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参考文献 1
Korb C L, Weng C Y. Differential absorption lidar technique for measurement of the atmospheric pressure profile [J]. Appl. Opt. 1983, 22(23), 3759-3770.
参考文献 2
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Riris H, Rodriguez M , Allan G R, et al. Pulsed airborne lidar measurements of atmospheric optical depth using the oxygen A-band at 765 nm [J]. Appl. Opt. 2013, 52(25): 6369-6382.
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参考文献 7
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参考文献 11
Wulfmeyer V. Ground-based differential absorption lidar for water-vapor and temperature profiling: development and specifications of a high-performance laser transmitter [J].Applied Optics,1998,37(18): 3804-3824
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Jens Bo¨senberg. Ground-based differential absorption lidar for water-vapor and temperature profiling: methodology [J]. Applied Optics, 1998, 37(18): 3845-3860
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Flamant C N, Schwemmer G K, Korb C L. Pressure measurements using an airborne differential absorption lidar. Part I: Analysis of the systematic error sources [J]. 1999, Journal of Atmospheric and Oceanic Technology, 16: 561-574
参考文献 14
Späth F, Metzendorf S, Behrendt A, et al. Online/offline injection seeding system with high frequency-stability and low crosstalk for water vapor DIAL [J]. Optics Communications. 2013,309:37-43
参考文献 15
Mahnke P, Klingenberg H H, Fix A, et al. Dependency of injection seeding and spectral purity of a single resonant KTP optical parametric oscillator on the phase matching condition[J]. Appl. Phys. B, 2007, 89: 1-7.
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参考文献 18
He Y, Baxter G W, Orr B J. Locking the cavity of a pulsed periodically poled lithium niobate optical parametric oscillator to the wavelength of a continuous-wave injection seeder by an “intensity-dip” method [J]. Rev. Sci. Instrum. 1999, 70(8): 3203-3213.
参考文献 19
Kobtsev S, Kandrushin S, Potekhin A. Long-term frequency stabilization of a continuous-wave tunable laser with the help of a precision wavelength meter [J]. Appl. Opt. 2007, 46(23): 5840-5843
参考文献 20
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(3): 9057-9068.
参考文献 21
Wulfmeyer V, B¨osenberg J, Single-mode operation of an injection-seeded alexandrite ring laser for application in water-vapor and temperature differential absorption lidar[J]. Optics Letters.1996,21(15): 1150-1152
参考文献 22
Kharchenko O V, Matvienko G G, Grishin A I, et al. Possibility of a meteorological lidar for measurement of basic meteorological parameters of the atmosphere [C]. 2005, Proceedings of SPIE , 5832, doi: 10.1117/12.619840
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Bruneau D, des Lions T A, Quaglia P, et al. Injection-seeded pulsed alexandrite laser for differential absorption lidar application [J].Appl. Opt. 1994, 33(18): 3941–3950.
目录 contents

    摘要

    介绍了一种氧气A带差分吸收激光雷达发射机,试图用于大气压力探测实验。该激光发射机是基于种子注入的光参量振荡器和光参量放大器的结构。作为从振荡器,采用一个环形腔KTP光参量振荡器。作为注入种子的主振荡器,即一个连续波外腔调谐二极管激光器。该连续波外腔调谐二极管激光器,由高精度的波长计构成的一个PID(Proportional-Integral-Derivative)伺服控制环,稳定其工作波长。向光参量振荡器的谐振腔注入连续波的种子激光,通过“Ramp-Hold-Fire”技术,锁定OPO(Optical Parametric Oscillator) 谐振腔的腔长。经过证实,这个激光发射机具有高的光频率稳定性(30 MHz/rms)、窄的线宽(傅立叶转换限)、高的脉冲能量(≥45 mJ)等性能,能够在工作期间得以保持。发射机系统以单纵模式工作,使得差分吸收激光雷达对后向散射光信号的窄带探测成为可能。因而此类系统具有精确探测大气压力的发展潜力。

    Abstract

    Here is a transmitter for an oxygen-band differential absorption lidar that is tried for atmospheric pressure detection experiment. The laser transmitter is based on the structure of a seed-injected optical parametric oscillator and an optical parametric amplifier. As a slave oscillator, a ring cavity KTP optical parametric oscillator is used. As the master oscillator, is a continuous wave external cavity diode laser. Operating wavelength of the continuous wave external cavity diode laser was stabilized, by a PID (Proportional-Integral-Derivative) servo control loop composed of a high-precision wavelength meter. A continuous wave seed laser is injected into the optical parametric oscillator cavity, and the cavity length of the OPO (Optical Parametric Oscillator) resonator is locked by the “Ramp-Hold-Fire” technique. This laser transmitter has been proven to have the following properties: high optical frequency stability (30 MHz/rms), narrow linewidth (Fourier transform limited), and high pulse energy (≥45 mJ),which can be maintained during operation. The transmitter system operates with a single longitudinal mode, making it possible for differential absorption lidar to detect narrowband backscattering signals. Such systems therefore have the developing potential to accurately measure atmospheric pressure.

