  We present a new exciton-based formalism for calculating the coherent
response of asymmetric semiconductor multiple quantum well structures to
ultra-short optical pulses valid to infinite order in the optical field and
including the self-generated intraband fields. We use these equations to
calculate and explain the oscillations with time delay of peaks in the
spectrally-resolved degenerate four wave mixing signals from biased
semiconductor superlattices, obtaining good agreement with experiment.
