A novel technique for suppression of parasitic superfluorescence in backward Raman amplifiers

The major parasitic modes of backward Raman amplifiers are different forms of superfluorescent forward Raman scattering. It is shown that spatial, and/or temporal variation of the Raman frequency in the scattering medium can inhibit amplification of spontaneously scattered light by creating off-resonant conditions in the forward direction. If the frequencies of the optical pulses are properly varied in time (the pulses are chirped), the required resonance condition for amplification of a counterpropagating Stokes wave can be maintained throughout the medium. Raman transition frequencies can be varied in an atomic vapor by application of a magnetic field via the Zeeman effect. The use of thallium (T1) metal vapors for scattering XeCl or KrF excimer laser radiation is considered as a particular example.