  • 引言

    大气压力参数,对于大气动力学研究和数值天气预报十分重要。如低气压、高气压、低压槽、高压脊、等压线等信息被引入大气数值模式;大气压力场,对于确定热带海洋气旋和台风的中心位置及其移动轨迹,对于改善中纬度地区的天气预报都是需要的。

    国际上在轨运行可以遥感大气压力的仪器,由于太阳天顶角的变化以及仪器光谱分辨率的限制,其地球表面压力探测精度还不能满足气象预报的需要,况且晚上没有太阳光的地表反射,亦不能工作。全球大气研究计划(Global Atmospheric Research Program)要求对地球表面大气压力遥感测量精度达到0.1%,对流层大气压力垂直廓线的探测精度达到0.3%。

    Laboratory for Atmospheres, NASA戈达德空间飞行中心的C. Lurence Korb 等专家,1983年就建议:基于氧气A带(图1)相邻两谱线之间凹槽处的吸收系数随大气压力而变化的思想,用差分吸收激光雷达探测大气压[1];Schwemmer等人于1987年再一次阐述了差分吸收激光雷达系统探测大气压力的原[2],并采用闪光灯泵浦翠宝石(Alexandrite)激光器发射位于波数13150 cm-1附近两种波长的激光脉冲,以差分吸收激光雷达探测了大气压力。1989年C. Laurence Korb[3]等人报道在美国东海岸进行了地基或机载差分吸收激光雷达的探测实验,他们宣称获得垂直分辨率30 m、水平分辨率2 km、大气压力精度2 mbr的实验结果 [4];美国在ASCENDS(Active Sensing of CO2 Emission over Nights, Days, and Seasons)计划中,为了精确测量CO2在干空气中的混合比,也特别关注大气压力的测量。NASA戈达德空间飞行中心的Mark Stephen,Michael Krainak,Haris Riris等人和Sigma Space Corporation的Graham R. Allan研究团队连续报道了他们于2007-2013[5,6,7]间以飞机为平台,激光发射机向下持续发出764.7 nm附近的N个波长的脉冲串,接收机接收地面对这些激光脉冲串的反射回波,多波长的脉冲串回波信号积累、绘制出764.7 nm附近氧气吸收谱曲线的凹槽片段,可由此透过率曲线计算出氧气的光学厚度,反演地面大气压力。发射机的核心是前置1529~1530 nm分布反馈(DFB)半导体激光器,和后置C波段掺铒光纤放大器(EDFA),再经过准相位匹配周期性极化的LiNbO3非线性光学晶体,输出倍频后的764.5~764.9 nm激光脉冲。发射机的优点是线宽很窄、脉冲重复(声光斩波)频率高,致命的弱点是单脉冲能量过小(<10 µJ)。

    图1
                            在标准大气压和两种温度296 k(红)276k(绿色)下,通过100 m长度大气,氧气A-吸收带透射光谱.

    图1 在标准大气压和两种温度296 k(红)276k(绿色)下,通过100 m长度大气,氧气A-吸收带透射光谱.

    Fig.1 Oxygen A-band transmission spectrum through 100 m of atmosphere at standard pressure and at two temperatures: 296 K (red) and 276 K (green).

    发展高性能的发射机一直是差分吸收激光雷达的一大挑战,并不是有较多成熟激光材料可以在A波段产生有效的、高功率的光辐射。最近几年,光参量振荡器(OPO)和光参量放大器(OPA),在效率、光谱质量和光束质量方面呈现了显著的进[]。OPO光源分别用半导体激光器作为种子,注入锁定OPO谐振腔,可以获得合适的波长、稳定的脉冲激光。两个波长相近的OPO/OPA光源各自担任online探测光源和offline参考光源。第3节,描述了氧气A吸收带、KTP晶体的光参量振荡器和光参量放大器。第4节讨论了种子注入锁定技术。

  • 1 激光雷达对大气压力探测的方法及其对发射机的性能要求

    大气压力可以从获得氧气A吸收带(758~770 nm)的大气光学厚度推演得到。“on”探测波长或 “off” 参考波长的光学厚度 (OD, Optical Depth) 是某一段大气透过率的自然对数,λonλoff 对应的垂直方向OD值之差 DOD = ODon - ODoff 可以看作大气压力差的函[1]。选择合适氧气A带“on”波长(765 nm附近),使得此波长的大气透过率随温度变化不敏感,同时该波长大气透过率对大气压力变化敏感,符合这一要求的光波长,取自两相邻的吸收谱线之间,不会选择吸收线的峰值波长。PonPoff是被探测到的online和offline波长的光功率,τ(z)是两种波长透过率的比值,式(1)当中有2,表示往返双光程。式(2)最右边是激光雷达可以测量的数据,式(2)最左边是跟大气压力相关的参数。

    τz=exp-2ODon-ODoff
    (1)
    ODon-ODoff=-12lnτz=-12lnPonzPoffzEoffEon .
    (2)

    德国马克斯-普朗克研究所(The Max-Planck-Institut fu¨r Meteorologie)的两位科学家Jens Bo¨ senberg和Volker Wulfmeyer,在1998年提出了利用氧气A吸收带差分吸收激光雷达探测大气温度廓线[11,12],对其发射机的技术要求做出了详细评估,认为激光线宽<130 MHz,光频稳定性(1σ)±75 MHz, 光谱纯度>99.9%;因为差分吸收激光雷达探测大气温度廓线和差分吸收激光雷达探测大气压力原理十分相似,同样都利用氧气A吸收带,所以这两位科学家的评估结果值得参考;如果大气压力探测的波长选自于相邻谱线的中间段,不象大气温度探测的波长选择谱线的峰值波长,那么服务于大气压力探测的激光线宽可以比服务于大气温度探测的激光线宽的要求更宽松一些,而温度和压力对光频稳定性和光谱纯度则要求相当。1999年,Cyrille N. Flamant,Geary K. Schwemmer And C. Laurence Korb在他们报[13]中提出,A吸收带差分吸收激光雷达探测大气压力,要求激光的线宽至少小于0.05 cm-1

