We investigate the effect of rapidly varying perturbations on the initial s
tage of the three-dimensional propagation of ultrashort pulses through a no
nlinear dispersive medium, in the limit and beyond the slowly varying envel
ope approximation. We have found a space-time coupling effect which distort
s and reduces the range for which sidebands appear on far-field and tempora
l spectrum. (C) 1999 Published by Elsevier Science B.V. All rights reserved
.