Ultrafast laser-absorption spectroscopy for single-shot, mid-infrared measurements of temperature, CO, and CH4 in flames

Abstract

This Letter describes the development of an ultrafast (i.e., femtosecond), mid-infrared (mid-IR), laser-absorption diagnostic and its initial application to measuring temperature, CO, and C H 4 in flames. The diagnostic employs a Ti:sapphire oscillator emitting 55 fs pulses near 800 nm that were amplified and converted into the mid-IR though optical parametric amplification at a repetition rate of 5 kHz. The pulses were directed through the test gas and into a high-speed mid-IR spectrograph to image spectra across a ≈ 30 n m bandwidth with a spectral resolution of ≈ 0.3 n m . Gas properties were determined by least-squares fitting simulated absorbance spectra to measured single-shot absorbance spectra. The diagnostic was validated with measurements of temperature, CO, and C H 4 in a static-gas cell with an accuracy of 0.7% to 1.8% of known values. Single-shot, 5 kHz measurements of temperature and CO column density were acquired near 4.9 µm in a laser-ignited HMX (i.e., 1,3,5,7-tetranitro-1,3,5,7-tetrazoctane) flame and exhibited 1 − σ precisions of 0.4% and 2.3%, respectively, at ≈ 2700 K . Further, temperature and C H 4 column density measurements were acquired near 3.3 µm in a partially premixed C H 4 -air flame produced by a Hencken burner and exhibited 1 − σ precisions of 0.3% and 1% respectively, at ≈ 1000 K .