Broadband amplitude-modulated laser source with high output power for FM/cw lidar transmitter

We are building a long-range FM/cw nonscanning imaging lidar breadboard. This lidar system achieves ranging based on a frequency modulation/continuous wave (FM/cw) technique, implemented by an amplitude modulated mid-IR diode laser transmitter with a linear frequency modulation (LFM) of the subcarrier. Firstly, various schemes of light beam modulation are analyzed. Secondly, we put forward a laser modulation scheme whose core was formed by a 1.55 μm electro-absorption modulated laser diode (EML) and an erbium-doped optical fiber amplifier (EDFA), then a corresponding experimental system architecture and components for light beam modulation and detection are established. Finally, a corresponding experiment of laser beam modulation is completed for the first time. In our experiment, the EML amplitude is modulated by a 200 MHz to 800 MHz LFM signal, whose amplitude value is 2.05 V. The average output power of the modulated laser obtained in the experiment is 10 W, peak power is 16.4 W, and the average modulation depth is 78%. The results of tests predict that this laser modulation scheme is likely to improve the imaging range of the FM/cw lidar.     

A Pseudo-Random Frequency Modulation Continuous Wave Coherent Lidar Using an Optical Field Correlation Detection Method

We propose a new detection method for a pseudo-random frequency modulation continuous wave (RM-CW) coherent lidar. The feature of this method is modulation of local beam with a time delayed pseudo-random sequence. Heterodyne detection and correlation detection between the received beam and the local beam are simultaneously carried out in an optical field. In the RM-CW coherent lidar using the optical field correlation detection method, the received equipment is greatly simplified. We carried out preliminary experiments and demonstrated that the new method is effective for detection of a CW coherent lidar.     

连续波差分吸收激光雷达测量大气CO_2

研制了一套接收硬目标回波的差分吸收激光雷达系统以用于全天候监测大气CO2浓度变化.系统采用10和12 kHz正弦波分别对处在CO2吸收峰内和吸收峰外的波长进行强度调制,利用单频检测技术提取回波信号.提出了一种利用激光扫频推算系统精度的方法,从而弥补了长期以来只能在理论上计算差分吸收激光雷达系统精度的不足,给实际系统自定标问题提供了一种解决方案.该系统采用全光纤结构,结构可靠,便于移动.利用此系统获得了上海市多天CO2浓度变化曲线,在450 m的积分路径长度上,1 s的积分时间取得了优于3.39×10-6的测量精度.     

双边缘测风激光雷达法布里-珀罗鉴频器控制方法

 提出一种双边缘测风激光雷达法布里-珀罗(F-P)干涉仪鉴频器的非线性比例-积分-微分(PID)控制方法。该方法利用发射激光在鉴频器校正通道的透过率作反馈进行F-P干涉仪鉴频器的稳定控制。首先给出F-P干涉仪鉴频器透过率的控制模型,由气压波动、温度变化和振动等引起的外界扰动被等效为施加在致动压电陶瓷上的扰动外力。为了进行扰动补偿,用新增的反正切函数设计了非线性PID控制器,提高了经典PID控制方法的反馈增益。仿真结果显示,与经典的PID控制相比,新的非线性控制方法可以使F-P干涉仪鉴频器在更短的时间达到稳定状态,并且稳态误差约减小到原来的1/20。     

载波调制激光雷达水下目标探测的仿真分析

激光雷达应用于水下目标探测,光信号受水体后向散射影响,导致目标对比度严重下降。一种以载波调制激光脉冲为探测信号的方法可实现对后向散射信号的抑制。利用基于蒙特卡罗的光子传输模型,模拟载波调制激光雷达水下目标探测的全过程。结果表明该方法对水下目标探测有明显改善,调制参数对探测性能有较大影响。     

