Godehard Hilfer

Stimulated Raman Scattering in the Transient Limit

Stimulated Raman scattering is considered in the transient limit where the pulse width is small compared with T2. In the case where diffraction, level saturation, anti-Stokes and higher-order Stokes radiation, and quantum noise can all be neglected, we show that there are three distinct evolution regimes: the I regime, where the Stokes pulse is small compared with the pump pulse; the transition regime, where the Stokes and pump pulses are comparable; and the J regime, where the pump pulse is small compared with the Stokes pulse. The characteristic behavior in each of these three regimes is explored by using a combination of analytical and numerical tools.

Observation in wave-number space of pump-beam replication in stimulated Raman scattering

Stimulated Raman scattering has been proposed for use in beam combining. In one configuration, several crossing pump beams are combined by amplifying a centrally seeded Stokes beam. Experiments show that a moderate level of pump-beam aberrations can give rise to beam replication, i.e., Stokes generation along the paths of the pump beams. The underlying physical mechanism could be nonlocal four-wave mixing or pump beam scintillation. A numerical scheme is used here that allows one to identify the individual gain contributions from different regions of wave-number space, which confirms that scintillation is primarily responsible for beam replication.

Asymptotic evolution of transient pulses undergoing stimulated Raman scattering

Propagation of short, transient pulses undergoing stimulated Raman scattering over long length scales is considered. It is shown that under common experimental circumstances the evolution has two different regimes: an I regime, at short lengths, where the pump changes little and the Stokes rapidly grows, and a J regime, at long lengths, where the Stokes intensity is close to saturation and the pump intensity decreases slowly as the square root of distance. The distance at which the J regime is reached is determined numerically.

Pump Replication In Stimulated Raman Scattering Using A Crossed-Beam Geometry

A theory of side beam replication in a crossing-beam geometry is reported. It is shown that side beam replication is not expected to occur when the Fresnel number of the aberrations (FNA) is large, while it is expected to occur when FNA is small, in accord with experiments. An analytic threshold is derived for the value of FNA at which side beam replication no longer occurs, and this threshold agrees well with the experiments. We propose a method for eliminating side beam replication at low values of FNA.