tailieunhanh - Nonlinear Optics - Chapter 9

Stimulated Brillouin and Stimulated Rayleigh Scattering Chúng tôi đã thấy tại mục ánh sáng tán xạ có thể xảy ra chỉ như là kết quả của sự biến động trong các thuộc tính quang học của một hệ thống vật chất. Một quá trình tán xạ ánh sáng được cho là được tự phát nếu các biến động (thường là hằng số điện môi) gây ra ánh sáng tán xạ được kích thích bởi nhiệt hoặc các hiệu ứng không điểm quantummechanical. Ngược lại, một quá trình tán xạ ánh sáng được cho là được kích thích nếu sự. | Chapter 9 Stimulated Brillouin and Stimulated Rayleigh Scattering . Stimulated Scattering Processes We saw in Section that light scattering can occur only as the result of fluctuations in the optical properties of a material system. A light-scattering process is said to be spontaneous if the fluctuations typically in the dielectric constant that cause the light-scattering are excited by thermal or by quantummechanical zero-point effects. In contrast a light-scattering process is said to be stimulated if the fluctuations are induced by the presence of the light field. Stimulated light scattering is typically very much more efficient than spontaneous light scattering. For example approximately one part in 105 of the power contained in a beam of visible light would be scattered out of the beam by spontaneous scattering in passing through 1 cm of liquid water. In this chapter we shall see that when the intensity of the incident light is sufficiently large essentially 100 of a beam of light can be scattered in a 1-cmpath as the result of stimulated scattering processes. In the present chapter we study stimulated light scattering resulting from induced density variations of a material system. The most important example of such a process is stimulated Brillouin scattering SBS which is illustrated schematically in Fig. . This figure shows an incident laser beam of frequency rnL scattering from the refractive index variation associated with a sound wave of frequency Q. Since the acoustic wavefronts are moving away from the incident laser wave the scattered light is shifted downward in frequency to the Stokes frequency rnS rnL Q. The reason why this interaction can lead to stimulated light scattering is that the interference of the laser Recall that the scattering coefficient R is of the order of 10 6 cm 1 for water. 429 430 9 Stimulated Brillouin and Stimulated Rayleigh Scattering Figure Stimulated Brillouin scattering. and Stokes fields contains a .