Quantum-cascade-laser-based dual-comb thermometry and speciation at high temperatures

Abstract

This work presents a methodology for using spectroscopic models to fit absorption-spectrum measurements made by a quantum-cascade-laser-based dual-comb spectrometer (QCL-DCS) for high-temperature kinetics research. A pair of quantum-cascade frequency combs was employed to detect methane’s ν 4 absorption features between 1270 and 1320 cm−1 in high-temperature shock-tube environments and extract methane mole fraction and gas temperature from the results. The methodology was first validated by comparing DCS measurements against modeled methane spectra at room temperature in a static cell, followed by assessing the fitting procedure in shock-heated mixtures of 2% methane in Ar at 1000 K. In both validation experiments, the tradeoffs between time resolution and measurement precision were explored. Measurements were achieved at a 4 µs measurement rate with 5% uncertainty for temperature and 4% uncertainty for mole fraction at 1000 K. Higher accuracy was achieved with longer measurement averaging, e.g. 1.8% uncertainty for temperature at 40 μ s resolution. Finally, the DCS spectral-fitting methodology was demonstrated to capture temperature and methane time-history evolution during the pyrolysis of iso-octane, a primary gasoline reference fuel. Good agreement was observed with kinetic models, and future applications for DCS kinetics research are discussed.

QCL Dual‐Comb Spectroscopy Matures for Applications

“…Combustion diagnostics has different challenges because widely used shock-tube model systems have typical test times of the order of a millisecond, experiments are not exactly repeatable and thermal emission from oxidizing fuel has to be separated from the light used for absorption spectroscopy. Due to these complications, laser-based diagnostics are often used in combustion applications and the added spectral bandwidth at short timescales that dual-comb spectroscopy offers has recently been used to study formation and decomposition time constants [11] and as a means of validating kinetic models [12].…”

A methane line list with sub-MHz accuracy in the 1250 to 1380 cm−1 range from optical frequency comb Fourier transform spectroscopy

“…The results were in good agreement with the earlier study by AlSaif et al [42,43], who recorded the ν1 fundamental band using a comb-referenced continuous-wave (CW) quantum cascade laser. While dual-comb spectroscopy with quantum-cascade lasers has been successfully applied to detect methane in the 8 μm spectral range [44][45][46], this method still relies on absorption lines with known frequencies for frequency calibration [44,47].…”

High-resolution spectroscopic measurements of cold samples in supersonic beams using a QCL dual-comb spectrometer*

“…Furthermore, dual-comb arrangements have been employed for gas-phase dynamics studies [33,34]. Quantum-Cascade-Laser (QCL) dual-comb spectroscopy has also been applied to gas-phase dynamics, with the spectral sampling determined by the repetition frequency of the QCL frequency combs [35,36]. To reduce the measurement point spacing, two approaches for interleaving QCL dual-comb measurements have been reported: rapid-sweep measurements relying on simultaneous tuning of the lasers [37] and step-sweep measurements relying on step-wise tuning of the lasers [38].…”