  • 2 光参量振荡器的种子源

    发射机中注入种子的结构(图2(a))与Florian Späth et al[14]在高功率脉冲钛宝石(Ti:sapphire)激光器中采用的种子注入的结构是相似的。作为稳定的注入种子光源,在这个结构里采用了Littman/Metcalf结构的外腔调谐二极管激光器(图2(b))。这两个ECDLs(Model TEC-520-0770-060,Sacher Lasertechnik GmbH)激光器,设计的波长范围位于755~780 nm区间,能提供30 mW左右的连续波、单纵模激光。该激光器也装备了压电换能器PZT用于调节反射器(图2(b)中的“Prism”)的角度。用了这个部件,激光器在被需要的光频率附近就具有30 GHz的自由跳模范围。此二极管激光器输出功率被保偏耦合进入99/1%单模交叉耦合光纤(XCOFs)。作为波长的监视端,交叉耦合光纤的1%端耦合进入光复用器 (OMP) (Model 4×1 optical switch)和其后的波长计(Model Angstrom WS8-2, HighFinesse,线宽测量精度100MHz,光频的测量精度2 MHz)。在外腔调谐二极管激光器稳定的过程中(在几秒时间内),复用器在铯Cs-852基准光源、激光器1、激光器2之间循环切换。我们的测量表明,经过光频率稳定的Cs-852基准光源(DFB801-852 Lasers, UniQuanta)的标定,波长计在30分钟内的稳定性记录优于±5 MHz。复用器的第二、第三个输入,经过Cs基准光源标定后的波长计的读出。交叉耦合器的99%端口耦合进入各自的光参量振荡器OPO1和OPO2。

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    图2 (a) 注入种子的建立。XCOF:交叉耦合光纤;OMP:光学复用器;DAQ:数据采集器;PFC:保偏光纤耦合器;(b) 外腔调谐二极管激光器的图片

    Fig. 2 (a) Injection-seeder setup. XCOF: cross-coupled optical fibers; OMP: optical multiplexer; DAQ: data acquisition; PFC: polarization maintenance fiber couplers. (b) Picture of external cavity diode laser

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    图3 852 nm(铯)高稳定度基准光源和高分辨率高精度波长计

    Fig. 3 Reference laser source 852 nm (Cs) with high stability and wavelength meter with high resolution and precision

    调谐过程分三个步骤。第一,激光器ECDL1和激光器ECDL2的波长,经由各自的电机驱动反射器调谐至λonλoff的5 GHz之内。第二,三角波扫描电压应用于激光器1和激光器2的压电控制器将它们的光频率调谐至设定光频率的0.5 GHz以内。外腔调谐二极管激光器的光频率满足了这些条件之后,第三个步骤才开始,一个稳定的闭环负反馈环节。在这个环节,波长计首先通过复用器读出Cs-852光源的光频率作为标准波长参考。这一读数补偿波长计可能的波长漂移。在波长计标定之后,激光器1和激光器2通过PID(proportional-integral-derivative)控制程序,以10 MHz的精度,将波长稳定在λonλoff处。在PID 控制程序中,设定的光频率νonνoff与波长计读出的激光器1或激光器2的实际光频率之间的差别,记作控制环的误差信号。误差信号乘以一个比例常数 (PID的P部分)加上一个修正信号,应用到激光器1和激光器2的压电控制器的电压上。PID 程序的I-部分和D-部分用以调整ECDL使其光频率的调谐得更稳定、更平滑。激光器的调谐精度受限于波长计的精度和压电的灵敏度。

    Nd:YAG泵浦激光器的1064nm波长的长时间漂移,则用另一个相同的算法程序锁定,读出高精度波长计(WS8-2, HighFinesse)测量的数值,以及用这个读数去锁定预定的波长。这一思想来源于 Kobtsev et al的工[19]。波长计关于几个激光脉冲的读数平均,结果与预定的波长比较。应用小的修正电压,让Nd:YAG 激光器的注入种子SI-2000的光频率,发生相反的改变,重新回到设置点。

  • 3 种子注入的光参量振荡器和放大器

    765 nm脉冲光源的光学布局,如图4所示,此OPO-OPA通过Continuum SureliteTM EX Nd:YAG脉冲激光器的二倍频532 nm泵浦,泵浦源-Nd:YAG激光器也是由SI-2000分布反馈稳频光纤激光器作为种子注入的,可单纵模输出1064 nm脉冲激光,它本身由闪光灯泵浦,脉冲重复频率10 Hz,横模端面强度分布接近平顶轮廓。使用的脉冲能量达220 mJ/532 nm。基波的脉冲宽度8~9 ns和二倍频的脉冲宽度7.5 ns。532 nm泵浦束直径经由缩束器减小至ϕ 4.5 mm。

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    图4 (a)单频和波长稳定的765 nm发射机关键部件框图。SMF:单模光纤; SHG:二次谐波发生器;APP:光楔;OI:光隔离器;BD:束收集器;PZT:压电换能器。(b) 光参量振荡器的图示和实物照片

    Fig.4 (a) Block diagram showing three key components of the single-frequency and wavelength-stabilized 765-nm laser transmitter. SMF: single-mode fiber; SHG: second harmonic generation; APP: anamorphic prism pair; OI: optical isolator; BD: beam dump; PZT: piezoelectric transducer;PD: photodetector;(b) Schematic and photograph of the physical OPO.

