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This manuscript presents an ultrafast-laser-absorption-spectroscopy (ULAS) diagnostic capable of providing calibration-free, single-shot measurements of temperature and CO at 5 kHz in combustion gases at low and high pressures. Additionally, this diagnostic was extended to provide 1D, single-shot measurements of temperature and CO in a propellant flame. A detailed description of the spectral-fitting routine, data-processing procedures, and determination of the instrument response function are also presented. The accuracy of the diagnostic was validated at 1000 K and pressures up to 40 bar in a heated-gas cell before being applied to characterize the spatiotemporal evolution of temperature and CO in AP-HTPB and AP-HTPB-aluminum propellant flames at pressures between 1 and 40 bar. The results presented here demonstrate that ULAS in the mid-IR can provide high-fidelity, calibration-free measurements of gas properties with sub-nanosecond time resolution in harsh, high-pressure combustion environments representative of rocket motors. 

This manuscript describes the first application of ultrafast-laser-absorption spectroscopy (ULAS) to characterizing high-pressure (up to 40 bar), multi-phase combustion gases. Single-shot measurements of temperature and CO were acquired at 5 kHz in AP-HTPB propellant flames with and without aluminum. An ultrafast light source was used to produce broadband pulses of light near 4.96 𝜇m at a repetition rate of 5 kHz and a high-speed mid-infrared imaging spectrometer was used to image the pulses across an 86 nm bandwidth with a spectral resolution of 0.7 nm. Measurements of temperature and CO concentration were obtained by least-squares fitting simulated absorbance spectra of CO to measured spectra. A system of corrective optics was used to diminish the e˙ect of beam steering during high-pressure experiments, greatly increasing the pressure capabilities of the diagnostic. The diagnostic was used to characterize AP-HTPB propellant flames in an argon bath gas at pressures of 1, 10, 20, and 40 bar. An aluminized AP-HTPB propellant was also characterized at 10 and 20 bar to demonstrate that ULAS can provide high-fidelity measurements in particulate-laden flames. The results demonstrate that ULAS is capable of providing single-shot temperature and species measurements at high pressures with 1-𝜎 precisions less than 1.1% and 3% for temperature and species respectively, despite non-absorbing transmission losses in excess of 90%.