Multi-line Boltzmann regression for near-electronvolt temperature and CO sensing via MHz-rate infrared laser absorption spectroscopy

A mid-infrared laser absorption technique is developed for sensing of temperature and carbon monoxide (CO) number density from 2000 K to above 9000 K. To resolve multiple rovibrational lines, a distributed-feedback quantum cascade (DFB-QCL) modulated across 80% its current range using trapezoidal waveform via bias-tee circuit. The attains spectral scan depth 1 cm $$^{-1}$$ , at frequency MHz, which allows simultaneous measurements four isolated CO transitions near 2011 (4.97 $$\mu$$ m) with lower-state energies spanning 3,000 42,000 . are calculated Boltzmann regression the population fractions. This method leverages information contained in each transition yields lower uncertainty than single line pair. sensor validated shock tube experiments over wide temperatures pressures (2300–8100 K, 0.3–3 atm). Measurements behind reflected waves compared kinetic model dissociation up 9310 shown recover equilibrium conditions. high able rapid species evolution electronvolt conditions making it suitable investigations high-speed flows, plasma applications, high-pressure detonations.