Phase sensitivity of perturbative nonlinear interactions
  Despite the current concentration on phase control in few-cycle pulses, it emerges that there exists a wide class of nonlinear optical interactions in which the carrier phase is essentially irrelevant, even for the shortest pulse profiles. Most parametric processes and most perturbative processes fall into this category, although others such as above threshold ionization (ATI) do not. In an envelope approach, the carrier oscillations are not part of the problem because they are removed at the outset. When they are reinstated at the end of the calculation, one is free to include arbitrary phase shifts -- within certain constraints. In many cases the constraints are relatively weak, and it follows that a single envelope solution can be used with an infinite range of choices for the carrier phase. 
P. Kinsler, G.H.C. New, J.C.A. Tyrrell