January 2023
Spotlight Summary by Ali Hosseinnia
Hybrid time–frequency domain dual-probe coherent anti-Stokes Raman scattering for simultaneous temperature and pressure measurements in compressible flows via spectral fitting
Acquiring thermodynamic information from rapidly changing and chaotic environments is a challenging task. However, hybrid femtosecond (fs)/picosecond (ps) rotational coherent anti-Stokes Raman spectroscopy (CARS) has proven to be an accurate and non-intrusive laser diagnostic technique for single-laser-shot thermometry and pressure measurements in harsh gas-phase environments. The approach enables delayed ps probing of Raman coherences that are derived by prompt fs excitation. Since the Raman linewidths are dominantly determined by pressure-dependent collisional dephasing, probing at short time delays allows for detection of collision-free spectra, enabling accurate thermometry. In the present work, Jonathan E. Retter and co-authors have harnessed these unique characteristics to simultaneously measure temperature and pressure in a one-dimensional probe volume (5-6 mm long) along the axis of an under-expanded sonic jet. A ps-laser pulse is divided into two pulses with variable time separation (τ1 = 0 and τ2 = 150-1000 ps) so that the temperatures acquired from the spectra at zero delay are utilized as input to obtain pressures from the delayed spectra in an iterative fitting process. Since both temperature and pressure abruptly change at the “Mach disk”, the researchers could measure the shock-layer thickness with impressive precision. Moreover, the effects of using different Raman linewidths for thermometry at low temperatures, down to 80K, is compared and discussed in the paper.
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Article Information
Hybrid time–frequency domain dual-probe coherent anti-Stokes Raman scattering for simultaneous temperature and pressure measurements in compressible flows via spectral fitting
Jonathan E. Retter, Matthew Koll, Chloe E. Dedic, Paul M. Danehy, Daniel R. Richardson, and Sean P. Kearney
Appl. Opt. 62(1) 50-62 (2023) View: Abstract | HTML | PDF