The Rotational Dual Broadband Approach and Noise Considerations in CARS Spectroscopy

Abstract

New experimental approaches to broadband CARS have recently been presented [1–3]. Rotational dual broadband CARS (RDBC) has shown to have several interesting properties for CARS thermometry [3]. By using an appropriate phase matching arrangement simultaneous recording of rotational and vibrational CARS spectra have been demonstrated. This is attractive for temperature determination in turbulent media with high temperature gradients. In rotational CARS there are no computational difficulties due to pressure induced narrowing as in vibrational Q-branch CARS where the Raman lines overlap at higher pressures. Non-Gaussian pump laser statistics [4,5] also does not have to be taken into account. Fitting to theoretical spectra for thermometry has been performed [6] in particular if instead of the complete spectral signature only the integrated intensities of the Raman resonances are fitted the computation time is reduced about two orders of magnitude. A quantitative analysis of the thermometry possibilities of RDBC is in progress [7]. Using RDBC a factor of two lower noise in non — Raman resonant CARS spectra than in a corresponding set-up using the conventional CARS approach has been observed [3]. There has also been a lively debate whether a single-mode or a multimode pump laser is the most appropriate one to use in CARS thermometry [8,9].