【作者】 胡明列；

【导师】 王清月；

【作者基本信息】 天津大学 ， 光学工程， 2004， 博士

【摘要】 本论文详细阐述了飞秒激光脉冲在光子晶体光纤中的传输特性,从理论和实验的角度分析和研究了飞秒激光脉冲与光子晶体相互作用时发生的多种非线性效应。首先综述了非线性光学及飞秒激光和光子晶体光纤的发展历程; 利用有限元法计算和分析了光子晶体光纤的色散特性、有效模面积、双折射度、模式折射率等参数; 利用分步傅立叶法数值模拟了广义的非线性薛定谔方程,分析研究了多种条件下超连续光谱的产生过程; 实验上系统研究了在多种结构的光子晶体光纤中超连续光谱的产生并详细讨论了超连续光谱的影响因素; 系统研究了光子晶体光纤中模式和偏振控制下的位相匹配过程。主要研究内容如下: 第一、利用有限元法分析和计算了不同空气比的光子晶体光纤的色散特性,论述了该方法的可靠性和精度,阐述了包层空气孔结构对光子晶体光纤色散特性的影响; 在此基础上分析和计算了不同空气比的光子晶体光纤的有效模面积并利用微扰法分析了双折射光子晶体光纤; 研究了增强数值孔径型光子晶体光纤的特性和大空气比光子晶体的高阶模折射率; 第二、利用分步傅立叶法对广义的非线性薛定谔方程进行了数值求解,在加入高阶色散和高阶非线性效应的情况下,模拟了飞秒激光脉冲在光子晶体光纤中的传输情况,分析了飞秒激光脉冲在正负色散区传输时的时域和频域展宽情况; 研究了高阶色散、自陡峭、脉冲内喇曼效应对光子晶体光纤中超短脉冲的传输以及最后超连续光谱产生的影响。第三、实验上系统研究了飞秒激光脉冲在具有相同芯径、不同包层结构的光子晶体光纤中超连续光谱的产生情况,指出光子晶体光纤可控色散特性在非线性光学研究中的重要性; 在多种结构的光子晶体光纤中获得了超连续光谱,并对比进行了分析; 在多芯集成的光子晶体光纤中,通过简单调整耦合位置在这种光纤中获得不同波段、不同带宽的超连续光谱; 在渐变数值孔径型的光子晶体光纤中,超连续光谱的输出功率明显提高; 分析讨论了飞秒激光脉冲参数对超连续光谱的影响以及脉冲不稳定性对超连续光谱产生的影响情况; 并首次研究了硅材料热效应对超连续光谱的影响,发现了光子晶体光纤中超连续产生过程中的温度效应。第四、利用单模双折射光子晶体光纤与纳焦耳量级的飞秒激光脉冲相互作用,首次在可见光波段高效率地获得了波长可调谐的反斯托克斯超短激光脉冲。所产生的反斯托克斯波脉冲的中心波长分别为490nm和510nm,并相应观察到明亮的蓝光和绿光基模输出。实验系统研究了泵浦光偏振方向和泵浦光强度对反斯托克斯波产生的影响。第五、利用多模双折射光子晶体光纤与纳焦耳量级的飞秒激光脉冲相互作更多还原

【Abstract】 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.更多还原