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选择常见的Nd:YAG固体脉冲激光器的三倍频输出作为多纵模激光雷达发射源。Nd:YAG晶体的荧光线宽大多为4~6 cm−1,即120~180 GHz,在无种子光注入的情况下将出现多纵模同时输出。当纵模间隔取7.2 GHz时,最多可出现26个纵模。这里以美国Continuum公司Powerlite 9030产品为例,其无种子注入时的线宽为1 cm−1(30 GHz),辐射线宽内将会同时出现五个纵模。设定探测垂直距离分辨率为30 m@0~12 km和60 m@12~20 km,时间分辨率为1 min,白天天空背景光亮度取0.3 W·sr−1·m−2 ·nm−1@355 nm,激光雷达比取20。
大气参数采用1976美国标准大气模型,同时为了更合理地模拟实际大气,在标准气溶胶模型的4 km和9 km高度附近分别加入了模拟的淡积云和卷云。模拟得到的大气分子和气溶胶后向散射系数如图7(a)所示,对应的后向散射比如图7(b)所示。采用表1所示的系统参数,对基于双级联FPI的多纵模测温激光雷达系统的探测性能进行仿真。图8给出了在0~20 km高度,匹配误差和锁定误差引起的温度测量偏差。可以看出,在没有出现云层的高度上,该温度测量偏差很小,在2 km以上完全可以忽略不计;当ΔνFSR =5 MHz、Δν01=2.5 MHz时,温度测量偏差在4 km和9 km高度处达到极值,分别为0.92 K和0.71 K;当ΔνFSR=Δν01= 2.5 MHz时,温度测量偏差在4 km和9 km高度处分别为0.26 K和0.2 K。显然,在出现云层、沙尘等天气条件下,由匹配误差和锁定误差造成的对应高度的温度测量偏差将会较大。此时,减小匹配误差才会显得非常重要。图9(a)和图9(b)分别给出了由信号噪声引起的温度测量误差和后向散射比相对测量误差(εR/Rβ)随高度的变化廓线。从图9(a)可以看出:在0~20 km高度范围,白天和晚间的温度测量误差分别小于3.7 K和3.5 K。从图9(b)可以看出:在0~20 km高度范围,白天和晚间的后向散射比相对测量误差分别小于0.40%和0.38%。激光雷达系统在白天与夜晚均可保证较高的参数测量精度。
Figure 7. Profile of simulated atmospheric parameters with altitude. (a) Backscatter coefficients of atmospheric molecules and aerosols; (b) Backscatter ratio
Parameter Value Parameter Value Wavelength 355 nm Filter peak transmission >60% Laser mode number 5 Laser energy/pulse 400 mJ Laser mode linewidth 90 MHz Laser mode interval 7.2 GHz Telescope/scanner aperture 25 cm Laser repetition frequency 30 Hz Optical efficiency >85% Field of view 0.1 mrad FPI free spectral range 7.2 GHz FPE-1 and FPE-2 separation 3.6 GHz FWHM of FPI-1, FPI-2 0.8 GHz Defect finesse of FPI 24 Effective reflectivity of FPI 0.707 Loss coefficient of FPI 0.2% Actual reflectivity of FPI 0.725 Detector quantum efficiency 23% Solar filter bandwidth 0.5 nm Detector dark count 100 CPS Table 1. Parameters of multi-mode temperature lidar system based on two-stage FPI
Multi-longitudinal mode temperature lidar technology based on two-stage Fabry-Perot interferometer
doi: 10.3788/IRLA20220573
- Received Date: 2022-08-05
- Rev Recd Date: 2022-10-27
- Publish Date: 2023-05-25
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Key words:
- lidar /
- atmospheric temperature /
- multi-mode pulse laser /
- Fabry-Perot interferometer /
- Rayleigh scattering
Abstract: