Propagation of ultrashort optical pulses in a medium with degenerate resonance levels with respect to the angular momentum projections is considered. Under the assumption that the Rabi frequency is much smaller than the transition frequency and without using the slowly varying envelope approximation, a new non-linear equation is obtained for describing this pulse dynamics. In the particular case when the pulse polarization is not changed, this is the modified Korteweg-de Vries equation. In the approximation of slowly varying enve-lopes, the reduced wave equation transforms into the vector nonlinear Schrbdinger equation.
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