基于NICE-OHMS技术进行大气压气样直接检测的理论分析

噪声免疫腔增强光外差分子光谱技术(NICE-OHMS)是目前世界上最灵敏的激光吸收光谱技术,其在低压环境中具有极高的探测灵敏度。然而当测量样品处于大气压时,NICE-OHMS系统的探测灵敏度会大幅下降。主要原因之一是大气压下获取最大NICE-OHMS信号幅度的条件与低气压下不同。通过对大气压NICE-OHMS理论进行分析,分析了影响信号幅度的参数,并通过数值模拟来寻找最佳的实验条件。本文着重讨论影响信号的主要参数包括光学腔腔长L,调制系数β,探测相位θ。其中,由于在NICE-OHMS中使用DeVoe-Brewer技术将调制频率ν_m锁定到Fabry-Parot(FP)腔的自由光谱区(FSR)。因此FP腔的腔长决定了ν_m,同时还作用于信号幅度S■。模拟结果显示,当腔长增大时,由于ν_m随之减小,载波和边带的光谱成分相互重叠部分增大,因此线型函数的幅度逐渐减小。而吸收信号幅度随着腔长的增加而逐渐增加,色散信号幅度先增大后减小,并且在腔长等于8 cm时达到最大值。调制系数β会影响频率调制后激光载波和边带的幅度大小,并且影响信号线型。随着腔长的增加,最大信号幅度对应的β值也随之增加。在相同腔长下,色散信号的最佳β值小于吸收信号,更容易使用电光调制器实现。最后分析了参数的可实现性,分析了不同种类激光器的频率调谐能力,压电陶瓷的扫描宽度等。以乙炔气体为例,大气压下NICE-OHMS的谱线半宽达到~3 GHz,而光谱覆盖范围大于10 GHz。分布反馈式半导体激光器(DFB)与外腔二极管激光器(ECDL)的频率调谐范围可以达到30 GHz以上,但是由于激光线宽宽,得到的PDH锁定性能欠佳。回音壁模式激光器(WGM)和掺饵光纤激光器(EDFL)线宽为百Hz量级,是目前高灵敏NICE-OHMS系统中常用的光源。但是WGM目前可以实现了5 GHz的激光频率调谐范围,而EDFL的外部电压可控制的调谐范围仅为3 GHz。使用精细度为55000的腔进行模拟,调制系数β=1,腔长大于8 cm时,可使用WGM激光器实现,腔长大于25 cm时,可以使用EDFL激光器实现。而对于在设计光学腔中常用的伸缩长度为25μm的PZT,随着腔长的增加,对应的腔模频移范围逐渐减小,在腔长为典型的40 cm时,扫描范围大于12 GHz。

Theoretical Analysis of Direct Measurement of Atmospheric Samples Based on NICE-OHMS Technology

Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy(NICE-OHMS)is the most sensitive laser absorption spectroscopy technology.It has obtained dozens of impressive detection sensitivities in a low pressure environment.However,when the measurement is taken at atmospheric pressure,its detection sensitivity becomes much worse.One of the dominant reasons is that the conditions for obtaining the maximum amplitude of NICE-OHMS signal at atmosphereare different from those under low pressure.In this paper,the theory of NICE-OHMS at atmospheric pressure is analyzed.The parametersthat affect the amplitude of the signal,including cavity length(L),modulation factor(β),and detection phase(θ),have been analyzed in order to figure out the best experimental conditions.Among them,due to the use of DeVoe-Brewer technology in NICE-OHMS,the modulation frequency(ν_m)is locked to the free spectral region(FSR)of the Fabry-Parot(FP)cavity.As a result,the cavity lengthnot only affects the NICE-OHMS signal amplitude,but also determines the value ofν_m.The results show that when the cavity length increases,the spectral components of the carrier and the sidebands overlap with each other due to the decrease ofν_m,which results in the decease of the amplitude of lineshape functions.And the amplitude of the NICE-OHMS signal at absorption phase increases gradually with the increase of the cavity length.While the amplitude at dispersion phase increases at the beginning,and reaches the maximum value when the cavity length is equal to 8 cm,then decreases with the cavity length.Modulation coefficientβaffects the magnitude of laser carrier and sidebands,as consequence,affects the signal lineshape.As the cavity length increases,theβvalue for the maximum signal amplitude also increases.At the same cavity length,theβvalue for the maximum amplitude of the dispersion signal is smaller than that for absorption signal,which is easier to achieve by using an electro-optic modulator.Finally,the feasibility of the parameters has been analyzed.The half-width at half-maximumof spectrum at atmospheric is determined by the pressure broadening,which isaround 3 GHz,and the spectral coverage is larger than 10 GHz.The frequency tuning range of the distributed feedback semiconductor laser(DFB)and external cavity diode laser(ECDL)can reach to 30 GHz,whiletheirlarge laser line width deteriorates the PDH locking performance.The line width of whispering wall mode laser(WGM)and erbium-doped fiber laser(EDFL),the conventional light source in high-sensitive NICE-OHMS system,is in the order of 100 Hz.However,as so far,the frequency tuning range of NICE-OHMS system based on WGM is only 5 GHz,while that based on EDFL is only 3 GHz.When the cavity length is longer than 8 cm,a WGM laser can be used.When the cavity length is greater than 25 cm,an EDFL laser can be used.For a PZT with a flexible length of 25μm,which is commonly used in the design of optical cavities,the frequency range of the corresponding cavity mode gradually decreases as the cavity length increases.At a typical cavity length of 40 cm,the frequency sweep range is greater than 12 GHz.