Spectral synthesis of coherent anti‐Stokes Raman spectra with broadband Stokes lasers of arbitrary line shapes

Abstract

AbstractAn accurate analysis of coherent anti‐Stokes Raman spectroscopy (CARS) relies on spectral convolutions over the relevant bandwidths. Although analytical solutions of CARS signals generated by pump and Stokes lasers with standard (i.e. Gaussian or Lorentzian) line shapes are well known, broadband Stokes lasers can deviate from the assumed profiles. To minimize the influence of such a deviation, it is customary to normalize the data to the previously measured background CARS spectrum reproducing the Stokes laser profile. However, no one has so far quantified the possible changes in the final outcome of a CARS experiment when the Stokes profile undergoes a distortion from standard line shapes. In an attempt to include this issue in the general treatment of CARS signals, the calculation of CARS spectra with arbitrary Stokes line shapes is described in this paper. To that end, the complexity of such problem is solved by linearizing the smooth profile of the broadband laser. Advantages of this new strategy are discussed in regard to an application to molecular hydrogen and signal corrections are found to be as high as 10% of the standard CARS intensities calculated for a Gaussian Stokes profile. For such signal variations, imprecision of standard CARS thermometry is expected to be about 3%. Comparison with background‐corrected standard intensities reveals, instead, an agreement on the spectral line strengths only; CARS line shapes and off‐resonance signal levels are in disagreement. Copyright © 2007 John Wiley & Sons, Ltd.