Finite-duration seeding effects in powerful backward Raman amplifiers.

In the process of backward Raman amplification (BRA), the leading layers of the seed laser pulse can shadow the rear layers, thus weakening the effective seeding power and affecting parameters of output pulses in BRA. We study this effect numerically and also analytically by approximating the pumped pulse by the "pi-pulse" manifold (family) of self-similar solutions. We determine how the pumped pulse projection moves within the pi-pulse manifold, and describe quantitatively the effective seeding power evolution. Our results extend the quantitative theory of BRA to regimes where the effective seeding power varies substantially during the amplification. These results might be of broader interest, since the basic equations are general equations for resonant three-wave interactions.