  We derive model equations for optical pulse propagation in a medium described
by a two-level Hamiltonian, without the use of the slowly varying envelope
approximation. Assuming that the resonance frequency of the two-level atoms is
either well above or well below the inverse of the characteristic duration of
the pulse, we reduce the propagation problem to a modified Korteweg-de Vries or
a sine-Gordon equation. We exhibit analytical solutions of these equations
which are rather close in shape and spectrum to pulses in the two-cycle regime
produced experimentally, which shows that soliton-type propagation of the
latter can be envisaged.
