Self-action of periodically modulated laser beams in doped waveguides

We report numerical modeling of self-action of a beam, whose amplitude or phase is periodically modulated in time, in a gradient waveguide medium doped with two-level saturable active centers. For frequencies close to the dopant absorption line and modulation periods close to its relaxation time the response of the medium is no more instantaneous, and the description in terms of susceptibility fails. We account for the delayed response of the medium by solving numerically the full set of Maxwell-Bloch equations. In case of modulation wavelength and the diffraction length larger than the propagation distance we also make use of a simplified scheme based oil synchronous interaction approximation. Using these schemes we study the influence of the non-stationary self-action of the beam on the transmission of modulation signal through the doped waveguide. It is shown that in the time dependence of the polarization and population difference the nonlinear distortions of the modulation signal can be much stronger than in the output field intensity.