通过实施光学条纹噪声抑制方法的激光波长调制光谱技术实现气体测量的高精度和高灵敏度检测

通过实施光学条纹噪声抑制方法的激光波长调制光谱技术实现气体测量的高精度和高灵敏度检测近日，来自安徽理工大学、安徽皖西学院、复旦大学大气与海洋科学学院、上海期智研究院的联合研究团队发表了《通过实施光学条纹噪声抑制方法的激光波长调制光谱技术实现气体测量的高精度和高灵敏度检测》论文。

Recently,thejointresearchteamfromAnhuiUniversityofScienceandTechnology,WestAnhuiUniversity,DepartmentofAtmosphericandOceanicSciences,FudanUniversity,ShanghaiQiZhiInstitutepublishedan

academicpapersHighprecisionandsensitivitydetectionofgasmeasurementbylaserwavelengthmodulationspectroscopyimplementinganopticalfringenoisesuppressionmethod.

可调谐二极管激光吸收光谱(TDLAS)已被开发用于痕量气体测量，因其高精度、高灵敏度和无需任何样品准备的原位自校准的独1特优势。通常，长光程的多次通过腔体(MPC)被应用于增强基于TDLAS的传感器的检测精度和灵敏度。然而，MPC中出现的意外光学干涉纹严重影响了传感器的检测精度和灵敏度。基于MPC的TDLAS传感器的检测精度和灵敏度通常受到光学干涉纹的限制，这些干涉纹由衍射、镜面表面瑕疵的散射、镜面畸变、热膨胀、冷收缩或应力变形引起。因此，MPC中观察到的光学干涉纹由不同的光学干涉纹组成。这些光学干涉纹主要是由于少量的激光以与主激光束相差ΔL的光程到达探测器所致。这些问题对于TDLAS是普遍存在的，尤其是在使用密集重叠斑点模式的MPC时，提出了一些不同的方法来消除光学干涉纹的负面影响。

TheTunableDiodeLaserAbsorptionSpectroscopy(TDLAS)hasbeendevelopedfortracegasmeasurement,asitsuniqueadvantagesofhighprecision,highsensitivityandself-calibrationinsituqualificationwith-outanysamplepreparation.Themulti-passcell(MPC)withalongopticalpathisusuallyappliedtoenhanceTDLAS-basedsensor’sdetectionprecisionandsensitivity.However,theunexpectedopticalfringesoccurringintheMPCoftenspoilthesensor’sdetectionprecisionandsensitivityseriously.ThedetectionprecisionandsensitivityoftheTDLAS-basedsensorscontaininganMPCareoftenlimitedbytheopticalfringesthatresultfromdiffraction,scatteringonthemirrorsurfaceimperfections,mirroraberration,thermalexpansion,coldcontraction,orstressdeformation.Therefore,thecomplexopticalfringeconsistingofdifferentopticalfringewillbeobservedintheMPC.TheseopticalfringesareduelargelytoasmallamountoflaserreachingthedetectorwithanopticalpathlengthdifferingbyΔLfromthemainlaserbeam.ThoseproblemsarecommonforTDLAS,especiallyusingdenseoverlappedspotpatternMPCandsomedifferentmethodsareproposedtoeliminatethenegativeinfluenceoftheopticalfringes.

研究团队提出了一种抑制可调二极管激光吸收光谱中光学条纹噪声的新方法，并将其应用于由光学条纹扰动的CH4气体传感器，以提高检测精度和灵敏度。

Inthiswork,anovelmethodtosuppressopticalfringenoiseinthetunablediodelaserabsorptionspectroscopyisproposedandappliedtotheCH4

gassensorperturbedbyopticalfringesforhigherdetectionprecisionandsensitivity.

所开发的CH4检测仪的示意图如图1所示。宁波海尔欣光电科技有限公司为此项目提供锁相放大器(HPLIA微型双通道调制解调锁相放大器)，从光电探测器输出的信号发送到锁相放大器，锁相放大器相对于同步信号对2f模式进行解调，锁相放大器的时间常数设为1ms。

TheschematicdiagramofthedevelopedCH4

detectioninstrumentisshowninFig.1.HealthyPhotonCo.,LtdprovidedaHPLIAMiniaturedual-channelmodulateddemodulationlock-inamplifierforthisproject.Thelock-inamplifierdemodulatesthesignalinthe2fmodewithrespecttothesyncsignal.Thetimeconstantofthelock-inamplifierissetto1ms.

Fig.1.SchematicdiagramofthedevelopedCH4detectionsystem

lock-inamplifier(HealthyPhoton,HPLIA)

对于被光学条纹和随机噪声干扰的20ppmCH4的二次谐波(2f)信号，通过该新方法，2f信号的信噪比(SNR)从17提高到182，优化平均光谱范围Δ���。与未经处理的原始信号相比，CH4测量精度改善了约1.5倍。相应的最小可检测浓度可从3ppb改善到0.78ppb。系统的相应噪声当量吸收灵敏度(NNEA)和噪声当量浓度(NEC)分别为6.13×10-11cm−1

WHz−1/2

and0.181ppm。

Forthe2nd

harmonic(2f)signalof20ppmCH4

spoiledbyopticalfringesandrandomnoise,bythenovelmethod,thesignal-to-noiseratio(SNR)ofthe2fsignalisimprovedabout6.5timesfrom17to182withanoptimalaveragingspectralrangeΔ���.A∼1.5timesimprovementinthemeasurementprecisionofCH4

isachievedcomparedtounprocessedrawsignal.Thecorrespondingminimumdetectableconcentrationcanbeimprovedfrom3ppbdownto0.78ppb.Thecorrespondingnoiseequivalentabsorptionsensitivity(NNEA)andthenoiseequivalentconcentration(NEC)ofthesystemis6.13×10-11cmW-1Hz-1/2

and0.181ppm,respectively.

Violetlinefromtraditionalaveragingmethodandmagentalinefromthenovelopticalfringenoisesuppressionmethod.

Histogramplotofthe20ppmCH4deviation.

20ppmCH4Allan-deviationstabilityofdevelopedoverlappedspotpatternMPC.

参考文献：

Reference:

YananCao,XinCheng,ZongXu,XingTian,GangCheng,FeiyanPeng,JingjingWang

Highprecision