MHz mid-infrared laser absorption sensor for carbon monoxide and temperature behind detonation waves

A mid-infrared laser absorption sensing strategy for carbon monoxide (CO) and temperature has been extended to MHz measurement rates for application to detonation studies. Bias-tee circuitry integrated with a distributed feedback quantum cascade laser near 4.98 micron enabled radio frequency wavelength scanning on the order of several MHz. A scan depth of approximately 0.55 cm-1 was achieved at 1 MHz, sufficient to capture a cluster of P-branch transitions in the fundamental bands with varying lower vibrational states. Temperature and CO mole fraction were inferred from a multi-line Voigt fitting routine. Accuracy of line intensities was evaluated on a shock tube facility at relevant combustion temperatures. MHz measurements were demonstrated behind ethylene-oxygen detonation waves on a single-pulse detonation facility at various mixture ratios and initial pressures, reflecting hydrocarbon-fueled rocket detonation exhaust conditions. The initial demonstration with a single laser configuration yields quantitative temperature and species over a range of conditions: 2500–3800 K and 1–5 atm. Temperature and mole fraction results were compared to detonation simulations.