    二极管种子激光束径扩展至大约ϕ 4.5 mm,或者略大于泵浦光束直径,使准直后的种子光束与泵浦光束最大限度地空间上重叠。光参量振荡器的信号谐振的设计,也是参考了P. Mahnke etal[15]之前的工作。光学谐振腔的建立,适合信号光的行波振荡,环形谐振腔包括四面平面反射镜。腔镜的透射光谱特性如图5所示。一对相同的KTP (KTiOPO4) 非线性光学晶体,以走离角补偿的结构形式安置在腔内。此谐振腔由平面镜M1 至 M4组成,其中M2平面镜是信号光耦合输出镜,也是种子激光注入OPO谐振腔的端口;耦合输出镜的薄膜涂层对于532 nm激光有高的反射率,对于信号光和种子光波段,具有40%~60%的反射率。平面镜M4背面安置PZT压电器件,用于主动稳定谐振腔的长度。532-nm泵浦脉冲通过M1进入谐振腔,穿过M3后离开谐振腔,泵浦脉冲单次穿过两块非线性晶体。四面镜片对于空闲光1746.7 nm均有较高的透过率,在这样的设计当中,空闲光透过各腔镜而被废弃,谐振腔称为信号波765 nm的单谐振共振腔,阻止固定的相位关系使得OPO参量变换的输出光束在下一块晶体上发生逆向转换(1746.7 nm+765 nm→532 nm)。这样的设计还有一个更大程度的优点,在OPO与泵浦源之间不需要光隔离器。所有的平面镜直径都是1英寸 (2.54cm)。谐振腔的每一个臂长都是4.5 cm,受限于常用镜片和晶体的支架的尺度,导致谐振腔的光学长度接近17.5 cm,自由光谱范围1.7 GHz。

    图5
                            光参量振荡器腔镜的透过率光谱曲线。腔镜薄膜涂层的入射角45°

    图5 光参量振荡器腔镜的透过率光谱曲线。腔镜薄膜涂层的入射角45°

    Fig. 5 Transmission curves for the OPO cavity mirrors. The mirrors are coated for a 45°angle of incidence.

    其中的KTP晶体为15-mm长,通光孔7.5 mm×7.5 mm、端面有一保护性薄膜涂层(p-type)对信号光、泵浦光以及空闲光具有最小反射率(<1%)。非线性光学模型软件程序 SNLO(Nonlinear Optics Software)计算的角度结果,晶体在XZ平面上切割角 (φ = 0 º)和θ = 55 º,这儿θ 是相对于Z轴的极角,φXZ平面上相对于X轴的方位角。II类相位匹配,信号光e-偏振(异常光),泵浦光o-偏振(寻常光)和空闲光o-偏振(寻常光)。532(o)→1746.7(o)+765(e)。o-偏振的方向在晶体中的指向平行于Y轴,e-偏振的方向在XZ 平面上。如图4a所示,765 nm种子光经过二分之一波片(2),并通过旋转的二分之一波片(2)调整种子光偏振方向,使得种子光的偏振态和信号光的偏振态一致,也使得泵浦光输出偏振和种子光输出偏振的方向,满足上述II类相位匹配对偏振态的要求。532nm泵浦光经过偏振分光镜,能量分成偏振方向垂直的两部分,旋转二分之一波片(1),可以改变偏振分光镜输出的532 nm激光偏振相互垂直的两个分量能量比例,即参量振荡器和参量放大器的泵浦能量比例,这个比例大约1:3左右;光参量放大级的532 nm泵浦激光的光程做了适当的延长,为了使得泵浦光脉冲和参量振荡级输出的信号光脉冲,到达放大级非线性光学KTP晶体上的时间较好地相重叠。

    相对于P. Mahnke et al[15]之前的工作,有两点不同,参量振荡级的泵浦光偏振方向与参量放大级的泵浦光偏振方向相互垂直(通过偏振束片PBS分开),参量振荡级的信号光偏振方向经过二分之一波片(3)旋转之后也垂直于原来的偏振方向,再送给参量放大级,目的在于使得放大级引起的信号光光斑椭圆形变轴向,垂直于参量振荡级引起信号光光斑的椭圆形变的轴向。其二,参量振荡级的谐振腔的两个非线性KTP晶体是位于环形光路四边形的相邻两个边上,而不是位于四边形面对面的两条边上(Mahnke et al[15]),这样谐振腔四块镜片M1、M2、M3和M4只需要两类光谱曲线(图5),而不是需要三类光谱曲线[16]

    OPO 的调谐,通过电机驱动的转台绕Y-轴旋转晶体保持方位角ɸ在0°。通过SNLO 软件仿真获得,每一个1 cm的KTP晶体对于信号光和空闲光的可接受角分别为1.4和2.0 mrad以内。

    当种子未注入时,其 OPO 的带宽在0.30 nm(90 GHz)量级。这是大大地宽于氧气特征吸收线

    的宽度(1~2 GHz)。窄线宽、连续波种子的注入,不仅可以压缩OPO输出信号光的光谱宽度、也可以提高信号光光谱纯度。OPO在种子注入和非种子注入情况下的比较如图6所示。种子输入OPO/OPA系统具有泵浦能量阈值42 mJ/脉冲,全能量泵浦(大约220 mJ)时,OPA终端产生接近60 mJ 能量输出,OPA光束质量M2 < 8,脉冲长度5.5~6.0 ns。对于激光雷达工作,OPA 输出能量一般使用到45 mJ/脉冲。在这些情况下,OPA的光谱纯度,通常用拍频方法监[17],可靠地优于99.9%。连续波种子激光和OPO/OPA输出脉冲信号光外差检测并傅立叶变换,萃取出信号光中765 nm主流光频分量的功率谱密度,它会高出剩下的边模分量(Side Mode Suppression Ratio)40 dB。外差拍频的方法测量光谱纯度,比测量多通道气体吸收池的剩余透过率的方法,更为粗略。

    图6
                            765 nm光参量振荡器/光参量放大器分别在种子注入和无种子注入条件下的辐射性能

    图6 765 nm光参量振荡器/光参量放大器分别在种子注入和无种子注入条件下的辐射性能

    Fig. 6 OPO/OPA radiation performance seeded and not seeded at 765 nm.

