  We study beating of a probe field with a time-varying susceptibility in a
coherently prepared Raman medium. We consider the general case of an arbitrary
variation of susceptibility, which corresponds to a superposition of an
arbitrary number of excited Raman transitions. We derive a general analytical
solution and conservation relations for this process. We show that the
interference between Raman polarizations may substantially affect frequency
modulation and pulse compression for the probe field.
