Minimally intrusive optical probe for in situ shock tube measurements of temperature and species via tunable IR laser absorption

An optical probe was developed for in situ laser absorption spectroscopy measurements of target species concentration and temperature near a shock tube endwall. The optical components of a single-ended sensor, including two sapphire rods with a separation of 2.8 cm (effective optical path length), were coupled with a custom-machined shock tube end cap. This hardware design—complemented by a scanned wavelength direct absorption (scanned DA) optical sensing scheme, targeting CO₂—enabled minimally invasive, sensitive and spatially resolved measurement of CO₂ along the shock tube axis in the reflected shock region. Scanned direct absorption of two adjacent, discrete, rovibrational features in the ν₃ fundamental band of CO₂ near 4.2 µm, with a single interband cascade laser, allowed measurement of temperature from the ratio of integrated absorbances. Several tests were conducted to validate the accuracy of the sensor and confirm that shock conditions were unperturbed by the probe. The probe-measured temperature and CO₂ mol fraction in the reflected shock region, accurate within ~ 1 and ~ 5%, respectively, at a rate of 20 kHz, in shocks with 1–7% carbon dioxide diluted in argon or nitrogen and at temperatures of 1200–2000 K and pressures of 0.7–1.2 bar. Agreement with multiple metrics for validation was confirmed. No evidence of perturbed shock conditions was found.