Parallel modes at pulsed two-frequency resonance in the Λ-scheme of degenerate quantum levels

We report the results of a theoretical study of the co-propagation of two elliptically polarised laser pulses resonantly interacting with quantum transitions in the Λ-scheme of degenerate energy levels. We consider the case of a weak high-frequency field, in which there arises the phenomenon of electromagnetically induced transparency, and the case of a high-power high-frequency field. It is shown that for both cases there are optimal conditions imposed on the input pulses of interacting radiations, under which the high-frequency field pulse experiences the least absorption and distortion in the medium. The first condition ensures a certain relationship for the eccentricities of the polarisation ellipses of the input fields. The second, less essential, condition is the parallelism of the main axes of these ellipses. Both conditions are predicted by the theory of normal modes of electromagnetically induced transparency in the considered Λ-scheme. Numerical simulation shows that the above conditions are quite well applicable not only in the case of weak high-frequency radiation, but also when high-frequency and low-frequency pulses have comparable peak intensities at the medium input.