25(5): 5264-5278. doi: 10.1364/OE.25.005264 [8] Kawahara T D, Nozawa S, Saito N, et al. Sodium temperature/wind lidar based on laser-diode-pumped Nd: YAG lasers deployed at Tromsø, Norway (69.6 N, 19.2 E) [J]. Optics Express, 2017, 25(12): A491-A501. doi: 10.1364/OE.25.00A491 [9] Li C, Wu D C, Deng Q, et al. Simulation and optimization of Fe resonance fluorescence lidar performance for temperature-wind measurement [J]. Optics Express, 2022, 30(8): 13278-13293. [10] Kaifler B, Büdenbender C, Mahnke P, et al. Demonstration of an iron fluorescence lidar operating at 372 nm wavelength using a newly-developed Nd: YAG laser [J]. Optics Letters, 2017, 42(15): 2858-2861. doi: 10.1364/OL.42.002858 [11] Lemmerz C, Lux O, Reitebuch O, et al. Frequency and timing stability of an airborne injection-seeded Nd: YAG laser system for direct-detection wind lidar [J]. Applied Optics, 2017, 56(32): 9057-9068. doi: 10.1364/AO.56.009057 [12] Nicklaus K, Morasch V, Hoefer M, et al. Frequency stabilization of Q-switched Nd: YAG oscillators for airborne and spaceborne lidar systems[C]//Solid State Lasers XVI: Technology and Devices. SPIE, 2007, 6451: 387-398. [13] 周军. 种子注入的全固态单纵模激光器研究[D]. 北京: 中国科学院研究生院, 2007. Zhou J. Study of injection-seeded single frequency all solid-state laser[D]. Beijing: University of Chinese Academy of Sciences, 2007. (in Chinese) [14] Wang J, Zhu R, Lu T, et al. Conductively cooled single frequency Nd: YAG laser for remote sensing[C]//International Symposium on Photoelectronic Detection and Imaging 2011: Laser Sensing and Imaging; and Biological and Medical Applications of Photonics Sensing and Imaging. SPIE, 2011, 8192: 832-839. [15] Gao Y, Zhang J, Zang H, et al. Stable single-mode operation of injection-seeded Q-switched Nd: YAG laser by sine voltage modulation [J]. Chinese Optics Letters, 2016, 14(7): 071401. doi: 10.3788/COL201614.071401 [16] She C Y, Yu J R. Simultaneous three-frequency Na lidar measurements of radial wind and temperature in the mesopause region [J]. Geophysical Research Letters, 1994, 21(17): 1771-1774. doi: 10.1029/94GL01417 [17] Gardner C S, Vargas F A. Optimizing three-frequency Na, Fe, and He lidars for measurements of wind, temperature, and species density and the vertical fluxes of heat and constituents [J]. Applied Optics, 2014, 53(19): 4100-4116. doi: 10.1364/AO.53.004100 [18] Henderson S W, Yuen E H, Fry E S. Fast resonance-detection technique for single-frequency operation of injection-seeded Nd: YAG lasers [J]. Optics Letters, 1986, 11(11): 715-717. doi: 10.1364/OL.11.000715 [19] 谢建东, 严利平, 陈本永等. 可调谐激光器激光波长宽范围自动偏频锁定[J]. 光学精密工程, 2021, 29(2): 211-219. doi: 10.37188/OPE.20212902.0211 Xie J D, Yan L P, Chen B Y, et al. Automatic offset-frequency locking of external cavity diode laser in wide wavelength range [J]. Optics and Precision Engineering, 2021, 29(2): 211-219. (in Chinese) doi: 10.37188/OPE.20212902.0211