Cavity-enhanced absorption spectroscopy with a ps-pulsed UV laser for sensitive, high-speed measurements in a shock tube

Intra-pulse laser absorption sensor with cavity enhancement for oxidation experiments in a rapid compression machine

A sensor based on a mid-IR pulsed quantum cascade laser (QCL) and off-axis cavity enhanced absorption spectroscopy (OA-CEAS) has been developed for highly sensitive concentration measurements of carbon monoxide (CO) in a rapid compression machine. The duty cycle and the pulse repetition rate of the laser were optimized for increased tuning range, high chirp rate, and small line width to achieve effective laser-cavity coupling. This enabled spectrally resolved CO line-shape measurements at high pressures (P ~10 bar). A gain factor of 133 and a time resolution of 10 μs were demonstrated. CO concentration-time profiles during the oxidation of highly dilute n-octane/air mixtures were recorded, illustrating new opportunities in RCM experiments for chemical kinetics.

Modeling the optical field in off-axis integrated-cavity-output spectroscopy using the decentered Gaussian beam model

Off-axis integrated-cavity-output spectroscopy (OA-ICOS) is widely used in trace gas detection and isotopic analysis for its enhanced sensitivity as well as robustness against optical instability. However, its sensitivity is ultimately limited by the spurious coupling noise formed in the cavity, and much of the design and optimization process relies on empirical iterations while quantitative analysis is lacking. In this paper, we develop a method to model the optical field in OA-ICOS based on the decentered Gaussian beam model, which is a generalization for large tilting angles as compared with previously developed models. From the optical field, the cavity transmission spectrum for different cavity configurations or input beam conditions can be calculated, and the fringe noise level can be derived. Results show that an optimum combination of input laser beam and off-axis alignment exists to fully suppress the interference fringes. Factors affecting the fringe noise level, including a mismatch between the input beam and the cavity, optical alignment conditions, and deviation from the re-entrant condition, are studied thoroughly. The developed method can serve to guide the design and optimization of OA-ICOS systems.

Time-Resolved Broadband Cavity-Enhanced Absorption Spectroscopy behind Shock Waves

A fast and sensitive broadband absorption technique for measurements of high-temperature chemical kinetics and spectroscopy has been developed by applying broadband cavity-enhanced absorption spectroscopy (BBCEAS) in a shock tube. The developed method has effective absorption path lengths of 60-200 cm, or cavity enhancement factors of 12-40, over a wavelength range of 280-420 nm, and is capable of simultaneously recording absorption time profiles over an ∼32 nm spectral bandpass in a single experiment with temporal and spectral resolutions of 5 μs and 2 nm, respectively. The accuracy of the kinetic and spectroscopic measurements was examined by investigating high-temperature reactions and absorption spectra of formaldehyde behind reflected shock waves using 1,3,5-trioxane as a precursor. The rate constants obtained for the thermal decomposition reactions of 1,3,5-trioxane (to three formaldehyde molecules) and formaldehyde (to HCO + H) agreed well with the literature data. High-temperature absorption cross sections of formaldehyde between 280 and 410 nm have been determined at the post-reflected-shock temperatures of 955, 1265, and 1708 K. The results demonstrate the applicability of the BBCEAS technique to time- and wavelength-resolved sensitive absorption measurements at high temperatures.