Resonantly enhanced Raman amplification of ultrashort pulses

We consider propagation of a pair of ultrashort coherent pulses in V-type medium. Both pulses are supposed to be resonant with corresponding atomic transitions and medium not prepared in a coherent state. It is shown that, in the system under consideration, lasing without inversion is impossible. However, the resonant character of the atom-field interaction yields several novel phenomena. If the strong pump pulse is taken in the form of a 2/spl pi/-soliton, and the medium is prepared in the ground state, then weak probe pulse propagates there without any absorption and dispersion. The system yields a pulse locking, in which the two pulses propagate with equal group velocities. The probe pulse evolves towards the locked state independently of its initial shape. If the medium is prepared in a state with small Raman inversion, then the probe pulse experiences high resonantly enhanced Raman gain, still accompanied by vanished linear absorption and dispersion. We demonstrate that, in the nonlinear stage of probe amplification, gain for the probe does not disappear or decrease, but considerably increases. Raman amplification of the probe pulse ends with complete vanishing of the pump, and all photons of the pump exchange on those of the drive one by one. Since the atom-field interaction is resonantly enhanced, this Raman scattering requires much weaker pump intensity in comparison with conventional Raman laser schemes working with far-detuned fields. We study various practical aspects of frequency up- and down-conversion utilizing resonant Raman scattering of ultrashort pulses. The physics behind the effect of pulse locking and resonant Raman amplification are also discussed. The proposed effect holds promise to yield coherent radiation in new frequency domains and improve performance of modern Raman lasers.