Dual-pump coherent anti-Stokes Raman scattering for simultaneous pressure/temperature measurement

A new dual-pump coherent anti-Stokes Raman scattering (CARS) technique for the simultaneous measurement of pressure and temperature is discussed. For these measurements the wavelengths of the two pump lasers and the Stokes laser are selected so that the vibrational and pure rotational spectra of nitrogen are simultaneously acquired. Because collisional narrowing is an important effect for the vibrational spectrum but not for the isolated pure rotational lines, the relative intensities of the vibrational and pure rotational spectra change markedly as the pressure changes. Consequently, pressure as well as temperature can be determined from the spectral shape of the dual-pump CARS spectrum. The pressure measurement capability of the technique was demonstrated in a room-temperature gas cell at pressures ranging from 0.1 to 20 atm. The pressures determined from averaged dual-pump CARS spectra are in good agreement with pressure transducer measurements over this entire pressure range, with an average absolute error of less than 8%. Simultaneous pressure and temperature measurements were also performed along the centerline of an underexpanded jet. Results agreed well with theoretical predictions. Single-laser-shot pressure and temperature measurements are discussed.