Research On Several Problems In Stimulated Brillouin Scattering

Stimulated Brillouin scattering (SBS) is accompanied with the effects of phase conjugation (PC), light slowing, optical filter et al. The phase conjugate effect can be used to eliminate real-time optical aberrations of laser beams and for coherent beam combinations as well, which both are important technologies in laser engineering. Slow light and optical filter via SBS, together with the application of SBS on storing light in optical fiber, are promising in optical buffers and information processing in the future fiber communications. SBS is still attracking lots of interests.This dissertation is focused on the issue of SBS, including the basic physical process, the mechanism of slow and fast lights, the PC mechanism and threshold reduction. The main work and conclusions are as follow.(1)We point out and verify therectically that Stokes wave can be anti-Stokes scattered by Brillouin sounds in SBS. The scattering can be seen as a reversal process with respect to the common Stokes scattering. The occurrence of the reversal scattering process can predict that there are tails in SBS transmitted waves. Although we did not observe a tail in our experiment because of restrictions in our lab, other researcher's experimental results provide strong evidence for it.(2)The optical distance of a Stokes pulse is enhanced due to the back-and-forth scatterings, resulting in light slowing. We calculate the additional distance and thereby the delaying time, which is proportional to the fiber length, pump intensity and the inverse of Brillouin bandwidth. The accompanying effects such as waveform broadening are qualitatively explained as well as the fast-light effect for anti-Stokes light.(3)We show that it is the interference of pump light and Stokes light in SBS that resonantly stimulates Brillouin sounds in SBS. Basing on the phase of the interference, we obtain the expression of the acoustic wavefront. When the light waves are conjugate to each other, Brillouin sound owns the same wavefront as the lights, which indicates that the PC Stokes mode can be resonant with and therefore get resonant gain from the pump in the entire volume. We also show that non-PC modes can only be resonant with a portion of transverse modes, and that the gain length is limited by the tranverse scale of the pump beam. Thus the PC Stokes mode becomes dominant in the scatterings.(4)The PC mechanism of Brillouin-enhanced four-wave mixing (BEFWM) is also explained based on the acoustic and optical wavefronts. The reasonable explanation indicates the rationality of the model we use for SBS. Basing on the model, we suggest that one can obtain coherent ultrasounds with arbitrary wavelength and wavefront in transparent media by two specific lights.(5)We propose reducing SBS threshold by enhancing initial Brillouin sounds. Calculations and theoretical analysis shows the feasibility of the proposal. We provide four schemes of enhancing the sounds, including by transducers, by electro-acoustic medium, by microwave radiation and by two lights.(6)We use the above schemes as SBS generators, therefore the threshold of the SBS amplifiers are reduced due to importing Stokes seeds by generators. Calculations show the threshold can be reduced by one magnitude.(7)We design an improved BEFWM scheme, in which two different materials are used to decouple and conjugate the pumps. To make signal wave resonant with backward wave, the angle between them is determined by the parameters of the media and lights. A zero-threshold, high reflectivity and high fidelity PC mirror can be achieved by this scheme.(8)The affect of optical breakdown on SBS in solid media is considered. We point out that the intensity of impurity, especially of ion, is a key factor that limits the maximum SBS reflectivity.(9)We deduce the nonlinear electric susceptibility for SBS. The imaginary part of the electric susceptibility with higher frequency is proportional to that of density fluctuations, while the susceptibility with Stokes frequence is proportional to the complex conjugate of the imaginary part of the fluctuation.