Cepstral analysis for baseline-insensitive absorption spectroscopy using light sources with pronounced intensity variations

A nonparametric point-by-point method to measure time-dependent frequency in wavelength modulation spectroscopy

A nonparametric point-by-point (NPP) method is presented for high-accuracy measurement of the time-dependent frequency (laser frequency) in tunable laser absorption spectroscopy, crucial for ensuring ultimate measurement accuracy. In wavelength modulation spectroscopy in particular, the parametric methods in current use for time-dependent frequency measurement are insufficiently accurate and are difficult to apply to complex modulation scenarios. Based on a multi-scale viewpoint, point-by-point measurement of the frequency is realized by linear superposition of the frequency information mapped from the interferometric signal on a unit scale and on a local scale. Validation experiments indicate that the measurement accuracy of the proposed NPP method is three times that of the existing parametric methods, while effectively immunizing against non-ideal tuning effects. Additionally, the NPP method is suitable for use with arbitrarily complex modulations such as square wave modulation, for which parametric methods are inapplicable.

Inverse fitting direct absorption spectroscopy Technology: Simplified implementation and enhanced performance

The conventional direct absorption spectroscopy (DAS) technique has been plagued by the difficulty of obtaining accurate baseline, which is caused by photoelectric drift and the absence of non-absorbing regions in the transmitted light intensity signal. An inverse fitting direct absorption spectroscopy (IF-DAS) technique has been proposed to address this difficulty. The technique leverages the intrinsic nonlinear intensity response of tunable lasers to achieve baseline-free concentration measurements. It offers the advantages of being straightforward to implement, baseline-free, calibration-free, and resistant to photoelectric signal drift. Its efficacy was validated using an example under ambient temperature and atmospheric pressure conditions. The performance of the IF-DAS technique was compared with that of the conventional DAS technique through standard experimental tests. The results demonstrate that the IF-DAS technique is less susceptible to fluctuations in light intensity, exhibits superior linearity and accuracy, with an R2 value of 0.99986 and an overall error of less than 2%. This technique shows potential for application in harsh scenarios such as reactive flow fields and long-term engineering applications.

Comparison of Frequency-Domain and Time-Domain Baseline Correction Approaches for Infrared Absorption Spectroscopy of Mixtures Containing Up to 464 Components

Many baseline correction approaches have been developed to address baseline artifacts observed in measured infrared (IR) absorption spectra during post-processing. These approaches offer distinct advantages and disadvantages, and the choice of which one to employ depends on the complexity of baseline artifacts present in a particular application. In this paper, we compare the performance of two baseline correction approaches: a frequency-domain polynomial fitting approach and a time-domain modified free induction decay approach, under various baseline scenarios, spectral resolutions, and noise levels for mixtures containing up to 464 species. Our results showed that the frequency-domain approach outperformed the time-domain approach by a factor of up to 16 when the baseline was represented by a sine wave with fewer than two cycles over the full spectral range. On the other hand, the time-domain approach performed up to 12 times better when the baseline featured two cycles of a sine wave. Additionally, we observed that the time-domain approach exhibited higher sensitivity to spectral resolution and underperformed when the noise level was high. The findings of this study emphasize the importance of numerically testing a few candidate approaches for a given application, taking into consideration baseline characteristics, as well as the spectral resolution and noise constraints of the application.

Quantitative gas pressure measurement by molecular spectroscopy using chip-based supercontinuum in the mid-infrared

We demonstrate the quantitative pressure measurement of gas molecules in the mid-infrared using chip-based supercontinuum and cepstrum analysis without additional measurements for baseline normalization. A supercontinuum generated in an on-chip waveguide made of chalcogenide glass having high nonlinearity passes through CO gas and provides a transmission spectrum. The gas absorption information is deconvoluted from the original supercontinuum spectral information containing temporal fluctuation by cepstrum analysis and extracted simply by applying a bandpass filter in the temporal domain. The gas pressure estimated from the extracted absorption information is consistent with the value measured by a pressure gauge within a difference of 1.25%, despite spectral fluctuations in the supercontinuum baseline comparable to the spectral depth of the gas absorption lines.

