High Nonlinear Dispersion Flatten Fiber Structure Design And Study Of Flatten Optical Frequency Comb

The optical frequency comb appears as a femtosecond pulse sequence with equal time intervals in the time domain;the frequency domain appears as a series of phase locked frequency lines with equal frequency intervals,and ultra-wide spectral range.Optical frequency comb has wide application prospects in many fields such as wavelength division multiplexing,microwave photonic signal processing,high-precision ranging,molecular state precise control,and astronomical instrument calibration.Based on these important applications values of optical frequency comb,it has become a research hotspot in many fields.The main work of this paper is to optimize the dispersion of the four-clad silica-based highly nonlinear optical fiber and lead silicate highly nonlinear optical fiber,and the generation of 1550 nm band tunable repetition rate and flatten optical frequency comb based on these two optical fibers.The research is carried out as follows:(1)Theoretical analysis of the pulse evolution and spectrum broadening caused by the combination of dispersion and nonlinear effects in the different dispersion regions of the nonlinear optical fiber is carried out.Using the split-step fast Fourier method to numerically solve the generalized Schr?dinger equation,the MATLAB simulation of the supercontinuum generation is carried out when the optical pulse propagate in the three regions of the large normal dispersion region,near-zero normal dispersion region,and near-zero anomalous dispersion region of the highly nonlinear optical fiber.The supercontinuum characteristics of these three regions are compared to study the effect of normal dispersion on the flatness of the optical frequency comb.(2)The optimization design of the silica-based near-zero normal dispersion flatten high nonlinear optical fiber based on the four-clad structure is carried out.Based on the coupling mode theory and finite element method,through using COMSOL and MATLAB two structures of five-clad type and core parabolic type are designed to solve the four-clad structure with small manufacturing tolerances which may cause the problem of positive and negative dispersion fluctuations.The effects of the cladding width and refractive index on the dispersion and nonlinear coefficient of the two optical fiber structures are explored separately,thereby the optimized normal near-zero dispersion flatten high nonlinear optical fiber is obtained.Furthermore,the flatten optical frequency comb based on the electro-optic modulator pulse source and the normal near-zero dispersion flatten high nonlinear optical fiber is simulated,and the time-frequency domain evolution process and coherence of the optical frequency comb are discussed.(3)The optimization design of lead silicate high nonlinear optical fiber with Lead Silica material SCHOTT SF57 as the core and SCHOTT LLF1 as the cladding is carried out.Through using COMSOL and MATLAB for simulation and choosing appropriate parameters of the step fiber,it solves the problem that the hundreds meters long silica-based fiber requirement for the optical frequency comb generation due to the small value of the nonlinear coefficient ? of the silica-based fiber.The influence of various parameters of the pulse,dispersion and loss on the optical frequency comb based on lead silicate high nonlinear optical fiber is analyzed.Finally,the performance of flatten optical frequency combs produced by two different platforms,silicabased optical fiber and lead-based optical fiber,is compared and analyzed.