Study On Supercontinuum Spectrum Generation In Optical Fibers

The origin of Supercontinuum Spectrum (SC) generation in optical fibers has been shown to be the interplay between fiber nonlinear effects (such as self phase modulation, cross phase modulation, stimulated Raman scattering and stimulated Brillouin scattering etc) and group-velocity dispersion (GVD). The dispersion characteristic of the fibers and the cross phase modulation effect on SC generation are studied. The thesis are composed of 7 chapters, among which are, Chapter 1 is a survey, Chapter 2 is the basic theory of this thesis, Chapter 3,4,5,6 are the introductions of my own work, and Chapter 7 are the conclusion and expectation.Chapterl:The development and applied significance of SC are introduced.Chapter2: In this chapter, the basic theory of the research work and the pulse-propagation equation are introduced.Chapter3:The generation of SC spectrum in the typical dispersion-decreasing fiber is theoretically researched. The research indicated that the dispersion characteristic of the fiber has much effect on pulse evolution and SC spectrum generation. While the pulse propagates in the fiber, it becomes asymmetrical and the asymmetrical SC spectrum is resulted eventually because the dispersion slope of the dispersion-decreasing fiber is not equal zero. The smaller the dispersion slope, the more asymmetrical the pulse and the SC spectrum are. It is m&fe suitable for broaden SC spectrum generation when the dispersion slope is smaller.Chapter4:The SC spectrum generated in the dispersion-flattened and decreasing fiber is theoretically researched. The research indicated that the pulse keeps symmetry while propagates in the fiber, and the symmetrical SC spectrum is resulted eventually because the dispersionslope of the fiber at the pump wavelength is equal zero. It is more suitable for the broader SC spectrum generation when the dispersion curve of the fiber is flatter and distance between the two zero-dispersion wavelengths of the fiber is farther from each other.Chapter5:The effect of cross-phase modulation (XPM) on pulse propagation and SC spectrum generation in a dispersion-flattened and decreasing fiber are researched. It is found that XPM could enhance the SC spectrum generation and increase the generation efficiency. Under certain conditions, in spite of their different modal indices the two polarized components of the pulse can trap one anther and move at a common group velocity due to XPM, the phenomenon referred as self-trapping occur. When the pump power of the input pulse was lower, the enhancement on SC spectrum generation due to cross-phase modulation effect is more significantly. The cross-phase modulation effect increased the SC spectrum generation efficiency greatly. By numerical analysis, we also knew that the Raman self-frequency effect has more significant effect on SC spectrum generation than other higher-order nonlinear effects.Chapter6:The generation of flat SC in non- dispersion-decreasing fiber is studied. The results show that the fiber with chromatic dispersion that is a convex function of wavelength in normal dispersion region is favorable for generating a flat SC spectrum. The flatness and the bandwidth of the SC spectrum are affected by the dispersion characteristic in a SC fiber. A flat and broad SC spectrum can be obtained by optimizing the chromatic dispersion design of the SC fiber.Chapter7: The conclusion and expectation.