We report the first application of cavity-enhanced absorption spectroscopy (CEAS) using a coherent light source for sensitive and rapid gaseous species time-history measurements in a shock tube. Off-axis alignment and fast scanning of the laser wavelength were used to minimize coupling noise in a low-finesse cavity. An absorption gain factor of 83 with a measurement time resolution of 20 µs was demonstrated for C2H2 detection using a near-infrared transition near 1537 nm, corresponding to a noise-equivalent detection limit of 20 ppm at 296 K and 76 ppm at 906 K at 50 kHz. This substantial gain in signal, relative to conventional single-pass absorption, will enable ultra-sensitive species detection in shock tube kinetics studies, particularly useful for measurements of minor species and for studies of dilute reactive systems.
Cavity-enhanced absorption spectroscopy (CEAS) using a mid-infrared DFB quantum-cascade laser is reported for sensitive time-resolved (10 μs) in situ CO measurements in a shock tube. Off-axis alignment and fast scanning of the laser wavelength were used to minimize coupling noise in a low-finesse cavity. An absorption gain factor of 91 was demonstrated, which enabled sub-ppm detection sensitivity for gas temperatures of 1000-2100K in a 15 cm diameter shock tube. This substantial improvement in detection sensitivity compared to conventional single-pass absorption measurements, shows great potential for the study of reaction pathways of high-temperature combustion kinetics mechanisms in shock tubes.
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.
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