瑞利散射多普勒激光雷达风场反演方法

介绍了瑞利散射多普勒激光雷达的风场探测原理和系统结构. 给出了Fabry-Perot(FP)标准具透过率曲线的校准方法. 指出对透过率采用Lorentz或Voigt拟合会产生较大误差,特别是采用Lorentz拟合最大将引起8%的误差. 提出了采用非线性最小二乘法拟合标准具的透过率函数,该方法可以有效消除拟合误差,提高风速测量精度. 考虑到温度不确定度在风场反演过程中的主导影响,提出了同时反演风速和大气温度的非线性迭代算法. 风场反演仿真试验结果表明:在不考虑米散射信号的影响下,该反演算法与传统的反演方法相 关键词： 激光雷达 瑞利散射 多普勒 风     

瑞利多普勒测风激光雷达的频率标定方法

频率标定是瑞利测风激光雷达的关键技术。瑞利测风激光雷达中,通过改变压电陶瓷管的电压实现连续调谐F-P标准具腔长,使出射激光频率处于双边缘透过率曲线的交点处。在连续调谐时,由于压电陶瓷管的磁滞效应引起腔长调谐非线性,从而导致系统误差。分析了该误差的原因及特性,提出了静态软件补偿和动态调频跟踪相结合的频率标定方法。若激光出射频率相对F-P标准具漂移小于100 MHz时,在数据反演时补偿该频率偏差;若相对频率漂移大于100 MHz时,将F-P标准具先退回预设腔长以下,通过逐步增加电压的方式,重新实现频率锁定,保证锁定过程处在磁滞回线的电压上升段,避免了磁滞效应引起的误差。多普勒激光雷达与无线电探空仪的两组对比实验中,在15~30km高度,风速最大偏差6.22m/s,平均偏差1.12m/s。     

基于单Fabry-Perot标准具的双频率多普勒激光雷达数据反演技术

本文导出了大气后向散射信号光入射到Fabry-Perot (F-P)标准具的有效透过率表达式. 在基于单F-P 标准具的双频率多普勒激光雷达系统中, 采用平均值法定量分析了瑞利后向散射信号对风场测量准确度的影响. 提出了同时反演风速和后向散射比的非线性迭代方法, 并通过仿真试验验证了该方法的有效性. 同时, 导出了径向风速和后向散射比测量误差的具体表达式, 并据此进行了仿真. 仿真结果表明: 若假定望远镜接收到的总后向散射光子数为50000, 径向风速测量误差随后向散射比的增大迅速减小, 在±25 m/s的风速测量动态范围内, 当R_β > 1.2时, 误差小于3 m/s; 后向散射比测量误差随后向散射比的增大而增大, 与径向风速大小几乎无关, 当R_β<10时, 相对误差小于13%. 关键词： 多普勒激光雷达 Fabry-Perot标准具 边缘技术 双频率     

基于扫描F-P标准具的高光谱分辨低平流层温度探测

报道了一种基于扫描F-P标准具的高光谱分辨低平流层大气温度探测技术。通过扫描F-P标准具,获得大气分子瑞利后向散射的透过率分布。对该透过率进行非线性拟合,由拟合得到的谱宽计算大气温度分布。为了减小频率不稳定引起的系统误差,采用静态的F-P标准具实时监测激光出射频率,并在数据处理中进行补偿。由时间分辨率2000s的激光雷达原始信号的信噪比,根据最大似然估计误差分析,该方法在30km以下的探测误差小于1.9K,50km以下的探测误差小于9.8K。在对比实验中,在18~36km高光谱分辨激光雷达与探空气球探测的温度廓线最大偏差4.7K;在27~34km,高光谱分辨激光雷达与瑞利积分激光雷达探测的温度最大偏差2.7K。在15~27km,由于气溶胶的污染,瑞利积分激光雷达的温度明显偏离其他两种探测结果,最大偏差达22.8K。     

