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The influence of acoustic-wave diffraction on nonstationary backward stimulated Brillouin scattering (SBS) in the presence of feedback for a Stokes electromagnetic wave is numerically investigated. The cases of an acoustic-waveguide and-antiwaveguide fibers are considered. The Kerr nonlinearity is taken into account. For fibers with a small core radius (r_0<3 μm), the influence of sound diffraction on the nonstationary SBS can lead to new features. The importance of the acoustic-mode structure of the fiber for the process of forming a pulse train is shown in the case of synchronous pumping in Brillouin fiber lasers. It has been found that the acoustic-antiwaveguide fibers with a small core, r_0<3 μm, can be preferable for obtaining a stable train of compressed pulses. Under a constant input pump, an acoustic diffraction can essentially reduce the threshold of the modulation instability under SBS in the presence of the feedback.

This paper deals, at first, with the non-linear parametric effects on both space charge and electromagnetic waves in GaAs semiconductors. If an external electric field is applied, at the critical field value, the mobility changes its sign and becomes negative, as a result, non-linear instabilities of the interactions are obtained. At the optimal value of electric field, Eopt, all non-linear processes are very effectives. The second point is the multiplication of the wave frequency, which is also effective thanks to the non-linearity. This process is used for carring waves into millimeter range instead of generation process, which in this range is non-effective. The Multiplication effect is based on the using space charge waves.

We show the possibility for nonlinear trapping of a Langmuir wave in the region of weak inhomogeneity of plasma under parametric interaction with a powerful electromagnetic wave. Under overcritical conditions in the presence of dissipation and imperfect reflection, space-time amplitude correlations emerge and wave soliton structures are then formed. Powerful regular oscillations appear in the system, leading to a giant modulation of the pump pulse. The Langmuir nonlinearity contribution increases with the above-threshold value, which results in a sharp increase and compression of solitary waves.