光谱学与光谱分析, 2019, 39 (3): 673, 网络出版: 2019-03-19   

一种时间相关吸收光谱技术分析新方法

A Novel Analytical Method of Time-Dependent Absorption Spectroscopy
作者单位
1 浙江师范大学数理与信息工程学院, 浙江 金华 321004
2 浙江金华广福医院, 浙江 金华 321004
摘要
时间相关吸收光谱技术, 如腔衰荡光谱技术(CRDS)和腔衰减相移光谱技术(CAPS), 是近三十几年发展起来的一类新型吸收光谱检测技术, 它具有探测灵敏度高、 响应速度快、 不受光源强度起伏变化影响等优点。 传统的吸收光谱技术都是基于Lambert-Beer定律, 如直接吸收光谱技术(DAS)、 波长调制光谱技术(WMS)和腔增强吸收光谱技术(CEAS)等, 这类光谱技术在探测物质微弱吸收的时候一旦遇到较强的背景光信号就变得难以测量, 而且光源的不稳定性也会对检测带来一定的限制。 时间相关吸收光谱技术由于其不受光源强度起伏变化的特点, 在很大程度上能够弥补传统吸收光谱技术所存在的缺陷, 但其也有自身的局限性。 首先在理论上, CRDS和CAPS这两种时间相关吸收光谱技术并不统一, 而且在现有光谱理论下, Pulse-CRDS在应用时使用的脉冲光源的脉宽必须远小于谐振腔本身的时间常数, 对于长脉宽的脉冲光或者反射率低(小于99.9%)的腔体, 现有理论将不再适用; CAPS在应用时光源调制信号必须是周期性的正弦信号或者方波信号, 对于其他类型的周期调制信号或者非周期性信号, 现有理论并没有涉及。 针对上述提到的时间相关吸收光谱技术的局限性, 提出了一种新的分析时间相关吸收光谱技术的方法, 即利用一阶传递函数, 将谐振腔视为一阶传感系统, 对时间相关吸收光谱技术理论进行统一解释, 在公式推导上证明新方法下的推导结果和现有理论结果的一致性。 针对Pulse-CRDS, 以高斯脉冲光为例, 给出一阶传感理论下的透射光强表达式, 并对一系列不同的脉冲宽度γ、 谐振腔时间常数τreal以及从输出信号中拟合而得的时间常数τanal进行了模拟仿真。 经过分析比较后发现, 当γ<0.3τreal时, τanal和τreal的偏差小于1%; 当γ>0.3τreal时, τanal和τreal的偏差渐渐变大, 将不再满足实验条件。 为了使Pulse-CRDS在长脉宽脉冲光下也能应用, 本文给出了修正函数, 使得在脉宽大于腔衰荡时间0.3倍的情况下, 经过修正补偿后, 衰荡时间的误差小于1%。 对于CAPS系统, 搭建相应实验平台, LED中心波长选用405 nm, 使用方波调制信号, 测量不同频率下的入射参考信号与探测信号的相位差和探测信号峰-峰值, 通过由一阶传递函数推导而得的相频特性和幅频特性, 拟合得到时间常数τ, 结果分别为7.24和7.25 μs, 残差范围分别为[-0.01, 0.02]和[-0.02, 0.025], 两者结果基本一致。 实验结果验证了一阶传感系统理论完全适用于时间相关光谱的信号分析, 并且一阶传感系统理论还使得时间相关光谱技术的理论得到了统一。
Abstract
Time-dependent absorption spectroscopy, such as cavity ring-down spectroscopy (CRDS) and cavity attenuated phase-shift spectroscopy (CAPS), is a new type of absorption spectroscopy technology developed in recent thirty years. It has the advantage of high detection sensitivity, fast response, and not being affected by the fluctuation of light source intensity. Traditional absorption spectroscopy is based on the Lambert-Beer law, such as direct absorption spectroscopy (DAS), wavelength modulation spectroscopy (WMS), cavity enhanced absorption spectroscopy (CEAS) and so on. The weak absorption of material is hard to be measured once the background light signal is strong. And the instability of the light source also brings some limits to the detection. The time dependent absorption spectroscopy can make up for the shortcomings of traditional absorption spectrometry to a large extent because of its characteristics of not being affected by the fluctuation of light source intensity, but it also has its own limitations. First of all, CRDS and CAPS are not theoretically unified. And the existing theory can only apply to Pulse-CRDS with short pulse light where the pulse width is far less than the time constant of the resonant cavity itself. For long pulse-width light source or low reflectivity (less than 99.9%) cavity, the existing theory will no longer apply. As for CAPS, the modulation signal of the light source must be periodically sinusoidal or square-wave modulated signal. And there are no other types of periodic modulation signals and aperiodic signals mentioned. In view of the limitations of the time-dependent absorption spectroscopy mentioned above, we present a novel analytical method on time-dependent spectroscopy in this paper. The resonant cavity is regarded as a first-order sensing system. We use the first-order transfer function to unify the theory of time-dependent absorption spectroscopy and prove the consistency between the existing theoretical results and the derivation results under the novel method on the formula derivation. For Pulse-CRDS, we use Gaussian pulse light to derive the expression of transmitted light intensity under first-order sensing theory and simulate a series of different pulse widths γ, resonant cavity time constants τreal, and fitted time constants τanal. After analysis and comparison, we find the deviation of τanal and τreal is less than 1% when γ<0.3τreal. And the experimental conditions will be no longer sufficient when γ>0.3τreal. In order to make Pulse-CRDS used with long pulse-width light, a correction function is given in this paper. And the error of the corrected ring-down time is less than 1% when the pulse-width is 0.3 times greater than the ring-down time. For CAPS system, we build an experimental platform with LED light source centered at 405nm and square wave modulated. Then we measure the phase difference and the peak value at different frequencies. The time constants τ calculated respectively by the phase-frequency and amplitude-frequency characteristic derived from the first-order transfer function are approximately the same, 7.24 and 7.25 μs with residual ranges of [-0.01, 0.02] and [-0.02, 0.025] respectively. The result shows that the theory of the first-order sensing system is fully applicable to the signal analysis of time-dependent spectroscopy. And the theory of first-order sensing system also makes the theory of time-dependent spectroscopy unified.

周海波, 邵杰, 钱惠国, 应朝福, 张逸彪. 一种时间相关吸收光谱技术分析新方法[J]. 光谱学与光谱分析, 2019, 39(3): 673. ZHOU Hai-bo, SHAO Jie, QIAN Hui-guo, YING Chao-fu, ZHANG Yi-biao. A Novel Analytical Method of Time-Dependent Absorption Spectroscopy[J]. Spectroscopy and Spectral Analysis, 2019, 39(3): 673.

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