摘要
基于离轴积分腔输出光谱技术(OA-ICOS)设计并搭建一套近红外二氧化碳(CO_(2))和甲烷(CH_(4))传感系统,用于快速、实时和原位监测环境中的CH_(4)和CO_(2)。通过加入射频(RF)噪声源减少离轴积分腔输出光谱中的残余腔模式,从而提高了基于OA-ICOS的CH_(4)和CO_(2)传感系统的信噪比、精度和测量灵敏度。结果表明:加入RF噪声源后OA-ICOS系统的测量精度相对于无噪声源的系统提高了2.74倍。由Allan方差结果可知,在加入RF噪声源的OA-ICOS系统中,CO_(2)和CH_(4)的Allan方差值始终优于无噪声源的OA-ICOS系统,在1000 s时CO_(2)和CH_(4)的探测极限分别为0.55×10^(-6)和5.78×10^(-9),相对于无噪声源的OA-ICOS系统探测极限至少提高了3倍。在5 s的平均时间下,加入RF噪声源的系统中CH_(4)和CO_(2)的噪声等效灵敏度分别为1.70×10^(-9)cm^(-1)·Hz^(-1/2)和1.07×10^(-9)cm^(-1)·Hz^(-1/2)。此外,对大气中CH_(4)和CO_(2)浓度水平进行了为期4 d的连续监测,以验证所发展的CH_(4)和CO_(2)传感器系统的稳定性和可靠性。
Objective Carbon dioxide(CO_(2))and methane(CH_(4))are major atmospheric greenhouse gases.In recent years,due to the continuous development of human activities and industrial production,the global greenhouse effect caused by the rising levels of CO_(2)and CH_(4) in the atmospheric environment has seriously affected human life and health.Therefore,accurate concentration detection is of significance for both environmental monitoring and human health.The off-axis integrating cavity output spectrum technology(OA-ICOS),has been widely concerned for its simple experimental operation,strong anti-interference ability,high sensitivity,and in-situ real-time measurement.It is often adopted in atmospheric and environmental science,medical diagnosis,industrial production engineering,and other research fields.Even when the incident light is fully off-axis,OA-ICOS still has some residual cavity modes that cannot be eliminated and become the main noise of the system.Injecting a radio frequency(RF)noise source into a laser current is a new method to suppress cavity modes.We optimize the CO_(2)and CH_(4) dual gas sensing system of OA-ICOS using an RF signal source.Methods We study the influence of different power RF noise sources on CH_(4) absorption signals and select the best RF noise source power.In realizing simultaneous measurement of CO_(2)and CH_(4),the output lights of the two DFB lasers are combined into a beam through a fiber coupler and coupled into the cavity.However,the time-division multiplexed scanning signal is designed by software to realize multiplex signal transmission and avoid interference between the signals during measurement.The RF noise source is injected into the near-infrared(NIR)distributed feedback laser,and the time-division multiplexing method is employed to collect dual gas signals at the same time for maximizing the signal-to-noise ratio(SNR)of the signals and the detection limit of the system.Meanwhile,we establish an OA-ICOS sensing system combined with TDM-DAS.Results and Discussions As shown in Table 1,the influence of different RF noise power values on the CH_(4) absorption spectrum is studied by analyzing SNR and absorption line width of the CH_(4) absorption spectrum.Considering the influence of SNR and broadening on absorption lines,−20 dBm is chosen as the white noise power of the system.The OA-ICOS systems without a noise source and with a−20 dBm RF noise source are utilized to measure CO_(2)and CH_(4) continuously for a long time.The stability and measurement accuracy of the two OA-ICOS systems are evaluated according to the experimental results of the two groups.As shown in the left side of Fig.5(a)and Fig.6(a),the average volume fraction of CO_(2)concentration is 5.8157×10^(-4) and 5.9895×10^(-4) in the system without a noise source and with an RF noise source.The average volume fraction of CH_(4) is 2.24×10^(-6) and 2.25×10^(-6).As shown in the right side of Fig.5(a)and Fig.6(a),the measurement accuracy of CO_(2)is 40.5200×10^(-6) and 14.7500×10^(-6) in the system without a noise source and with an RF noise source,respectively.The measurement accuracy of CH_(4) is 0.2716×10^(-6) and 0.0997×10^(-6) in the system without a noise source and with an RF noise source,respectively.Compared with the system without an RF noise source,the system measurement accuracy of the system with an RF noise source is increased by 2.74 times.Figs.5(b)and 6(b)show the analysis of Allan variance results.In the systems without a noise source and with an RF noise source,the CO_(2)detection limits at 1000 s are 1.85×10^(-6) and 5.50×10^(-7),with the detection limits of CH_(4) at 1000 s being 1.61×10^(-8) and 5.78×10^(-9).The Allan variance values of CH_(4) and CO_(2)are always lower for OA-ICOS with an RF noise source than OA-ICOS without a noise source,and the system detection limit is at least three times higher.Adding an RF noise source can improve the stability and detection limit of the OA-ICOS system.Under the average time of 5 s,the NEAS of CH_(4) and CO_(2)in the system without adding a noise source is 4.98×10^(-9) cm^(-1)·Hz^(-1/2) and 2.14×10^(-9) cm^(-1)·Hz^(-1/2) respectively.By adding an RF noise source,the NEAS in the system for CH_(4) and CO_(2)is 1.70×10^(-9) cm^(-1)·Hz^(-1/2) and 1.07×10^(-9) cm^(-1)·Hz^(-1/2) respectively.Conclusions We present a near-infrared OA-ICOS dual gas detection sensing system for continuous and real-time CO_(2)and CH_(4) detection.By adding an RF noise source to the laser drive current,the cavity mode noise is suppressed,and the SNR,accuracy,and measurement sensitivity of the OA-ICOS system are enhanced.The results show that the measurement accuracy of the OA-ICOS system with an RF noise source is improved by a factor of 2.74 relative to that of the system without a noise source.According to the analysis of Allan variance results,in OA-ICOS systems with an RF noise source,the Allan variance values of CO_(2)and CH_(4) are always better than those without noise sources,and the detection limits of CO_(2)and CH_(4) at 1000 s are 5.50×10^(-7) and 5.78×10^(-9).The system detection limit is at least three times higher than that without noise sources.Under the average time of 5 s,the noise equivalent sensitivities of CH_(4) and CO_(2)in the system with an RF noise source are 1.70×10^(-9) cm^(-1)·Hz^(-1/2) and 1.07×10^(-9) cm^(-1)·Hz^(-1/2) respectively.Additionally,the CH_(4) and CO_(2)concentrations in the atmosphere are continuously monitored for four days to verify the stability and reliability of this system.
作者
李文婷
吴涛
闫宏达
高萌钒
张珂豪
李志林
Li Wenting;Wu Tao;Yan Hongda;Gao Mengfan;Zhang Kehao;Li Zhilin(School of Measuring and Optical Engineering,Nanchang Hangkong University,Nanchang 330063,Jiangxi,China)
出处
《光学学报》
EI
CAS
CSCD
北大核心
2023年第24期127-135,共9页
Acta Optica Sinica
基金
国家自然科学基金(61965013,42175130)
江西省自然科学基金重点项目(20232ACB202002)
江西省高层次高技能领军人才培养工程。
关键词
大气光学
直接吸收光谱
离轴积分腔
射频噪声源
双气体探测
时分复用
atmospheric optics
direct absorption spectrum
off-axis integrating cavity
RF noise source
dual gas detection
time-division multiplexing
