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.

The inversion of lidar returns from homogeneous atmospheres has been done customarily through the well-known slope method. The logarithmic operation over the range-corrected and system-normalized received signal used in this method introduces a bias in the statistics of the noise-affected processed signal that can severely distort the estimates of the atmospheric attenuation and backscatter coefficients under measurement. It is shown that a fitting of the theoretically expected exponential signal to the range-corrected received one, using as the initial guess the results provided by the slope method and a least-squares iterative procedure, can yield enhanced accuracy under low signal-to-noise ratios and especially in moderate-to-high extinction conditions.

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