  • 4 OPO腔锁定和光频稳定

    只有当谐振腔的腔长是种子光波长的整数倍,OPO才会有最好的实现;偏离此精确匹配,将影响转换效率和引起种子光频率和输出信号光频率的差异;为了OPO稳定地工作在长程傅立叶变换限状态下,需要主动稳定其腔长,去补偿光的、机械的、或热的等因素引起的腔长漂移或抖动。总之,仅仅让连续波种子激光在谐振腔腔内环绕是不够的。一般来说,当相位匹配的OPO谐振波长等于种子光波长,获得的光频率啁啾最小。我们发现“ramp, hold, fire”方[18]对解决我们的问题,效果较好。

    通过扫描PZT压电器件的电压,腔长随之改变,我们可以观察到光电探测器(图4(a) PD-photodector)监测到的来自谐振腔镜M2的连续波种子源的反射光强度的变化。显示出一系列清晰可辨的强度极小值,它对应于腔长与种子光波长之间的谐振点(如图7)。典型的调制深度 40%~65 %,取决于在这个特殊波长下腔内的损耗(如耦合镜的透过率、晶体的吸收率、晶体的散射损失)。具体而言,通过一个“Sweep, Hold, and Fire” 周期,种子光被OPO腔镜反射的连续波光强,受到光电探测器的持续监测,压电器件PZT的正确位置,由种子光反射束(图4(a)中的M2镜片)强度产生的一个极小值而得到确定 (图7)。

    图7
                            OPO环形腔腔镜M3背面的扫描压电元件。每一个最小值对应于注入种子波长的谐振腔的谐振结构。

    图7 OPO环形腔腔镜M3背面的扫描压电元件。每一个最小值对应于注入种子波长的谐振腔的谐振结构。

    Fig. 7 Scanning of cavity fringes of the OPO ring cavity observed as the PZT element on mirror M3. Each dip corresponds to a resonant configuration of the resonator with respect to the injected seed wavelength.

    首先,来自延迟发生器的触发脉冲发送到位于控制计算机上的数据采集卡,去启动锁定循环 (图7)。计算机随之从一个边沿扫描(“sweep”)压电器件PZT上的电压,光电探测器的信号同时被数据采集卡记录,可以确定对应于腔反射最小值的PZT电压。一个孔径用来限制到达光探测器的光通量。光探测器具有较大的光敏面和中等损伤阈值。一个低压放大器用在数据采集卡的低通滤波输出和压电器件PZT之间。在扫描之后,压电器件PZT的电压被调节到产生腔最小反射的那个值。并且此电压保持(“hold”)几个毫秒钟,泵浦激光脉冲就在这段时间内发射(“fire”),随之OPO的谐振腔输出与种子光共振的信号光波。随后,压电器件PZT又被重新设置(reset)到初始值,新的循环再一次实施,循环重复频率等于泵浦激光器的脉冲重复频率(10 Hz)。这种方式有一个微妙的优点,避免了对PZT压电器件应用高电压,去给种子波长做大范围的扫描。

    压电器件PZT始终复置相同的初值。通过“Ramp, Hold, and Fire”技术,仅在种子波长附近扫描较小的范围,OPO每次总是锁定在最近的腔纵模上(图8)[18]

    Fig. 8 Ramp, hold, and fire locking scheme. Upper trace: piezo voltage; lower trace: PD signal. The discrepancy (indicated by the dashed curve) between the “found” PZT voltage corresponding to the dip in the PD (Photodetector) signal and the “hold” voltage where the pump fires is attributed to hysteresis of the piezo and is accounted for in the locking software.

    注:图8.“扫描,保持,和发射”的锁定结构。上面的轨迹:压电电压;下面的轨迹:光电二极管信号。“found”阶段压电器件对应于探测器光谐振点的电压和“hold”阶段泵浦光发射时的压电元件的电压迟滞,在锁定软件中的差异

    这一技术特点,来自于我们在压电器件PZT扫描过程中持续地监视连续波种子激光对腔镜(图4(a), M2)的反射。有效地将OPO腔当作一个诊断用的、低精度的、类似于Fabry–Perot扫描(腔镜M2是入射端,也是反射端)干涉仪;实际上,在压电器件运动中,回到“Hold Position”状态时,它总有一个小的迟滞 (~15%)。以经验数据确定修正因子,在LabVIEW™ 程序工作中,可以非常好地补偿伺服环的这一滞后。图9显示了用这种方式获得的信号光稳频(方差σ)效果。这个结果是通过WS8-2超高精度波长计测得的。

    图 9
                            WS8-2测量的波长稳定性

    图 9 WS8-2测量的波长稳定性

    Fig. 9 Wavelength stability measured with WS8-2

    压电调谐标准具(FPI 100-0750-3V0,TOPTICA)对光参量振荡器的信号光进行扫频,图10显示信号光的线宽180 MHz(FWHM),横坐标MHz,纵坐标为被测光束透过F-P标准具的相对强度。

    图10
                            自由光谱范围1 GHz/分辨率400标准具对光参量振荡器的信号光进行扫频显示信号光的线宽180 MHz(FWHM)

    图10 自由光谱范围1 GHz/分辨率400标准具对光参量振荡器的信号光进行扫频显示信号光的线宽180 MHz(FWHM)

    Fig.10 FSR-1GHz/F-400 etalon(TOPTICA,FPI 100-0750-3V0) trace of the signal wave showing a signal linewidth of 180 MHz FWHM

