ACTIVE MEDIA: Two-photon Raman processes in spectral ultrabroadening of intense ultrashort light pulses

The spectral evolution of an intense electromagnetic pulse in the course of a coherent two-photon Raman interaction with a medium is analyzed under conditions such that the approximation of slow envelopes is not valid for the field or polarization of the medium. There exist threshold power levels at which a pronounced self-broadening of the field spectrum occurs if the field spectrum includes the frequencies Ω, where \hslashΩ=E₂ - E₁ is the energy difference between the levels involved in the stimulated Raman scattering. The results found here agree with experimental data on the generation of a supercontinuum in various media. In particular, there is agreement in terms of the spectral self-transformation of optical solitons in optical fibers.