相干多普勒测风激光雷达低信噪比区域回波信号的估计方法

相干多普勒测风激光雷达通常会采用周期图最大值法（PM）提取不同距离门信号的多普勒频移（对应风速）信息。由于噪声和相干效率的影响,个别距离门信号会出现信噪比（SNR）突然降低的情况,从而导致系统的探测概率降低,影响系统整体的探测性能。为了解决个别距离门信号多普勒频移的错误估计问题,提出了一种新的非线性自适应多普勒频移估计方法。该方法利用风速的连续性,标定错误距离门,并自适应地利用强信噪比区域的多普勒频移统计数据来弥补信噪比变差而出现的估计错误。分别利用了仿真模型和一套1.54μm全光纤相干激光雷达系统获得了风场测量数据,对比了使用该技术前后反演得到的风速趋势,证明该方法能够有效地解决上述问题。     

差分吸收测污激光雷达光路分束设计

在AML-2车载测污激光雷达中,对装有不同气体的拉曼管进行拉曼激发获得相应的强、弱吸收关波长。针对差分雷达激光发射光路转换的特点及提高雷达监测的实时性能要求,基于激光偏振调制原理提出了激光雷达光路分束设计的新方法,对该方法可行性进行了理论分析,并对改进光路的能量损失进行了估算,模拟结果显示改进后的系统延长了仪器的使用寿命,提高了测量的实时性。为提高差分激光雷达实时监测性能提供了一种新方法。     

Rayleigh散射Doppler激光雷达风场反演方法改进

在Rayleigh散射Doppler激光雷达的风场反演过程中,除了温度、压强等因素之外,风速反演结果的 准确性还受到Mie散射信号的影响．当Mie散射信号较强时,尤其是遇到高层云或火山灰等情况,如果仍不考虑 气溶胶信号,由于温度不确定度和气溶胶信号的综合影响,风速反演结果将与真值偏差很大．本文提出了利用 激光雷达在垂直方向的测量信号同时反演后向散射比和大气温度的非线性迭代算法,并优化给出了最佳的 初始发射激光工作点．仿真试验结果表明:该方法可以准确有效地反演后向散射比;将该方法结合非线性迭代 风速反演方法,可以有效消除气溶胶后向散射信号的影响,进一步提高大气风速和温度的反演精度．     

Photon-counting chirped amplitude modulation lidar system using superconducting nanowire single-photon detector at 1550-nm wavelength

We demonstrate a photon-counting chirped amplitude modulation(CAM) light detection and ranging(lidar) system incorporating a superconducting nanowire single-photon detector(SNSPD) and operated at a wavelength of 1550 nm.The distance accuracy of the lidar system was determined by the CAM bandwidth and signal-to-noise ratio(SNR) of an intermediate frequency(IF) signal. Owing to a short dead time(10 ns) and negligible dark count rate(70 Hz) of the SNSPD, the obtained IF signal attained an SNR of 42 d B and the direct distance accuracy was improved to 3 mm when the modulation bandwidth of the CAM signal was 240 MHz and the modulation period was 1 ms.     

一种新型的用于差分吸收激光雷达中脉冲式光学参量振荡器的种子激光器的频率稳定方法

提出了一种新型的用于差分吸收激光雷达中脉冲式光学参量振荡器的种子激光器的频率稳定方法. 详细介绍了该稳频方法的工作原理和实验装置, 并在理论上对该方法的稳频精度及其影响因素进行了分析. 利用该方法, 在实验中将种子光激光器稳定在水汽的吸收峰中心(935.6849 nm)处, 频率抖动的标准差小于8 MHz. 测试了种子注入后的光学参量振荡器输出的脉冲光的频率抖动, 测试结果表明, 脉冲光频率能够与种子光保持一致, 频率抖动的标准差小于28.7 MHz, 该稳频结果完全能够满足差分吸收激光雷达的需求.     

光谱稳定性对直接探测多普勒测风激光雷达的影响研究

从种子注入固体激光器的不稳定和多普勒频移检测干涉仪的光谱漂移出发,模拟和分析其对基于双边缘探测技术的直接探测多普勒激光雷达风速测量准确度的影响. 模拟结果显示,在5 min积分时间的30 000个脉冲内,如果达到风速准确度1 m/s,要求激光器出现多纵模的脉冲不能超过总脉冲个数的0.06％.在干涉仪光谱稳定方面,使用两级温控可以将干涉仪温度控制在±0.002℃,对应风速误差为±0.226 m/s.同时提出通过监视种子注入过程中的脉冲建立时间和干涉仪温度,可以在数据反演时,消除激光频率跳动和干涉仪光谱漂移对风速测量准确度的影响.     

