A mid-infrared laser absorption sensing method has been developed to quantify gas properties (temperature, pressure, and species density) at MHz measurement rates, with application to annular rotating detonation rocket flows. Bias-tee circuitry is integrated with distributed feedback quantum cascade and interband cascade lasers in the 4−5 m range enabling diplexed radio frequency (RF) wavelength modulation on the order of several MHz while yielding sufficient scan depth to capture multiple rovibrational transitions in the fundamental vibrational bands of CO and CO 2. Sub-microsecond spectrally-resolved CO absorption lineshapes provide for inference of temperature and species from a two-line area ratio and pressure from collision line-width. CO 2 column density is inferred from peak-to-valley differential absorption at the bandhead near 4.19 m. A field demonstration on a methane-oxygen rotating detonation rocket engine was performed utilizing an in situ single-ended retro-reflection optical configuration aligned at the exhaust plane. The target gas properties are temporally-resolved at up to 3 MHz across rotating detonations with up to 20 kHz cycle frequency.

Methods to extend the spectral tuning range and signal-to-noise ratio via waveform optimization were examined using diplexed RF-modulation with continuous-wave distributed-feedback lasers, with relevance to MHz-rate laser absorption spectroscopy. With a bias-tee circuit, laser chirp rates are shown to increase by modulating the AC input voltage using square waves instead of sine waves and by scanning the laser below the lasing threshold. The effect of square waveform duty cycle and leading edge ramp rate are examined. Scan depths on the order of 1 cm −1 at a rate of 1 MHz are achieved with continuous-wave DFB quantum cascade lasers. The attenuation of the high-frequency laser signals due to detector bandwidth are also examined, and limitations are noted. Based on common detection system limitations, an optimization approach is established for a given detection bandwidth, with a representative optimized waveform determined for a 200 MHz system. This waveform is validated for accuracy across the scan range and for scan-to-scan repeatability using room temperature laser absorption measurements. The method is then deployed in a detonation tube to measure temperature, pressure, and CO concentration at MHz rates.

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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.