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

Nicolas Minesi (1) , Miles Richmond (1) , Christopher Jelloian (1) , Nicholas Kuenning (1) , Anil Nair (1) , R. Mitchell Spearrin (1)

1 UCLA - University of California [Los Angeles]

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

Domaines: Génie mécanique [physics.class-ph]

Nicolas Minesi, Miles Richmond, Christopher Jelloian, Nicholas Kuenning, Anil Nair, et al.. Multi-line Boltzmann regression for near-electronvolt temperature and CO sensing via MHz-rate infrared laser absorption spectroscopy. Applied Physics B - Laser and Optics, 2022, 128 (12), pp.214. ⟨10.1007/s00340-022-07931-7⟩. ⟨hal-03862194⟩