Quantum-cascade-laser-absorption-spectroscopy diagnostic for temperature, pressure, and NO X 2 Π 1/2 at 500 kHz in shock-heated air at elevated pressures

The design, validation, and application of a quantum-cascade-laser-absorption-spectroscopy diagnostic for measuring gas temperature, pressure, and nitric oxide (NO) in high-temperature air are presented. A distributed-feedback quantum-cascade laser (QCL) centered near 1976c m ^-1 was used to scan across two transitions of NO in its ground electronic state (X ² Π _1/2). A measurement rate of 500 kHz was achieved using a single QCL by: (1) performing current modulation through a bias-tee, and (2) targeting closely spaced transitions with a large difference in lower-state energy. The diagnostic was validated in a mixture of 95% argon and 5% NO, which was shock-heated to ≈2000 to 3700 K. The average mean percent differences between laser-absorption-spectroscopy (LAS) measurements and predictions from shock-jump relations for temperature, pressure, and NO mole fraction were 3.1%, 4.1%, and 6.5%, respectively. The diagnostic was then applied to characterize shock-heated air at high temperatures (up to ≈5500K) and high pressures (up to 12 atm) behind either incident or reflected shocks. The LAS measurements were compared to theoretical predictions from shock-jump relations, pressure sensors mounted in the wall of the shock tube, and equilibrium values of the NO mole fraction. The average mean percent differences between LAS measurements and their aforementioned reference values were 3.2%, 10.8%, and 10.4% for temperature, pressure, and NO mole fraction, respectively. Last, a comparison between a measured NO mole fraction time history and a time-stepped homogeneous reactor simulation performed using two different chemical kinetics mechanisms is presented.

A selective laser-based sensor for fugitive methane emissions

A mid-infrared laser-based sensor is reported for the quantification of fugitive methane emissions. The sensor is based on a distributed feedback inter-band cascade laser operating near 3.3 μm. Wavelength tuning with cepstral analysis is employed to isolate methane absorbance from (1) fluctuations in the baseline laser intensity, and (2) interfering species. Cepstral analysis creates a modified form of the time-domain molecular free-induction-decay (m-FID) signal to temporally separate optical and molecular responses. The developed sensor is insensitive to baseline laser intensity imperfections and spectral interference from other species. Accurate measurements of methane in the presence of a representative interfering species, benzene, are performed by careful selection of the scan index (ratio of laser tuning range to spectral linewidth) and initial and final time of m-FID signal fitting. The minimum detection limit of the sensor is ~ 110 ppm which can be enhanced with an optical cavity. The proposed sensing strategy can be utilized to measure methane leaks in harsh environments and in the presence of interfering species in environment-monitoring applications.

Ultrafast-laser-absorption spectroscopy in the mid-infrared for single-shot, calibration-free temperature and species measurements in low- and high-pressure combustion gases

This manuscript presents an ultrafast-laser-absorption-spectroscopy (ULAS) diagnostic capable of providing calibration-free, single-shot measurements of temperature and CO at 5 kHz in combustion gases at low and high pressures. Additionally, this diagnostic was extended to provide 1D, single-shot measurements of temperature and CO in a propellant flame. A detailed description of the spectral-fitting routine, data-processing procedures, and determination of the instrument response function are also presented. The accuracy of the diagnostic was validated at 1000 K and pressures up to 40 bar in a heated-gas cell before being applied to characterize the spatiotemporal evolution of temperature and CO in AP-HTPB and AP-HTPB-aluminum propellant flames at pressures between 1 and 40 bar. The results presented here demonstrate that ULAS in the mid-IR can provide high-fidelity, calibration-free measurements of gas properties with sub-nanosecond time resolution in harsh, high-pressure combustion environments representative of rocket motors.