1_5_m激光雷达相干探测CO_2与风场系统设计与仿真_英文_

文章编号 :100425929 (2008) 0120091206 1 . 5μm激光雷达相干探测 CO2 与风场系统 设计 领导形象设计圆作业设计ao工艺污水处理厂设计附属工程施工组织设计清扫机器人结构设计 与仿真 陶小红 , 胡以华 , 赵楠翔 , 雷武虎 (电子工程学院 , 合肥 230037) 摘 　要 : 探测低空大气 CO2 浓度的同时可以探测大气风场。采用相干探测较非相干探测具有更高的信噪 比 ,而目前 1. 5μm 波长由于在人眼安全、系统设计简单廉价等方面存在一定优势 ,使 1. 5μm 可能成为未 来探测气溶胶激光雷达的主流波长。本文阐述了激光雷达相干探测大气 CO2 与风场的原理 ,设计了 1. 5 μm 相干探测激光雷达系统 ,并对系统的信噪比进行了估算 ,得出结论 :1. 5μm 相干探测风场与 CO2 是可 行的 ,经过 3 分钟的脉冲积累 ,在 3km 处仍具有高于 10 的信噪比值。 关键词 : 相干探测 ;激光雷达 ;风场 ;CO2 浓度 中图法分类号 : TN958 　　　文献标识码 : A Design and Simulation of Coherent Lidar System for Measurements of CO2 and wind f ield with 1. 5 Microns TAO Xiao - hong , HU Yi - hua , ZHAO Nan - xiang , L EI Wu - hu ( Elect ronic Engineering Instit ute , Hef ei 230037 , China) Abstract : Concentration of CO2 and wind field in low altitude can be measured by coherent li2 dar simultaneously , with better signal - to - noise ratio ( SNR) than incoherent detection. At present the 1. 5 microns laser is both eye - safe and low - cost for building a lidar system . The 1. 5 microns wavelength will also be the best candidate for the future aerosol lidar. In this pa2 per , the principles of detecting concentration of CO2 and wind field with coherent lidar are de2 scribed and the design details of coherent lidar with 1. 5 microns are given. From the simulated SNR , it can be concluded that heterodyne detection of wind field and concentration of CO2 with 1. 5 microns is feasible and the estimated SNR at 3km distance in daylight can be better than 10 for 3 minutes integraton. Key words : Heterodyne detection ; Coherent lidar ; Wind field ; Concentration of CO2 收稿日期 : 2007207231 ; 收修改稿日期 : 2007210218 作者简介 : 陶小红 ,男 ,汉族 ,1980 - ,电子工程学院博士研究生 ,研究方向为空间信息处理. 通信地址 :安徽省合肥市电子工程学 院 506 室 邮编 :230037. E2mail : txheei @163. com1 　IntroductionConcentration of CO2 in atmosphere is closelylinked with our living environment and has be2come a concern to scientists because of its“Green2 house Effect”. Many characteristics of atmospher2ic CO2 are still not well known , thus it is neces2sary to continue to carry out measurements of CO2concentration profiles using lidar technology. . Thewind field’s detection is also important to avia2 第 20 卷 　第 1 期 光 　散 　射 　学 　报 Vol120 　No11 2008 年 3 月 THE JOURNAL OF L IGHT SCATTERIN G Mar1 2008 © 1994-2008 China Academic Journal Electronic Publishing House. All rights reserved. http://www.cnki.net tions , in application areas such as atmospheric turbulence and wind shear warning to aircrafts. It has been shown previously that coherent lidar can be used to detect concentration of CO2 and wind field simultaneously[1 ] . The use of laser wave2 length at 1. 5 microns has merits of high eye safe2 ty threshold of 1 J / cm2 , no photochemistry dam2 age and low sky radiance interference. Based on 1. 5 microns fiber laser , the lidar system is simple and stable. So 1. 5 microns will be a popular wavelength of aerosol lidar in the future[2 ] . 2 　Heterodyne detection of wind f ield and con2 centration of CO2 　　 Heterodyne detection can provide better SNR than direct detection and its noise - equiva2 lent - power (N EP) is close to the theoretical de2 tection limit . Intermediate f requency ( IF) signal of heterodyne detection can shorten the receiver bandwidth so that f requency detection is easy[3 ] . Heterodyne detection of wind field is based on laser Doppler theory[4 ] . When the transmitted laser signal is backscattered by the atmosphere , the backscattered photons will have frequency shift. The frequency shift is in direct proportion to the wind radial velocity , i. e. Δv = 2 V c v (1) Where , v is the laser f requency , V is wind radial velocity , c is light velocity andΔv is Doppler f re2 quency shift . The return signal will be mixed with local oscillator (LO) laser beam at the detec2 tor. After f requency differentiation , the wind ve2 locity can be obtained. With the 1. 