Study On Flatten Optical Frequency Comb Generation Based On The Normal Dispersion Flatten High Nonlinear Silica Fiber

Optical frequency comb(OFC)has become the research target of many researchers because of its wide application.High peak power mode-locked optical pulse injected into the high nonlinear fiber can generate wideband optical frequency comb,and the span of OFC depends on the dispersion characteristics and nonlinear effect of the fiber.When the high peak power pulse is injected into the near zero anomalous dispersion region of fiber,it will generate high-order solitons.The span of OFC is extended greatly by the splitting of high-order solitons.When the high peak power pulse is injected into the normal dispersion region of fiber,the generation of OFC is mainly affected by self-phase modulation(SPM)and optical wave-breaking(OWB),and the OFC has good coherence.In this paper,I have further study on the optimal design of normal dispersion flattened high nonlinear silica fiber and the generation of tunable high-repetition rate,broadband flat optical frequency comb at 1550nm.The details are as follows:(1)The pulse evolution and spectrum broadening principle of pulse in normal dispersion region and anomalous dispersion region were described respectively.Based on the split-step Fourier method for solving the generalized nonlinear Schrodinger equation,MATLAB simulation was carried out for the generation of supercontinuum spectrum in three regions of highly nonlinear silica fiber with large normal dispersion,near zero normal dispersion and near-zero anomalous dispersion.By comparing the characteristics of supercontinuum spectrum under the three conditions,the near-zero normal dispersion region was determined as the main research scope.(2)We optimized the structure of near zero normal dispersion flat high nonlinear silica fiber.Based on coupling mode theory and finite element method,COMSOL software and MATLAB were used to simulate the effects of various parameters in the multi-cladding fiber structure on the parameters such as fiber dispersion and nonlinear coefficient.A high nonlinear fiber with ideal dispersion and nonlinear parameters was designed for the generation of broadband flatten optical frequency comb.(3)We studied the generation of broadband flatten optical frequency comb based on electro-optical modulator and near-zero normal dispersion flatten high nonlinear silica fiber.We analyzed the effect of optical-wave breaking on the performance of optical frequency comb during pulse evolution.Through MATLAB simulation,we studied the effects of the parameters on the characteristics of broadband flatten optical frequency comb,such as peak pulse power,pulse width,fiber transmission length,initial pulse chirp.(4)The improvement scheme of the bandwidth and flatteness performance of the optical frequency comb was studied.Different optical fiber combinations were designed to achieve the pulse chirp compression to increase the peak power of the pulse,and then the optical frequency comb spectrum in bandwidth and flatness was improved.