Wideband pulse propagation: single-field and multi-field approaches to   Raman interactions
  We model the process of ultra broadband light generation in which a pair of laser pulses separated by the Raman frequency drive a Raman transition. In contrast to the usual approach using separate field envelopes for the different frequency components, we treat the field as a single entity. This requires the inclusion of few-cycle corrections to the pulse propagation. Our single-field model makes fewer approximations and is mathematically (and hence computationally) simpler, although it does require greater computational resources to implement. The single-field theory reduces to the traditional multi-field one using appropriate approximations. 
P. Kinsler and G.H.C. New