5 microns fiber laser , the transmitted signal power can be ampli2 fied by an erbium doped fiber amplifiers ( EDFA) . Detection of the concentration of CO2 is based on differential absorption lidar ( DIAL ) . After the laser pulses are transmitted and scat2 tered by aerosol , the return signal and LO laser beam are mixed and detected. After the mixed signals are processed , the concentration of CO2 can be obtained. The principle of DIAL consists of using two different wavelengths with the first one in resonance with the peak of an absorption line (λon) of CO2 and the second one at the line wing (λof f ) . The two wavelengths must be care2 fully chosen to prevent interference from other at2 mospheric molecule absorption lines , minimize temperature dependence , and optimize optical depth. The absorption curve of CO2 near 1. 5 mi2 crons is shown in Fig. 1. In this paper the wave2 lengths are chosen to be 1571. 276 nm (λon ) at the peak of absorption curve and 1571. 434 nm (λof f ) at the nearest line wing. Fig. 1 　Absorption curve of CO2 near 1. 5 microns The lidar return signals can be written as[5 ] : P( R) on = P0 (λon)ηβ(λon , R)ΔR ( A / R2) · exp [ - 2∫R0 [ N w ( z )σw (λon) +α(λon , z ) ]d z ] (2) P ( R) of f = P0 (λof f )ηβ(λof f , R)ΔR ( A / R2) · exp [ - 2∫R0 [ N w ( z )σw (λof f ) +α(λof f , z ) ]d z ] (3) Where , ΔR is the spatial resolution , N w is the concentration of CO2 , R is the detection distance , P( R) x ( x denotes on or of f ) is the return power from R to R +ΔR , P0 (λ) is the transmitted laser power , η is the optical efficiency , A is the effective receiver area , and β(λ, R) is the backscattering coefficient . The 29 光 　　散 　　射 　　学 　　报 　第 20 卷 © 1994-2008 China Academic Journal Electronic Publishing House. All rights reserved. http://www.cnki.net atmospheric extinction of the laser includes atmospheric scattering and absorption extinction , i. e. N w ( z )σw (λ, z ) + α(λ, z ) ·σw (λ) is the absorption cross section of CO2 , α(λ, z ) is the atmospheric absorption coefficient besides CO2 . Because λon is very close to λof f , it can be shown that : β(λon , R) = β(λof f , R) , α(λon , z ) = α(λof f , z ) After division of Eq. 2 by Eq. 3 , and taking the logarithmic and differential operation , the concentration of CO2 will be obtained from equation 4 below [6 ] . N w ( R) = 12[σw (λon) - σw (λof f ) ] · d d R ln p ( R) of f p ( R) on (4) 3 　System design of coherent measurements of wind and CO2 　　Shown in Fig. 2 is the coherent lidar system for detection of wind field and concentration of CO2 . In the transmission portion , an external cavity diode laser ( ECDL) is used as the master oscillator of λon , with wavelength at 1571. 276 nm. The ECDL is composed of a diode laser , a collimation lens , a diff raction grating and a back reflection mirror. One surface of the diode laser is a full - reflecting film is coated while the other surface a half - reflecting. The full - reflecting film and the back reflection mirror formed a reso2 nance cavity. The diff raction grating is used not only for selecting the frequency but also to give a feedback to compress the line - width of laser. In order to stabilize the λon , a portion of light is t ransmitted from the coupler to the wavelength - meter to monitor the changes of the laser wave2 length. Actually the wavelength - meter is a stan2 dard absorption cell filled with atmospheric CO2 , and the feedback signal can be used to adjust the wavelength of ECDL so that the laser can be locked at the center of absorption peak of CO2 . A dist ributed feedback laser (DFB) is used as the master oscillator of λof f , whose wavelength is 1571. 