    自由光谱范围1 GHz、分辨率400的标准具的带宽比被测激光束的线宽窄得多;通过调谐标准具的压电PZT的电压,可以改变F-P标准具的中心波长,因此扫描压电PZT的电压,就能扫描被测激光束的透过频谱,相当于超窄带F-P标准具对被测光束的功率频谱进行持续取样,F-P标准具的中心波长(光频)和被测激光束中心波长(光频)一致时,F-P输出光强度最大;扫描标准具的压电PZT的电压使得被测激光束透过F-P标准具的强度等于其最大值的一半时,此时F-P标准具指示的中心光频率则为被测激光束频谱的半宽度处的光频率,它与最大值处的光频率之差,反映了激光束的线宽(频谱宽度)。

    光束远场轮廓用一个CCD相机成像在球面镜1-m长的焦平面上,如图11所示。从这个束斑像计算的发散角接近8.5 mrad,标准方差0.25 mrad。765-nm中心位置标准方差27 μrad在X-轴和28 μrad在Y-轴。OPA终端输出光束质量因子M2为7.6,此时泵浦光能量在190 mJ上下。发射之后,还要经过10×倍,12×倍的扩束,以减小束发散角。

    图11
                            765 nm束远场轮廓的输出

    图11 765 nm束远场轮廓的输出

    Fig. 11 Far-field beam profile of the 765-nm output

    与法国The Service d'Adronomie du Centre National de la Recherche Scientifique[23]、德国Max-Planck-Institut fu¨r Meteorologie[21],和俄国Institute of atmospheric optics[22],以及美国ASCENDS机载实[7]的类似光源的作性能对比,如表1

    表1 位于氧气A吸收带的几个差分吸收激光雷达的光源参数对比

    Table 1 Comparison of light source parameters of several Lidars based on oxygen A-band

    光源类型激光线宽脉冲能量波长光频稳定性 (rms)光谱纯度脉冲长度脉冲重复频率年代
    Alexandrite激光器(法)99MHz(3.3×10-3 cm-1)30 mJ760.65 nm(13146.6cm-1 )40MHz (±1.3 × 10-3 cm-1)99.9%500ns-----1994
    Alexandrite 激光器(德)<40 MHz>40 mJ720-780 nm15 MHz99.99%200ns15Hz1996
    Ti:Sapphire 激光器(俄)900MHz(0.03cm-1)30mJ768 nm101.7 MHz(0.2 pm)----------30Hz2005
    半导体激光器+光纤放大器(美)5MHz1.5-2.0μJ764.5-764.9 nm扫频------250 ns10 kHz2013
    OPO/OPA (中)180 Mz45 mJ765 nm30 MHz99.9%5.5~6ns10 Hz2018
  • 5 总结

    以Cs-852基准源标定的高精度波长计测量误差信号的PID伺服控制环,可以稳定种子激光的波长;通过“Ramp, Hold, and Fire”技术,种子注入能够锁定光参量振荡器的谐振腔的腔长。光参量振荡器-光参量放大器输出单脉冲能量高达45 mJ,脉冲宽度5.5~6.0 ns,脉冲重复频率10 Hz,线宽180 MHz。从Nd:YAG 的532nm泵浦激光向OPO-OPA的765 nm信号光的转换斜度效率(Slope Efficiency)大约23.6%。单纵模Nd:YAG 激光器的二次谐波提供的泵浦能量大约190 mJ,泵浦脉冲宽度7.5 ns。种子注入,降低了光参量振荡的阈值,并提高了OPO能量转换效率。全固态二极管激光器结构紧凑,是OPO特别理想的种子源。

    如果用DFB(分布反馈式)二极管激光器或DBR(分布布拉格反射式)二极管激光器(EYP-DFB-0764-00010-1500-BFY02-0002,Eagleyard Photonics GmbH)以取代ECDLs外腔调谐二极管激光器做注入种子,因为没有运动元件(如外腔调谐光栅Diffraction Grating和反射镜Prism),则该系统将更加牢靠。如果使用767 nm 39K(钾,Vescent Photonics, Inc. D2-100 DBR Laser)参考源波长来代替Cs(铯)-852 nm参考源波长,因为参考基准波长(767 nm)更接近被测量的波长(765 nm),那么波长计读数精度会更高,种子波长控制的更稳定。

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      Mahnke P, Klingenberg H H, Fix A, et al. Dependency of injection seeding and spectral purity of a single resonant KTP optical parametric oscillator on the phase matching condition[J]. Appl. Phys. B, 2007, 89: 1-7.

    • 16

      Velarde L, Engelhart D P, Matsiev D, et al. Generation of tunable narrow bandwidth nanosecond pulse in the deep ultraviolet for efficient optical pumping and high resolution spectroscopy [J]. Rev. Sci. Instrum. 2010, 81: 063106.

    • 17

      Mammez D, Dherbecourt J B, Raybaut M, et al. Linewidth, and real-time spectral analysis of an amplified pulsed nested cavity optical parametric oscillator [C]. 2015,European Union,978-1-4673-7475-0

    • 18

      He Y, Baxter G W, Orr B J. Locking the cavity of a pulsed periodically poled lithium niobate optical parametric oscillator to the wavelength of a continuous-wave injection seeder by an “intensity-dip” method [J]. Rev. Sci. Instrum. 1999, 70(8): 3203-3213.

    • 19

      Kobtsev S, Kandrushin S, Potekhin A. Long-term frequency stabilization of a continuous-wave tunable laser with the help of a precision wavelength meter [J]. Appl. Opt. 2007, 46(23): 5840-5843

    • 20

      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(3): 9057-9068.

