Abstract
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.
Highlights
Experimental investigations of high-temperature (T > 1000 K) gas-phase kinetics have been substantially augmented by measurement techniques based on laser absorption spectroscopy [1]
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
Summary
Experimental investigations of high-temperature (T > 1000 K) gas-phase kinetics have been substantially augmented by measurement techniques based on laser absorption spectroscopy [1]. Such experiments are usually performed under controlled thermodynamic conditions in homogeneous environments, such as shock tubes and rapid compression machines (RCM). These conditions are perfectly suited to path-averaged laser absorption techniques which can achieve high sensitivity in species concentration measurements by means of strong rovibrational transitions in the mid-infrared wavelength region. Our implementation avoids the experimental complexity of active mode-locking via optical feedback to an RCM-mounted cavity by utilizing an offaxis beam alignment, which reduces the cavity free spectral range (FSR) and enhances spurious mode coupling
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