Interplay of pulse bandwidth and spectral resolution in spectral-focusing CARS microscopy

We present a theoretical analysis of the relationship among the spectral resolution, the degree of chirp-matching, and pulse bandwidth in coherent anti-Stokes Raman scattering microscopy implementations based on spectral focusing (SF-CARS). We find that, despite allowing better ultimate spectral resolution when chirp-matching is attained, the use of the broadest bandwidth pulses can significantly worsen the spectral resolution if the pulses are not chirp-matched. We demonstrate that the bandwidth of the detected anti-Stokes signal is minimized when the pump is twice as chirped as the Stokes, meaning that-contrary to our initial presumption-a narrow anti-Stokes bandwidth does not imply good spectral resolution. We present approximate expressions that relate the bandwidths of the pump, Stokes, and anti-Stokes pulses to the degree of chirp-matching and outline how these could be used to estimate the amount of glass needed to attain chirp-matching. These expressions also permit a useful estimate of the group delay dispersion from other optics in the pump or Stokes beam path. This work aims to clarify several technical aspects of SF-CARS microscopy and provides researchers with valuable information to consider when working with SF-CARS microscopy systems. (C) 2018 Optical Society of America