    • 21

      Wulfmeyer V, B¨osenberg J, Single-mode operation of an injection-seeded alexandrite ring laser for application in water-vapor and temperature differential absorption lidar[J]. Optics Letters.1996,21(15): 1150-1152

    • 22

      Kharchenko O V, Matvienko G G, Grishin A I, et al. Possibility of a meteorological lidar for measurement of basic meteorological parameters of the atmosphere [C]. 2005, Proceedings of SPIE , 5832, doi: 10.1117/12.619840

    • 23

      Bruneau D, des Lions T A, Quaglia P, et al. Injection-seeded pulsed alexandrite laser for differential absorption lidar application [J].Appl. Opt. 1994, 33(18): 3941–3950.

洪光烈

机 构:

1. 中国科学院空间主动光电技术重点实验室,上海 200083

2. 中国科学院上海技术物理研究所,上海 200083

Affiliation:

1. Key Laboratory of Space Active Optoelectronic Technology, Chinese Academy of Sciences, Shanghai 200083, China

2. Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China

邮 箱:glhong@mail.sitp.ac.cn

作者简介:(Biography):(Biography):洪光烈(1966-),男,安徽庐江人,研究员,主要从事激光雷达大气遥感与大气探测方面研究。E-mail:glhong@mail.sitp.ac.cn

王钦

机 构:

1. 中国科学院空间主动光电技术重点实验室,上海 200083

3. 中国科学院大学,北京100049

Affiliation:

1. Key Laboratory of Space Active Optoelectronic Technology, Chinese Academy of Sciences, Shanghai 200083, China

3. University of Chinese Academy of Sciences, Beijing 100049, China

肖春雷

机 构:

1. 中国科学院空间主动光电技术重点实验室,上海 200083

2. 中国科学院上海技术物理研究所,上海 200083

Affiliation:

1. Key Laboratory of Space Active Optoelectronic Technology, Chinese Academy of Sciences, Shanghai 200083, China

2. Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China

孔伟

机 构:

1. 中国科学院空间主动光电技术重点实验室,上海 200083

2. 中国科学院上海技术物理研究所,上海 200083

Affiliation:

1. Key Laboratory of Space Active Optoelectronic Technology, Chinese Academy of Sciences, Shanghai 200083, China

2. Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China

王建宇

机 构:

1. 中国科学院空间主动光电技术重点实验室,上海 200083

2. 中国科学院上海技术物理研究所,上海 200083

Affiliation:

1. Key Laboratory of Space Active Optoelectronic Technology, Chinese Academy of Sciences, Shanghai 200083, China

2. Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China

角 色:通讯作者

Role:Corresponding author

邮 箱:jywang@mail.sitp.ac.cn

作者简介:E-mail: jywang@mail.sitp.ac.cn

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光源类型激光线宽脉冲能量波长光频稳定性 (rms)光谱纯度脉冲长度脉冲重复频率年代
Alexandrite激光器(法)99MHz(3.3×10-3 cm-1)30 mJ760.65 nm(13146.6cm-1 )40MHz (±1.3 × 10-3 cm-1)99.9%500ns-----1994
Alexandrite 激光器(德)<40 MHz>40 mJ720-780 nm15 MHz99.99%200ns15Hz1996
Ti:Sapphire 激光器(俄)900MHz(0.03cm-1)30mJ768 nm101.7 MHz(0.2 pm)----------30Hz2005
半导体激光器+光纤放大器(美)5MHz1.5-2.0μJ764.5-764.9 nm扫频------250 ns10 kHz2013
OPO/OPA (中)180 Mz45 mJ765 nm30 MHz99.9%5.5~6ns10 Hz2018

图1 在标准大气压和两种温度296 k(红)276k(绿色)下,通过100 m长度大气,氧气A-吸收带透射光谱.

Fig.1 Oxygen A-band transmission spectrum through 100 m of atmosphere at standard pressure and at two temperatures: 296 K (red) and 276 K (green).

图2 (a) 注入种子的建立。XCOF:交叉耦合光纤;OMP:光学复用器;DAQ:数据采集器;PFC:保偏光纤耦合器;(b) 外腔调谐二极管激光器的图片

Fig. 2 (a) Injection-seeder setup. XCOF: cross-coupled optical fibers; OMP: optical multiplexer; DAQ: data acquisition; PFC: polarization maintenance fiber couplers. (b) Picture of external cavity diode laser

图2 (a) 注入种子的建立。XCOF:交叉耦合光纤;OMP:光学复用器;DAQ:数据采集器;PFC:保偏光纤耦合器;(b) 外腔调谐二极管激光器的图片

Fig. 2 (a) Injection-seeder setup. XCOF: cross-coupled optical fibers; OMP: optical multiplexer; DAQ: data acquisition; PFC: polarization maintenance fiber couplers. (b) Picture of external cavity diode laser

图3 852 nm(铯)高稳定度基准光源和高分辨率高精度波长计

Fig. 3 Reference laser source 852 nm (Cs) with high stability and wavelength meter with high resolution and precision

图3 852 nm(铯)高稳定度基准光源和高分辨率高精度波长计

Fig. 3 Reference laser source 852 nm (Cs) with high stability and wavelength meter with high resolution and precision

图3 852 nm(铯)高稳定度基准光源和高分辨率高精度波长计

Fig. 3 Reference laser source 852 nm (Cs) with high stability and wavelength meter with high resolution and precision

图4 (a)单频和波长稳定的765 nm发射机关键部件框图。SMF:单模光纤; SHG:二次谐波发生器;APP:光楔;OI:光隔离器;BD:束收集器;PZT:压电换能器。(b) 光参量振荡器的图示和实物照片

Fig.4 (a) Block diagram showing three key components of the single-frequency and wavelength-stabilized 765-nm laser transmitter. SMF: single-mode fiber; SHG: second harmonic generation; APP: anamorphic prism pair; OI: optical isolator; BD: beam dump; PZT: piezoelectric transducer;PD: photodetector;(b) Schematic and photograph of the physical OPO.

