Nonlinear Mechanism And Experimental Study Of Femtosecond Pulse Propagating In Photonic Crystal Fiber

Photonic crystal fiber has many excellent characteristics that traditional optical fiber does not have,such as: highly nonlinearity,adjustable dispersion,high birefringence and large mode field area,etc.The combination of high peak intensity of ultrashort pulse laser and the photonic crystal fiber produces all kinds of optical nonlinear phenomenon,and the broadening spectrum generated by the nonlinear effect plays a wide role in research and application,such as the industry,communication and medical,etc.Although the photonic crystal fiber generates all kinds of nonlinear spectrum,but it is relatively lagging behind in the highly efficient frequency conversion,deep ultraviolet supercontinuum generation,widely tunable mid-infrared soliton and mid-infrared supercontinuum and so on.These issues have attracted considerable attention in the area of nonlinear spectrum.This thesis launches the corresponding characteristic research mainly around the above applications,main research content in this paper are described as follows.First,combined with the maxwell's equations,nonlinear schrodinger equation about the nonlinear effect of photonic crystal fiber is established.The group velocity dispersion,and nonlinear effect of photonic crystal fiber,such as self phase modulation,cross phase modulation,four wave mixing,solitons,and the the stimulated Raman scattering are given.According to the simulation results,we can analyze the effect of photonic crystal fiber length,pulse width,pulse peak power,pulse wavelength,initial chirp on photonic crystal fiber frequence spectrum.Second,the finite element method is used to simulate the various parameters of photonic crystal fiber and high nonlinear,double zero dispersion and birefringence characteristics are found.The photonic crystal fiber is pumped by femtosecond laser pulses in the anomalous dispersion and highly efficient and broadband dispersion wave are generated.The ratio of the dispersive wave and residual pump intensity can be up to 42.67,the spectral width of dispersion wave is broadened to 81 nm.Conversion efficiency of dispersive wave can exceed 50%,the residual pump left only less than 10%,which achive highly efficient and broad band frequency conversion using the low average power.Attenuation plate is then used to adjust the average power and half wave plate is used to change input pulse polarization direction,which have an effect on intensity and center wavelength of dispersion wave of the high nonlinear and birefringent photonic crystal fiber.The changes of dispersive wave are analyzed.The research results can promote the high speed optical communication and the generation of the new device in the ultrafast photonics.Third,a photonic crystal fiber with double cladding structure and a photonic crystal fiber with large air holes formed section are studyed about the middle infrared Raman soliton through the experiment.When the photonic crystal fiber with double cladding structure is pumped by femtosecond laser,the first mid-infrared soliton moves from 1 933 nm to 2 403 nm and the adjustable range lasts for up to 470 nm with the pump wavelength working at 827 nm and the pump power increasing from 0.1 W to 0.42 W,which provides a good condition for silica-based photonic crystal fiber to generate the broadband tunable ultrashort pulse soliton laser source.Most of photonic crystal fibers are using the fiber core transmission in current research and application.However,when a photonic crystal fiber cladding has large air holes,its mode area Surrounded by three air hole is smaller than the fiber core and the corresponding nonlinear coefficient is larger.so it can form rich nonlinear effect.Fourth,deep ultra-violet supercontinuum broadening effect of photonic crystal fiber is studied through the experiment and its nonlinear mechanism is analyzed.When the pump wavelength is fixed at 860 nm,the broadening range of deep ultra-violet supercontinuum is gradually broadened with the increase of pump power;when the pump power is fixed at 0.4 W,the cross-phase modulation between the solitons and the dispersive waves can effectively extend the fundamental mode deep ultra-violet supercontinuum into the wavelength of 212 nm.The increase of pump wavelength not only broadens supercontinuum range but also greatly improves the conversion efficiency of deep ultra-violet supercontinuum.The experiment result can provide a good solution for deep ultra-violet sources needed by optical detection and optical information processing.In addition,when the pump wavelength is far from zero dispersion wavelength of photonic crystal fiber,this article adopts the method of the post-processing of photonic crystal fiber.The tapered technology is used to change the position of zero dispersion wavelength of photonic crystal fiber,which can help to generate the ultra-violet supercontinuum.Finally,based on the value of mid-infrared supercontinuum,one kind of photonic crystal fiber is developed to produce mid-infrared supercontinuum.Supercontinuum range extending from 610 nm to 2 500 nm is got from the experiment,which can be used as mid-infrared supercontinuum light source,spectral analysis and atmospheric detection,etc.