Study On Propagation Of Femtosecond Laser Pulses In Photonic Crystal Fibers～+

The propagation of femtosecond laser pulses in photonic crystal fiber is demonstrated in this dissertation. The experimental and numerical results for nonlinear evolution of femtosecond pulses during propagation in photonic crystal fibers are presented. The phenomena known as supercontinuum generation and other nonlinear optical processes are investigated with the focus mainly on their occurrence in photonic crystal fibers. It is analyzed how Finite Element Method (FEM) calculations are done to find the frequency dependent propagation constant, dispersion curve, effective area, birefringence, modal refractive index of the fibers. The processes responsible for the generation of a supercontinuum are identified though the simulation of the nonlinear Schrodinger equation by Split-step Fast Fourier Transform Method. Efficient intermodally phase-matched processes are generated in birefrigent photonic crystal fibers and controlled by the polarization of the pumping femtosecond pulses. The details are described as follows: 1 The FEM calculations provide dispersion curves of the photonic crystal fibers with different air filling fractions. The precision and error of this method is discussed. Base on the FEM calculation, the birefringence is obtained with the perturbation theory. The characters of the enhanced-numerical-aperture photonic crystal fibers and the higher-oder refractive indices of the large core size photonic crystal fibers are discussed here. 2 The nonlinear Schr?dinger equation with higher-order dispersions and nonlinearity has been applied to describe the propagation of femtosecond laser pulses and supercontinuum generation in photonic crystal fibers. Numerical simulation results show that the supercontinuum bandwidth varies as the femtosecond laser pulses propagating in different dispersion ranges and the role of Self-Steepening and Raman Shock is highlighted. 3 Experimental results show that the nonlinearities can be efficiently harnessed to generate supercontinuum and laser light at new wavelengths through appropriate choice of the dispersion curve. The ability to control light on the femtosecond timescale, by manipulating the dispersion profiles precisely over a broad wavelength band, defines a new territory in nonlinear optics. And an array of fused silica waveguiding channels with randomly distributed transverse sizes in a disordered phtonic crystal fiber is shown to have an ability to generate a highly efficient and broadly tunable supercontinuum by low-energy ultrashort laser pulses, which dispersion can be switched in such waveguide arrays by coupling the pump field into waveguiding wires with different diameters. And a new kind of photonic crystal fiber with very high numerical aperture is used to enhance the efficiency of supercontinuum generation. The instability of the femtosecond laser pulses which can wash out the spectral fine structure and degrade the coherence of the generated supercontinuum has also been discussed here. Slowly-expanding supercontinuum generation phenomena were observed and proved to be the result of the temperature of the fiber end, arising from heat flow into the fiber from the in put light. 4 Birefringent microstructure fibers are shown to allow efficient generation of frequency-tunable Anti-Stokes line emissions as a result of nonlinear-optical spectral transformation of unamplified femtosecond Ti: sapphire laser pulses. The pumping femtosecond pulses polarized along the fast and slow axes of the elliptical core of a photonic crystal fiber generate intense blue-shifted lines centered at 490 and 510 nm, respectively, observed as bright blue and green emissions at the output of a 10-cm length microstructure fiber. 5 Nonlinear-optical spectral transformation of unamplified 30-fs Ti:sapphire laser pulses in a birefringent fused silica photonic-crystal fiber is shown to result in the efficient generation of a doublet of two physically distinct states of the anti-Stokes radiation field. The central wavelength of frequency-upconverted radiation and its transverse intensity profile are controlled in our experiments through a selective coupling of the pump field into higher order modes of the fiber and the polarization-sensitive phase matching in parametric four-wave mixing, allowing the central wavelength of the Anti-Stokes output to be tuned and its transverse intensity profile to be modified by varying the polarization state of the input pumping field.