3.93μm激光外差光谱系统光学结构设计与N_(2)O测量

Optical Structure Design of 3.93μm Laser Heterodyne Spectrometer and N_(2O)Measurement

激光外差光谱(LHS)系统具有光谱分辨率高、探测灵敏度高和体积小等特点,在高分辨率太阳光谱和整层大气透过率测量等应用中受到关注。以3.93μm分布反馈式带间级联激光器(DFB-ICL)作为本振光源搭建了激光外差光谱测量系统,以太阳光作为信号光,利用Zemax光学仿真软件对太阳光与激光的耦合进行设计和仿真。基于开普勒望远镜原理设计了太阳光整形结构。光斑整形后有效提高了外差耦合效率,将系统信噪比提高至162.1,比无整形条件下获得的信噪比提高了一倍。利用搭建的激光外差系统实测了合肥地区整层大气N_(2)O吸收光谱,采用最优估算法反演了N_(2)O的柱浓度,均值为0.311×10^(-6),反演结果与EM27/SUN的实测结果进行比较,两种方式获得的N_(2)O柱浓度相关性为0.856。测量结果表明,激光外差光谱测量系统的光学结构经设计和优化后,光学耦合效率和系统信噪比均得到有效提升,基于实测太阳光谱反演获得的N_(2)O柱浓度也与商用仪器观测结果具有较好的一致性。

Objective Laser heterodyne spectrum measurement technology has the characteristics of high spectral resolution,high detection sensitivity and short sampling time.This technology can not only obtain the high-resolution spectral information of the whole layer of atmospheric molecules,but also facilitate the observation of gas concentration in different scenarios due to its small size and easy integration.Therefore,it has been widely concerned by researchers.At present,domestic research on laser heterodyne spectrum measurement system mainly focuses on solar tracking,spectral resolution and inversion algorithm.Due to the low power of the detection signal received by the heterodyne system,the signal-to-noise ratio(SNR)of the heterodyne system is low in actual operation.Therefore,based on the principle of Kepler telescope,a set of sunlight beam shaping structure is designed to improve the sunlight power,and the system SNR is improved by matching the size of the two beams.Methods In this paper,a 3.93μm distributed feedback interband cascade laser(DFB-ICL)is used as the local oscillator light source to build a laser heterodyne spectrum measurement system,and sunlight is used as the signal light.The coupling of sunlight and laser is designed and simulated by Zemax optical simulation software.In the experiment,considering the distance between the shaping lens and the photosensitive surface of the detector,the coupling efficiency and the performance of the detector,the incident sunlight diameter is set to 4.5 mm,the plane-convex lens 1 with a focal length of 750 mm and the plane-convex lens 2 with a focal length of 500 mm are selected to form the Kepler telescope structure for shaping the sunlight.The absorption spectrum of N_(2)O in the range of 2542.9-2545.0 cm^(-1)is measured by studying the beam reduction rule of the sunlight and the SNR of the system.The optimal estimation method is used to inverse the measured spectra,and the N_(2)O column concentration is obtained.Finally,the inversion results of the laser heterodyne spectrum measurement system and the commercial Fourier transform spectrometer are compared and analyzed.Results and Discussions After the laser is emitted from the collimator,the spot diameter is 3.0 mm.Zemax simulation software is used to simulate the structure for sunlight beam reduction.After the simulation and optimization of Kepler telescope structure,the input parameters are as follows:the distance between the two lenses is 1320.155 mm,and the radius of sunlight after 1.5 times beam reduction is 1499.76μm(Fig.7).These meet the required spot size requirements.The diameter of the sunlight spot before beam shaping is 4.5 mm,and the SNR of the system is 80.6 when compared with the laser beat frequency result of 3.0 mm diameter sunlight spot.In this case,the size of the two spots can be matched while improving the sunlight power.The optical power of the two beams at the beat frequency is fully utilized,and the heterodyne coupling efficiency is the best.Therefore,the SNR of the system can reach the highest,which is 162.1[Fig.9(a)].According to the best SNRs of the system with the diameter of 2.2,2.5,3.0,3.4 and 4.1 mm after the sunlight beam is reduced through the lenses,the more matched the diameters of the two facula,the higher the SNR of the system(Fig.8).The N_(2)O absorption spectrum in the range of 2542.9-2545.0 cm~(-1)was measured before and after the beam shaping structure was added.There are two N_(2)O absorption spectral lines in this band[Fig.9(b)].The measurement results show that the amplitude of the spectral signal after beam shaping is significantly improved when only the size of the sunlight spot is changed,which can provide more accurate spectral data for the subsequent inversion of N_(2)O concentration profile and column concentration.Comparing the measured spectrum with the inversion fitting spectrum,the residual error of the two curves is within±0.08 V(Fig.10).The N_(2)O column concentration results obtained by the laser heterodyne spectrum measurement system are compared with the measured results of commercial Fourier transform spectrometer EM27/SUN.The variation trend of N_(2)O concentration measured by the two methods is relatively consistent,and the measurement results obtained using the two methods show a correlation coefficient of 0.856(Fig.12).Conclusions In this paper,a set of high-resolution laser heterodyne system is built with a 3.93μm laser as the local oscillator light source,the sunlight is taken as the signal light,and the Kepler telescope structure and Zemax optical simulation software are used to shape the sunlight,so that the size and focus angle of the light spot incident on the photosensitive surface of the detector are smaller than the effective receiving area and field of view of the detector,respectively.The beam reduction of the sunlight in the free space and the size matching of the two spots on the photosensitive surface are realized.The experimental results show that the single-pass SNR of the system is up to 162.1 after the sunlight is shaped and matched with the laser beam,which is twice as high as that of the system without beam shaping.At the same time,the absorption spectrum of N_(2)O was measured,the optimal estimation method was used to realize the inversion of N_(2)O column concentration,and the inversion results were compared with those measured by the Fourier transform spectrometer EM27/SUN.The variation trend of N_(2)O column concentration obtained by the two methods is relatively consistent,and the measurement results obtained using the two methods show a correlation coefficient of 0.856.Through the research on the 3.93μm laser heterodyne spectrum measurement system,the main factors affecting the SNR of the heterodyne optical path are grasped.The follow-up research will be carried out on the system signal processing and instrument linear function optimization to further improve the SNR and provide favorable conditions for the subsequent high-sensitivity detection of greenhouse gases in the atmosphere.