Research On Supercontinuum Generation Based On Few Mode Photonic Crystal Fiber

Supercontinuum is widely used in many fields due to its ultra-wide continuous spectral range,and high coherence.It is the preferred source of optical coherence tomography and effectively solves the problem of the light source in the ultra-high-rate,ultra-large-capacity combined multiplexing optical fiber transmission system.Due to their periodic cladding structure,flexible adjustment of dispersion and high nonlinearity,photonic crystal fibers have unique advantages in the media which are pumped to generate high-quality supercontinuum spectra,furthermore the mode characteristics of the few-mode photonic crystal fibers is advantageous to produce wider supercontinuum spectrum.Thus the study of the supercontinuum spectra generated by few-mode photonic crystal fibers has important academic and application value.The main research of this paper is as follows:?1?The research background of supercontinuum spectroscopy is described,and the transmission characteristics and nonlinear characteristics affecting the generation of supercontinuum spectra are introduced.Combining Maxwell's equation with Helmholtz,the single-mode nonlinear Schr?dinger equation is derived.It is extended to the few-mode nonlinear Schr?dinger equation and the numerical simulation method used to solve the equation is expounded.?2?In this paper,a new method of generating low-pump mid-infrared supercontinuum spectra is proposed by the mode characteristics of few-mode photonic crystal fibers.The material Ge_11.5As₂₄Se_64.5 with high transmittance and high nonlinear coefficient in the mid-infrared region is selected as the base material of the photonic crystal fiber,and six small air holes are added around the innermost air hole to enhance the coupling effect and the nonlinear effect,and reduce the loss of the first-order mode in the fiber.A high nonlinearity,low loss and flat dispersion few-mode photonic crystal fiber has designed.The theoretical model of supercontinuum generation of few-mode photonic crystal fibers is simplified.The Step-by-step Fourier numerical method is used to calculate the influence of various nonlinear effects on the supercontinuum spectrum.When self-phase modulation dominates,the spectrum is almost symmetrically extended around the center wavelength of the pump light,when four-wave mixing and cross-phase modulation dominate,the spectrum is asymmetric extended to the long-wave direction,and the spectral width becomes wider,four-wave mixing has a larger effect on spectral width extension than cross-phase modulation.?3?On the basis,a system which the mid-infrared supercontinuum spectra is generated in the low pumped few-mode photonic crystal fiber is constructed.The structure parameters affecting dispersion and loss properties of photonic crystal fiber are optimized from air hole layer number,duty cycle and smaller air hole diameter.Then the working parameters affecting the spectrum are optimized from the fiber length,the peak power of pump light.In the conclusion:When the center wavelength of the pump source is ?₀=3?m,the initial pulse width is f₀=250fs,the peak power of the pump light is P=120W,the FM-PCF fiber length is L=3cm,it can produce a supercontinuum spectrum with the output of-30.5dB,and good flatness.The spectral range can be from near-infrared to almost mid-infrared,and fits the needs of low-pump mid-infrared supercontinuum sources.