图4 (a)单频和波长稳定的765 nm发射机关键部件框图。SMF:单模光纤; SHG:二次谐波发生器;APP:光楔;OI:光隔离器;BD:束收集器;PZT:压电换能器。(b) 光参量振荡器的图示和实物照片

Fig.4 (a) Block diagram showing three key components of the single-frequency and wavelength-stabilized 765-nm laser transmitter. SMF: single-mode fiber; SHG: second harmonic generation; APP: anamorphic prism pair; OI: optical isolator; BD: beam dump; PZT: piezoelectric transducer;PD: photodetector;(b) Schematic and photograph of the physical OPO.

图4 (a)单频和波长稳定的765 nm发射机关键部件框图。SMF:单模光纤; SHG:二次谐波发生器;APP:光楔;OI:光隔离器;BD:束收集器;PZT:压电换能器。(b) 光参量振荡器的图示和实物照片

Fig.4 (a) Block diagram showing three key components of the single-frequency and wavelength-stabilized 765-nm laser transmitter. SMF: single-mode fiber; SHG: second harmonic generation; APP: anamorphic prism pair; OI: optical isolator; BD: beam dump; PZT: piezoelectric transducer;PD: photodetector;(b) Schematic and photograph of the physical OPO.

图5 光参量振荡器腔镜的透过率光谱曲线。腔镜薄膜涂层的入射角45°

Fig. 5 Transmission curves for the OPO cavity mirrors. The mirrors are coated for a 45°angle of incidence.

图6 765 nm光参量振荡器/光参量放大器分别在种子注入和无种子注入条件下的辐射性能

Fig. 6 OPO/OPA radiation performance seeded and not seeded at 765 nm.

图7 OPO环形腔腔镜M3背面的扫描压电元件。每一个最小值对应于注入种子波长的谐振腔的谐振结构。

Fig. 7 Scanning of cavity fringes of the OPO ring cavity observed as the PZT element on mirror M3. Each dip corresponds to a resonant configuration of the resonator with respect to the injected seed wavelength.

Fig. 8 Ramp, hold, and fire locking scheme. Upper trace: piezo voltage; lower trace: PD signal. The discrepancy (indicated by the dashed curve) between the “found” PZT voltage corresponding to the dip in the PD (Photodetector) signal and the “hold” voltage where the pump fires is attributed to hysteresis of the piezo and is accounted for in the locking software.

图 9 WS8-2测量的波长稳定性

Fig. 9 Wavelength stability measured with WS8-2

图10 自由光谱范围1 GHz/分辨率400标准具对光参量振荡器的信号光进行扫频显示信号光的线宽180 MHz(FWHM)

Fig.10 FSR-1GHz/F-400 etalon(TOPTICA,FPI 100-0750-3V0) trace of the signal wave showing a signal linewidth of 180 MHz FWHM

图11 765 nm束远场轮廓的输出

Fig. 11 Far-field beam profile of the 765-nm output

表1 位于氧气A吸收带的几个差分吸收激光雷达的光源参数对比

Table 1 Comparison of light source parameters of several Lidars based on oxygen A-band

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图8.“扫描,保持,和发射”的锁定结构。上面的轨迹:压电电压;下面的轨迹:光电二极管信号。“found”阶段压电器件对应于探测器光谐振点的电压和“hold”阶段泵浦光发射时的压电元件的电压迟滞,在锁定软件中的差异

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      Späth F, Metzendorf S, Behrendt A, et al. Online/offline injection seeding system with high frequency-stability and low crosstalk for water vapor DIAL [J]. Optics Communications. 2013,309:37-43

    • 15

      Mahnke P, Klingenberg H H, Fix A, et al. Dependency of injection seeding and spectral purity of a single resonant KTP optical parametric oscillator on the phase matching condition[J]. Appl. Phys. B, 2007, 89: 1-7.

    • 16

      Velarde L, Engelhart D P, Matsiev D, et al. Generation of tunable narrow bandwidth nanosecond pulse in the deep ultraviolet for efficient optical pumping and high resolution spectroscopy [J]. Rev. Sci. Instrum. 2010, 81: 063106.

    • 17

      Mammez D, Dherbecourt J B, Raybaut M, et al. Linewidth, and real-time spectral analysis of an amplified pulsed nested cavity optical parametric oscillator [C]. 2015,European Union,978-1-4673-7475-0

    • 18

      He Y, Baxter G W, Orr B J. Locking the cavity of a pulsed periodically poled lithium niobate optical parametric oscillator to the wavelength of a continuous-wave injection seeder by an “intensity-dip” method [J]. Rev. Sci. Instrum. 1999, 70(8): 3203-3213.

    • 19

      Kobtsev S, Kandrushin S, Potekhin A. Long-term frequency stabilization of a continuous-wave tunable laser with the help of a precision wavelength meter [J]. Appl. Opt. 2007, 46(23): 5840-5843

    • 20

      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(3): 9057-9068.

    • 21

      Wulfmeyer V, B¨osenberg J, Single-mode operation of an injection-seeded alexandrite ring laser for application in water-vapor and temperature differential absorption lidar[J]. Optics Letters.1996,21(15): 1150-1152

    • 22

      Kharchenko O V, Matvienko G G, Grishin A I, et al. Possibility of a meteorological lidar for measurement of basic meteorological parameters of the atmosphere [C]. 2005, Proceedings of SPIE , 5832, doi: 10.1117/12.619840

    • 23

      Bruneau D, des Lions T A, Quaglia P, et al. Injection-seeded pulsed alexandrite laser for differential absorption lidar application [J].Appl. Opt. 1994, 33(18): 3941–3950.


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