Optimal Design Of Highly Nonlinear Photonic Crystal Fibers And Research On Raman Soliton Self-Frequency Shift

Highly nonlinear photonic crystal fibers (PCFs) have unique properties compared with the traditional optical fibers due to the wavelength of the order periodic or quasi-periodic distribution of air holes in the cladding region. PCFs have very good applications in optical communications, optoelectronics, nonlinear optics and so on, which is an excellent performance optoelectronic device. Because the geometry structure of refractive index of the light guide PCFs can be flexibly designed, it is possible to control the dispersion characteristic of PCFs. By adjusting reasonable the structure of the PCFs, this PCF with a broadband and flattened dispersion profile can be obtained. The PCF is applicated very extensive in the field of parametric amplification and supercontinuum generation, and highly nonlinear can play an important role in the development of nonlinear optical devices, and the highly nonlinear and flattened dispersion PCFs have farther application value and very broad space for development.In this paper, the dispersion and nonlinear characteristics of PCFs, numerical analysis method of the PCFs and their application in Raman soliton self-frequency shift (RSSFS) are numerically studied. By comparatively analyzing the optical properties of PCF, the method on designing the highly nonlinear PCF with the optimal structure is given. And two novel highly nonlinear and broadband dispersion flattened PCFs (HN-BDF-PCFs) are designed. We deduce Generalize Nonlinear Schrodinger Equation (GNLSE), and research the properties of ultrashort pulses propagation in highly nonlinear fiber. The GNLSE was modified for the subwavelength size of nonlinear effect, and the RSSFS of highly nonlinear PCFs are studied deeply. The main innovation of research and results are as follows:(1) The structure and properties of PCFs with traditional rule-based hexagonal distribution of air holes are analyzed theoretically, and performance parameters of PCFs associated with the structure are simulated that the optimal design method of HN-BDF-PCFs is given. By this method, a HN-BDF-PCF is designed, and the nonlinearity and dispersion characteristics are simulated and analyzed. By further optimizing the structure, another improved HN-BDF-PCF is designed, and the performance is simulated and analyzed. By comparing the structural parameters and performance of two HN-BDF-PCFs, the dependence of performance on the structure parameters of HN-BDF-PCF can be obtained.(2) In order to accurately describe the propagation dynamics of ultrashort pulses in HN-BDF-PCFs, GNLSE is deduced detailed in this paper, and the new nonlinear phenomena is analysised in the PCF, considering the properties of electromagnetic vector caused by optical field enhancement mechanism, the effects of nonlinear in the spectrum of the pulse with strong dispersion, the complex change of vector mode contour, the mode field distribution changed by frequency and the sensitivity of the transverse field distribution with frequency. The modified GNLSE is deduced detailed. Compared to GNLSE, the revised-Nonlinear Schrodinger Equation (R-NLSE) adds subwavelength size of the nonlinear terms, and still considers the self-steepening effects and the Raman effects. Within a certain frequency range, the subwavelength size of nonlinear effect has a certain degree of limit or promotion on the certain nonlinear effects.(3) In order to better analyze the nonlinear effects in HN-BDF-PCFs, the split-step Fourier method (SSFM) is used to solve the R-NLSE, so that the whole operation speed is faster. SSFM is used to study the nonlinearity induced by the femtosecond optical pulses and supercontinuum generation in HN-BDF-PCFs. The influence of high-order dispersion and nonlinear effects on the shape and bandwidth of supercontinuum generation referring to the RSSFS is analysed. It is demonstrated that inter-pulse Raman scattering combined with self-phase modulation led to the emergence of ultra-fine structure in the continuous spectrum.(4) The mechanism of RSSFS is analysised, and the effects of pulse parameters and the fiber length on RSSFS range are discussed theoretically. By simulating to realize RSSFS that the subpicosecond pulses are propagated in HN-BDF-PCF and improved HN-BDF-PCF, and the effects of pulse parameters and the fiber length on RSSFS range and output pulses are analyzed and compared respectively, so the reasonable suggestion about choicing the frequency shift of the optical fiber is get. By the simulation, it is demonstrated that the subwavelength size of nonlinear effect can have a limit on RSSFS. The related details are studied when the short pulses with different peak power and widths are propagated in different transmission distance.