434nm. DFB is a complicated servo device in a feedback frequency - locking configuration and composed of an etalon and a photodiode , and a cooler with heat - sensitive resistance. The spectral width of DFB is wider than ECDL , but much lower price. Fig. 2 　Schematic diagram of coherent detection system 39第 1 期 陶小红 : 1. 5μm 激光雷达相干探测 CO2 与风场系统设计与仿真 © 1994-2008 China Academic Journal Electronic Publishing House. All rights reserved. http://www.cnki.net 　　 In this system the output of DFB and ECDL are continuous wave ( CW) laser. The two CW lasers are turned into a pulse lasers by using a2 cousto - optic modulators ( AOM) . The control f requency of the two AOMs are set to 10 KHz and their pulses are alternanting. In order to avoid disturbances among echo pulses , the pulses should not be very close together , so the repetition fre2 quency is set below 20 KHz. The switches are controlled to alternate between the λon and λof f pulses. Narrow - band filter is also used to filter out the sky background noise. As LO signal , a portion of the laser light f rom the master oscillator is t ransmitted by a long fiber and mixed with the return signal at the detector. After the signal am2 plification , and digitization , the acquired data are processed , the concentration of CO2 are calculated from the mixed signal. After f requency differenti2 ation , wind velocity can also be obtained. In GaAs PIN photodiode is used as the detector. The length of the long fiber should be chosen to be about the same as the detection distance for better coherence. 4 　Simulation of system Heterodyne detection needs two balance de2 tectors. That is to say the two photodiodes should be identical. When LO signal power is higher than background noise power , the noise of hetero2 dyne detection mainly comes from shot noise. As2 suming that the responsivity is χ, the bandwidth of detector is B , LO signal power is Pl , echo sig2 nal power is Ps , background radiance power is Pb , the number of pulses accumulation is M , work temperature is T and resistance is R , the SNR of heterodyne detection can be written as[7 ] S M R = M · 　 (2χ Ps Pl ) 2 2 B [χe ( Ps + Pl + Pb) + 2 K T/ R) ] (5) Where e is electronic charge , K is Boltzman con2 stant . The echo signal power can be written as , Ps ( R) = P0 cτ2 η′β( R) ( A / R 2) · exp [ - 2∫R0 [ N w ( z )σw (λ) +α(λ, z ) ]d z ] (6) Where η′is optical efficiency (quantum efficiency not included) , c is the speed of light , τ is pulse duration. Atmospheric extinction at 1. 5 microns can be deduced from that of 532 nm. As has studied , atmospheric molecular backscattering coefficient of 532 nm can be written asβm ( z ) = 1 . 54 ×10 - 3 exp ( - z / 7) and aerosol backscattering coefficient can be described as βa ( z ) = 2 . 47 ×10 - 3 exp ( - z / 2) + 5. 13 ×10 - 6exp ( - ( - z - 20) 2/ 36) . At2 mospheric molecular extinction coefficient can be expressed asαm ( z ) =βm ( z ) ×8π/ 3 and aerosol extinction coefficient can be given by αa ( z ) =βa ( z ) ×50 [8 ] . The relationship among molecular backscattering coefficient , aerosol backscattering coefficient and wavelength can be shown as[9 ] βa (λ1 , z ) βa (λ2 , z ) = λ1 λ2 - 1 (7) βm (λ1 , z ) βm (λ2 , z ) = λ1 λ2 - 4 (8) And the relationship between backscattering coefficient of 1571 nm and that of 532 nm is given by βa (1571 , z ) = 0 . 34 ×βa (532 , z ) , βm (1571 , z ) = 0 . 0131 ×βm (532 , z ) From the above equations , the atmospheric backscattering coefficient and absorption coefficient of 1571 nm can be written as , 　β( R) = 0 . 8398 ×10 - 3exp ( - R/ 2) + 1 . 74 ×10 - 6exp ( - ( R - 20) 2/ 36) + 2 ×10 - 5exp ( - R/ 7) (9) 　α( z ) = 4 . 2 ×10 - 2exp ( - z / 2) + 8 . 7 ×10 - 5exp ( - ( z - 20) 2/ 36) + 1 . 