Propagation of optical pulses under two-photon resonance absorption conditions

An investigation is made of optical pulse propagation in a two-photon-absorbing medium in the incoherent regime. Simple analytic expressions describing the initial stage of evolution of arbitrarily shaped radiation pulses are derived in the long-wavelength approximation. The steady-state pulse shape is determined. In cases of self-modulation instability, the steady-state pulse is a harmonic wave whose period is governed by the properties of the medium and does not depend on the intensity of the propagating radiation. A qualitative investigation is made of the formation of steady-state pulses.