Numerical Investigation On The Characteristics And Application Of Vector Field In Photonic Crystal Fiber

Photonic crystal fiber?PCF?is a kind of fiber with periodic structure.The characteristics of endless single mode,adjustable mode-field size and tailorable dispersion can be obtained by changing the parameters of the fiber structure,which is widely used in the regulation of optical field and nonlinear fiber optics,and has important applications in supercontinuum generation?SCG?.The vector beam field in fiber is a kind of non-uniformly polarized light field whose polarization state varies with the spatial distribution.The polarization state of the vector beam is different everywhere on the monochromatic cross section of the beam,so it shows many new optical phenomena and has become one of the research hotspots in optical field.In this thesis,the characteristics of vector beams are analyzed numerically in an As₂Se₃ PCF and a silica PCF with small central hollow core?SCHH?,including the mode field distribution,confinement loss,effective refractive index(n_eff),and chromatic dispersion.The SCG in As₂Se₃ PCF is simulated using the fundamental mode(HE₁₁)and high-order modes(TE₀₁,HE₂₁,TM₀₁)pumping.The main contents of this research are as follows:Firstly,we design the PCF structure and establish a physical model of the fiberstructure.By controlling the structural parameters to obtain the ideal effective refractive index,dispersion,and loss curve of the fiber.The influence of SCHH?diameter from 0to 1.5?m?and duty cycle?d/?from 0.70 to 0.87?on PCF is analyaed numerically.Based on the As₂Se₃ PCF and silica PCF,when the d/?is equal to 0.75,the effective refractive index difference(?_neff)between the fundamental mode(HE₁₁)and high-order modes(TE₀₁,HE₂₁,TM₀₁)decreases as the SCHH diameter increases,while the?_neffbetween the three high-order modes increases gradually.In the silica PCF,when the SCHH diameter 1.0?m,the?_neff between adjacent modes increases with the increase of duty cycle.When the duty cycle is 0.70,the maximum?_neff between the fundamental mode and the high-mode is 0.03053,while the maximum?_neffbetween the higher mode is 0.01199 at the wavelength of 2?m.In As₂Se₃ PCF,field enhancement occurs in SCHH of the fundamental mode at longer wavelengths because the central air hole size?0.5-1.5?m?is much smaller than the transmitted wavelength range?5-10?m?and the length is far below the diffraction limit.When the duty cycle is 0.75,the?_neff between the fundamental mode and the high-order mode is 0.03098,and the?_neff between the high-order mode is 0.06867 at the wavelength of 5?m.The dispersion characteristic fluctuates greatly with the structure,and the dispersion curve of the fundamental mode is much more stable than that of the high-order modes.The anomalous dispersion regions of the high-order modes are also wider than that of the fundamental mode.This characteristic has an important impact on SCG.The loss of As₂Se₃ PCF can be kept within 1 d B/m in the wavelength range from 1-9?m after optimizing,which ensures the application of PCF in mid-infrared band.Secondly,based on the As₂Se₃ PCF,the coherence of SCG and the SCG under the fundamental mode(HE₁₁)and higher-order modes(TE₀₁,HE₂₁,TM₀₁)pumped is studied.The SCHH diameter of the fiber is simulated from 0 to 1.5?m at intervals of 0.5?m.When the pump is in the HE₁₁ mode,the range of SC is 1.2-6.0?m,1.9-8.5?m,2.0-8.0?m,and 1.7-7.3?m.When the pump is in the TE₀₁ mode,the range of SC is 1.2-6.0?m,1.2-5.5?m,1.2-6.0?m,and 1.4-6.5?m.When the pump is in the HE₂₁ mode,the range of SC is 1.2-7.5?m,1.2-5.0?m,1.2-5.0?m,and 1.2-5.5?m.When the pump is in the TM₀₁ mode,the range of SC is 1.2-8.1?m,1.2-7.0?m,1.2-6.4?m,and 1.1-6.6?m.The first zero-dispersion wavelength?ZDW?of the HE₁₁ mode is longer than other high-order vector modes,which is advantageous to generate longer wavelengths by the soliton effects.The second ZDW of the TM₀₁ mode is shorter than other vector modes,so the wavelength interval between the two ZDWs is shorter,which is conducive to the generation of dispersion waves in the longer-wavelength direction.Therefore,the HE₁₁and TM₀₁ modes produce relatively wide SCs.As the peak power is too high and the noise is amplified,resulting in generally poor spectral coherence of the SC.