Research On Design And Application Of Flat-dispersion Photonic Crystal Fiber

The flat-dispersion photonic crystal fiber（FD-PCF）is a kind of total internal reflection photonic crystal fiber.By changing the cross-section structure and size of the FD-PCF,its optical properties can be well adjusted,so that it can be applied to various scenarios.Therefore,the research on FD-PCF has been enduring.Its applications can be divided into two different fields of linearity and nonlinearity.The former requires that the signal frequency does not change during transmission,while the latter requires that the signal frequency be controlled and converted to different frequency range.Linear applications are mainly used in communication systems,especially in dense wavelength division multiplexing（DWDM）systems to increase transmission capacity by adding channels at different frequencies.An important direction of nonlinear applications is the generation of supercontinuum,especially in the mid-infrared.Supercontinuum has been widely used in optical communication,spectroscopy,micro measurement,remote sensing and medical diagnosis etc due to its wide spectrum,high brightness and high coherence.This paper designed fiber structures based on the important applications of FD-PCFs in linear and nonlinear fields,the finite element method was used to study the optical properties of the PCF,and the nonlinear Schrodinger equation was used to carry out numerical simulation of the propagation of the optical pulse in the PCF.Finally,two kinds of TIR-PCFs,line PCF and DC-PCF（dispersion compensation-PCF）,without crossing-zero wavelengths in DWDM wavelength band were designed and proved to be suitable for long-distance and u WB（ultra-wide-band）transmission.In addition,the FD-PCFs with three zero-dispersion wavelengths（ZDWs）were designed based on Si O₂ and As₂Se₃,respectively,for the generation of supercontinuum,and good near-infrared and mid-infrared supercontinuums（MIR-SCs）were obtained by numerical simulation.The main research contents and innovative achievements of this paper are as follows:1.An improved PCF model with three-layer air holes was proposed,and two PCFs with ultra-flat dispersions were designed and applied in DWDM system.Within the minimum attenuation frequency band of silica,one PCF had a near-zero,all-positive and ultra-flat dispersion curve,which was suitable for line transmission in the DWDM system,and the other one had a low negative and ultra-flat dispersion curve,which was suitable for dispersion compensation module in the DWDM system.The PCF model with special structure was proposed to obtain the third extreme point of dispersion curve by adding three holes between the standard lattices of odd layers.The dispersion curve tended to be flat by controlling the filling rate of air holes,the size of optical fiber and the diameter of air holes in different layers of cladding.In DWDM system,the designed optical fiber could support super-long-haul transmission of 1600km and without any dispersion compensation.If the DC-PCF designed in this paper was used for dispersion compensation,it could transmit 498km more,and the dispersion compensation efficiency of the DC-PCF was 33 times.This design method not only brought more bandwidth to DWDM system,but also reduced the number of dispersion compensation modules,improved the dispersion compensation efficiency,and effectively reduced the network construction cost of DWDM system.The research on these two kinds of PCFs provided a scheme of low dispersion compensation cost and low signal equalization cost for the construction of super-long-haul and ultra-large bandwidth DWDM system.2.Considering the transmission constant and the results of supercontinuum spectrum simulation should be presented with wavelength as independent variable,we started from the definition of the transmission constant of optical pulse,combined with the relationships between the speed of light,wavelength and frequency in vacuum,the general term formula of the transmission constant of any order based on wavelength was derived by using the derivative method of composite function.This general term formula could provide any orders of transimission constants for the simulation of optical pulse in photonic crystal fiber.3.A quantitative control method for dispersion parameters of PCF was proposed.Silica PCF with three ZDWs and flat dispersion was designed,and ultra-wide coherent supercontinuum spectrum was obtained by adjusting the dispersion slope of PCF.In order to make dispersion slope the only dispersive factor affecting supercontinuum formation in numerical simulation,a new method was proposed to design PCF with different dispersion slope by fixing the three ZDWs.Two PCFs with the same three ZDWs and different dispersion slopes were designed by this method.And on this basis,the effects of different dispersion slopes on the width and flatness of supercontinuum were analyzed.By pumping 50 fs high-energy optical pulse into the first anomalous dispersion region near the first ZDW,it was found that the PCF with small dispersion slope was more favorable to generate a flatter and wider supercontinuum,but it needed a longer evolution distance.And a new physical mechanism named direct soliton spectrum tunneling（DSST）was found that it stemmed from the mismatch between self-phase modulation and the decreasing dispersion in abnormal dispersion regime.Then,the prevailed SPM greatly compressed the first fundamental soliton,caused its spectrum extended greatly to span the first anomalous dispersion region,the second normal dispersion region and the second anomalous dispersion region,and made it eventually lose soliton properties and inhibit the production of redshift dispersive wave（R-DW）.This study provided experimental and theoretical basis for the generation of ultra-wide coherent supercontinuum spectrum on the right side of transparent window in silica PCF,and further enriched the category and content of soliton dynamics in nonlinear optics by proposing the concept of direct soliton spectrum tunneling.And it can be used for the generation of coherent supercontinuum.4.A circular As₂Se₃ PCF with flat dispersion and three ZDWs was designed to generate a big bandwidth and high flat MIR-SC.By analyzing the dispersion curve and ultra-high nonlinear coefficient of As₂Se₃ material,the PCF with big core diameter was determined to be taken to shift the first ZDW of material dispersion to 4.055μm,which,at the same time,made the dispersion curve flatter and the nonlinear coefficient lower,so the first fundamental soliton could be manipulated to produce DSST in the process of self-frequency shift of the soliton.The numerical simulation results showed that the DSST and other nonlinear processes could extend the MIR-SC to almost the whole mid-infrared band（2.535μm～16.6μm）.Meanwhile,the effects of propagation length,peak power,initial chirp and pulse width on the generation characteristics of MIR-SC were analyzed in detail,which effectively avoided the adverse factors such as optical shocking and four-wave mixing that affected the expansion of mid-infrared supercontinuum in highly nonlinear PCF,and finally increased the range and degree of flatness of Mir-SC.