Goldenstein Christopher S, Mathews Garrett C, Cole Ryan K, Makowiecki Amanda S, Rieker Gregory B
Appl Opt. 2020 Sep 10;59(26):7865-7875. doi: 10.1364/AO.399405.
This paper presents a data-processing technique that improves the accuracy and precision of absorption-spectroscopy measurements by isolating the molecular absorbance signal from errors in the baseline light intensity () using cepstral analysis. Recently, cepstral analysis has been used with traditional absorption spectrometers to create a modified form of the time-domain molecular free-induction decay (m-FID) signal, which can be analyzed independently from . However, independent analysis of the molecular signature is not possible when the baseline intensity and molecular response do not separate well in the time domain, which is typical when using injection-current-tuned lasers [e.g., tunable diode and quantum cascade lasers (QCLs)] and other light sources with pronounced intensity tuning. In contrast, the method presented here is applicable to virtually all light sources since it determines gas properties by least-squares fitting a simulated m-FID signal (comprising an estimated and simulated absorbance spectrum) to the measured m-FID signal in the time domain. This method is insensitive to errors in the estimated , which vary slowly with optical frequency and, therefore, decay rapidly in the time domain. The benefits provided by this method are demonstrated via scanned-wavelength direct-absorption-spectroscopy measurements acquired with a distributed-feedback (DFB) QCL. The wavelength of a DFB QCL was scanned across the CO P(0,20) and P(1,14) absorption transitions at 1 kHz to measure the gas temperature and concentration of CO. Measurements were acquired in a gas cell and in a laminar ethylene-air diffusion flame at 1 atm. The measured spectra were processed using the new m-FID-based method and two traditional methods, which rely on inferring (instead of rejecting) the baseline error within the spectral-fitting routine. The m-FID-based method demonstrated superior accuracy in all cases and a measurement precision that was ≈1.5 to 10 times smaller than that provided using traditional methods.
本文介绍了一种数据处理技术，该技术通过使用倒谱分析将分子吸收信号与基线光强（）中的误差隔离开来，从而提高吸收光谱测量的准确性和精度。最近，倒谱分析已与传统吸收光谱仪一起用于创建时域分子自由感应衰减（m-FID）信号的改进形式，该信号可以独立于进行分析。然而，当基线强度和分子响应在时域中不能很好地分离时，就无法对分子特征进行独立分析，这在使用注入电流调谐激光器[例如可调谐二极管和量子级联激光器（QCL）]以及其他具有明显强度调谐的光源时很常见。相比之下，本文提出的方法几乎适用于所有光源，因为它通过在时域中将模拟的m-FID信号（包括估计的和模拟的吸收光谱）与测量的m-FID信号进行最小二乘拟合来确定气体特性。该方法对估计中的误差不敏感，这些误差随光频率缓慢变化，因此在时域中迅速衰减。通过使用分布反馈（DFB）QCL进行的扫描波长直接吸收光谱测量证明了该方法的优势。以1 kHz的频率扫描DFB QCL的波长，使其穿过CO P(0,20)和P(1,14)吸收跃迁，以测量CO的气体温度和浓度。在1个大气压下，在气室和层流乙烯-空气扩散火焰中进行测量。使用基于新的m-FID的方法和两种传统方法对测量光谱进行处理，这两种传统方法依赖于在光谱拟合程序中推断（而不是拒绝）基线误差。基于m-FID的方法在所有情况下都表现出更高的准确性，其测量精度比使用传统方法时小约1.5至10倍。
