Understanding the sensitivity of cavity-enhanced absorption spectroscopy: pathlength enhancement versus noise suppression
Understanding the sensitivity of cavity-enhanced absorption spectroscopy: pathlength enhancement versus noise suppression
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DOI：
10.1007/s00340-012-5178-3
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发表时间：
2012-09
期刊：
影响因子：
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通讯作者：
Bin Ouyang;Roderic L. Jones
中科院分区：
文献类型：
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作者：
Bin Ouyang;Roderic L. Jones
腔增强吸收光谱技术作为一种高灵敏度的微弱光谱吸收观测技术已得到广泛应用。腔内的光子在反射率R接近1的反射镜之间来回反射，因此（平均而言）利用的吸收路径长度L比单程测量长1/（1 −R）。正如Beer-Lambert定律所表明的，L的增加导致吸光度A的增加（由α L给出，α是吸收系数），这反过来有利于检测弱吸收。然而，与此同时，只有（1 −R）的入射光可以进入腔[假设镜面透射率T等于（1 −R）]，因此由分子吸收引起的透射光强度降低Δ I等于实际上不存在腔时的情况。因此，CEAS获得的A = ΔI/I（其中I是总透射光强度）的增强不是来自ΔI的增加，而是I的急剧减小。本文计算了引入腔前后这两项的大小，旨在从这种可观测的角度（即Δ I和I）解释腔增强吸收光谱提供的灵敏度提高。它首先表明，存储在腔中的光子能量是在最好的输入光源一样强烈，这意味着任何吸收样品内的腔暴露于相同或甚至更低的光强度后，腔形成。因此，被样品吸收或散射的光的强度（其对应于前述Δ I项）永远不会大于单次通过测量的情况。然后示出，虽然该“分子”项没有改进，但“分母”项I显著减少;因此，在本发明中，对比度增加ΔI/I仅由透射的背景光I的衰减贡献，并且最终归结为与其相关的任何测量噪声的抑制。最有效抑制的噪声分量是其幅度与光成线性比例的类型强度I，这是由环境不稳定性引起的典型噪声，其次是散粒噪声，散粒噪声与I的平方根成比例。对于与I无关的噪声源，不能实现抑制，一个值得注意的例子是检测器或检测电子设备的热噪声。这种“噪声抑制”论点的有用性在于，它将腔体提供的灵敏度增益与系统中存在的测量噪声的性质联系起来，并且清楚地表明，可实现的灵敏度取决于腔体对各种噪声分量的“抑制”效率。
Cavity-enhanced absorption spectroscopy is now widely used as an ultrasensitive technique in observing weak spectroscopic absorptions. Photons inside the cavity are reflected back and forth between the mirrors with reflectivitiesRclose to one and thus (on average) exploit an absorption pathlengthLthat is 1/(1 −R) longer than a single pass measurement. As suggested by the Beer-Lambert law, this increase inLresults in enhanced absorbanceA(given byαLwithαbeing the absorption coefficient) which in turn favours the detection of weak absorptions. At the same time, however, only (1 −R) of the incident light can enter the cavity [assuming that mirror transmissionTis equal to (1 −R)], so that the reduction in transmitted light intensity ΔIcaused by molecular absorption equates to that would be obtained if in factno cavity were present. The enhancement inA= ΔI/I, whereIis the total transmitted light intensity, achievable from CEAS therefore comes not from an increase in ΔI, but a sharp decrease inI.In this paper, we calculate the magnitudes of these two terms before and after a cavity is introduced, and aim at interpreting the sensitivity improvement offered by cavity-enhanced absorption spectroscopy from this observable-oriented (i.e. ΔIandI) perspective. It is first shown that photon energy stored in the cavity is at best as intense as the input light source, implying that any absorbing sample within the cavity is exposed to the same or even lower light intensity after the cavity is formed. As a consequence, the intensity of the light absorbed or scattered by the sample, which corresponds to the ΔIterm aforementioned, is never greater than would be the case in a single pass measurement. It is then shown that while this “numerator” term is not improved, the “denominator” term,I, is reduced considerably; therefore, the increase in contrast ratio ΔI/Iis solely contributed by the attenuation of transmitted background lightIand is ultimately down to the suppression of any measurement noise that is associated with it. The noise component that is most effectively suppressed is the type whose magnitude scales linearly with light intensityI, as is typical of noise caused by environmental instabilities, followed by the shot noise which scales as square root ofI. No suppression is achievable for noise sources that are independent ofI, a notable example being the thermal noise of a detector or of detection electronics. The usefulness of this “noise suppression” argument is that it links the sensitivity gain offered by a cavity with the property of measurement noise present in the system, and clearly suggests that the achievable sensitivity is dependent on how efficient the various noise components are “suppressed” by the cavity.