The Numerical Simulation Of Sbs In Water Based On FEM

The underwater detection technology based on stimulated Brillouin scattering(SBS) belongs to a frequency modulation method, the backward scattering signal can be obtained from the interaction between pump beam and acoustic wave. Based on the SBS spectra, the location information of underwater object can be judged. SBS method presents a significant advantage in underwater object detection, so it is very important to improve the performance of SSB detection system. In this dissertation, the SBS physical model of single cell focalizing structure has been numerically simulated to explore the influence rules of pump parameters, medium parameters and focusing parameters on SBS features, such as energy reflectivity, phase conjugation fidelity, steady-state gain coefficient. Based on the SBS physical model, some appropriate parameters were selected for improving the performance of SBS detection system.In this dissertation,the three-dimensional transient coupled wave equations of SBS with the non-plane waves, transverse diffraction and medium absorption were presented based on Maxewll’s equations and Navier-Stokes equation. According to the SBS physical model of ingle cell focalizing structure and laboratory conditions, the expression of Gaussian beam has been transformed from three-dimension to two-dimension. A steady state two-dimensional model of SBS was given based on the steady state approximation.Numerical solution was given of the mathematical model by using the finite element method, and then the influence of plus parameters(single pulse energy, bandwidth, wave length), medium parameters(phonon lifetime, interaction length) and focusing parameters(focal length) on SBS characteristics(energy reflectivity, phase conjugation fidelity, the steady-state gain coefficient) was analyzed.The simulated results indicate that, the increase of the single pulse energy of pump light can improve the SBS energy reflectivity and phase conjugation fidelity; the bandwidth of pump light has a limited effect on SBS steady-state gain coefficient, the greater the bandwidth of pump light is, the smaller SBS steady-state gain coefficient is; within the range of SBS effective length, the longer the interaction length is, the higher the SBS energy reflectivity and the phase conjugation fidelity are; the phonon lifetime is proportional to the SBS energy reflectivity and maximum steady-state gain coefficient; there exists an optimum focal length, which can produce the highest SBS energy reflectivity, and when the focal length is below or above the optimum focal length the SBS energy reflectivity will reduce.