Accuracy analysis and stabilization design of the Fabry–Perot frequency discriminator in double-edge wind lidars

The measurement error of a double-edge wind lidar caused by a disturbed Fabry–Perot interferometer (FPI) is analyzed. Several error sources such as air pressure variations, temperature changes, and mechanical vibrations are considered in the measurement-error model. The simulation results show that a double-edge wind lidar is so sensitive to environmental variations that the measurement error reaches ±60 m/s if the FPI is not stabilized. In order to compensate the external disturbance acting on the FPI, a nonlinear proportional–integral–derivative (PID) control scheme is designed based on the transmission measurement of the calibration channel. An arc tangent function is used to improve the feedback gain of the usual PID control design. The results show that with the new controller the measurement accuracy of the wind lidar can be improved 4–5 times in comparison with the usual control design, and the range of the measurement error is only ±3 m/s.     

拉曼激光雷达探测低对流层大气二氧化碳分布

介绍了中国科学院安徽光学精密机械研究所研制成功的我国第一台测量低对流层大气CO2时空分布的拉曼激光雷达系统,选用波长355nm的紫外激光作为光源,利用光子计数卡双通道采集大气中N2和CO2的拉曼后向散射信号。详细分析了拉曼激光雷达系统的定标方法,提出采用Li7500型H2O/CO2分析仪与拉曼激光雷达系统进行对比与标定,结果显示激光雷达与CO2分析仪数据变化趋势一致性较好,激光雷达具有很高的探测灵敏度与准确性,通过线性拟合水平方向标定误差小于0.2%,垂直方向小于1.4%。由标定关系反演出大气中CO2的时空分布,给出了合肥西郊低对流层大气CO2水平方向0～2.0km与垂直方向0～2.5km分布的典型测量结果。     

Low tropospheric wind measurement with Mie Doppler Lidar

A double edge Mie Doppler lidar at 1064 nm was developed in Hefei, China in 2005 for low tropospheric wind measurement. Intercomparison experiments with a wind profiler and a wiresonde were held. Intercomparisons of concurrent lidar and other instrument observations show good agreement with expected measurement accuracy. The examples of validated lidar wind profiles obtained during these experiments as well as an example of continuous wind observations are presented. The instrument has demonstrated the capability of atmospheric wind field measurement from 0.2 to 5 km altitude, achieving below 2 m/s accuracy with 1 min averaging and 21.2 m vertical resolution.     

Accuracy Analysis of the Fabry–Perot Etalon Based Doppler Wind Lidar

A direct detection Doppler wind lidar with the dual Fabry–Perot technique has been built in Hefei, China, and the system is described in this paper. A dual, air-spaced Fabry–Perot etalon is designed as the frequency discriminator and the transmission characteristics are measured with a stabilized cw laser and a pulse laser. The experimental data are found to be in agreement with the designed parameters. The Doppler shift is measured by taking the ratio of the transmitted intensities from the dual etalon. Analysis of the accuracy of Doppler measurement is made by considering the factors of the number of iterations in converting the measured ratio to the Doppler shift, the laser intensity fluctuation and the signal-to-noise ratio. In the sufficient signal-to-noise ratio the velocity accuracy of the Doppler lidar system is estimated to be better than 0.5 m/s.     

Seed injection and frequency-locked Nd:YAG laser for direct detection wind lidar

The single longitudinal mode operation and frequency stability are essential for the laser transmitter used in the Doppler lidar. We devise a seed injection, frequency tunable and locked Q-switched Nd:YAG laser for the direct detection Doppler lidar. By implementation of the dual-wavelength seed laser and iodine-based PID frequency-locking technique, the frequency-stabilized seed laser is robust to interference and can be locked within 200 kHz for 3 h. The stable output of single longitudinal mode of the frequency-doubled pulsed laser makes it possible to achieve operational wind measurement.