67 ×10 - 4exp ( - z / 7) (10) In order to simulate the echo signal , concentration of atmospheric CO2 at low altitude can be given by , N w ( z ) = N 0exp ( - z / 7) (9) Where , N 0 is 1. 048 × 1016 (cm - 3) [10 ] . The absorption cross sections of CO2 at the wavelength 49 光 　　散 　　射 　　学 　　报 　第 20 卷 © 1994-2008 China Academic Journal Electronic Publishing House. All rights reserved. http://www.cnki.net of λon andλof f are 6. 36 ×10 - 23 (cm2) and 4. 56 × 10 - 24 (cm2) respectively[11 ] . The background radiance power can be expressed as , Pb = A π 4θ 2η′S b (λ)Δλ (10) Where ,θ is the field of view ( FOV) , S b (λ) is background spectrum radiance and Δλ is the bandwidth of narrow - band filter. With the typi2 cal parameters given by IPG Company , the SNR of this lidar system is estimated. System parame2 ters are given in Table 1. Table 1 　Parameters of lidar system Parameter Value Parameter Value Output power of EDFA 5w LO power 2. 5mw Pulse duration 100ns Repetition rate 10 KHz Diameter of telescope 350mm Optic efficiency 0. 5 FOV 0. 5mrad Detection responsivity 0. 95A/ w Bandwidth of filter 1nm Detection bandwidth 50MHz Background spectrum radiance 1w/ m2srum Pulse accumulation time 3min Temperature 273 K Resistance 50Ω 　　 The simulated SNR of heterodyne detection as a function of distance are shown in Fig. 3. The solid curve corresponds to the SNR of λon and the dashed curve is that ofλon . From Fig. 3 , it can be seen that the SNR at 3 km distance in daylight is still better than 10 after 3 minutes of pulses accu2 mulation. The SNR will be improved for longer integation , measurement taken at night or detec2 tors with better sensitivities are used. Fig. 3 　Variation of SNR with distance after 3 min pulse accumulation References : [ 1 ] 　Grady J . Koch , Bruce W. Barnes , Mulugeta Petros , et al. Coherernt differrntial absorption lidar measure2 ments of CO2 [J ] . Applied Optics , 2004 ,43 (26) : 5092 - 5099. [2 ] 　G G Gimmestad , D W Roberts. 1. 5 microns : the future of unattended aerosol lidar [J ] . IEEE , 2004 : 1944 - 1946. [3 ] 　Wu Song - hua. Key technologies of high spectral resolution wind measurement by laser with high sta2 bility[D ] . Qingdao : China Ocean University , 2004. [4 ] 　Dai Yong - jiang. Principle of lidar [ M ] . Beijing : National Defence Industry , 2002. [5 ] 　Yan Ji - xiang , Gong Shun - sheng , Liu Zhi - sheng. Lidar of environmental survey[ M ]. Beijing : Science , 2001. [6 ] 　Sun Jing - qun. Atmospheric measurements by laser [ M ] . Beijing : Science , 1986. [7 ] 　Xiong Hui - feng. Laser radar [ M ] . Beijing : Space Navigation , 1992. [8 ] 　Y Sasano , T Kobayashi. Feasibility study on space lidars for measuring global atmospheric environment [ Z] . 1995. 59第 1 期 陶小红 : 1. 5μm 激光雷达相干探测 CO2 与风场系统设计与仿真 © 1994-2008 China Academic Journal Electronic Publishing House. All rights reserved. http://www.cnki.net [9 ] 　Markus Henriksson. Detection probabilities for photon - counting avalanche photodiodes applied to a laser radar system[J ]. Applied Optics , 2005 , 44 ( 6) : 5140 - 5146. [10 ] 　Hong Guang - lie , Zhang Yan - chu , Hu Shun - xing. Near infrared micro pulse lidar of profiling at2 mospheric CO2 [ J ] . Journal of Infrared and Mil2 limeter Waves , 2004 , 23(5) : 384 - 388. [11 ] 　Paolo Francesco Ambrico , Aldo Amodeo , Paolo Di Girolamo , Nicola Spinelli. Sensitivity analysis of differential absorption lidar measurements in the mid - infrared region [J ] . Applied Optics , 2005 , 39 (36) : 6847 - 6865. [12 ] 　Andrew Y S Cheng , Peter Voelger. Monte carlo simulation of lidar return signals I multiple scatter2 ing intersities from homngenous haze [J ] . Chinese Jounal of Light Scattering , 2001 , 13 (2) : 70 - 77. 69 光 　　散 　　射 　　学 　　报 　第 20 卷 © 1994-2008 China Academic Journal Electronic Publishing House. All rights reserved. http://www.cnki.net