【作者】 张继皇；

【导师】 陆培祥；

【作者基本信息】 华中科技大学 ， 物理电子学， 2007， 硕士

【摘要】 微结构光纤是近年来出现的一种新型光纤,其特点是包层具有规则或随机分布的波长量级的微结构。包层中的微结构使得这种光纤能够呈现出许多传统光纤不具备的特性,如:无截止的单模特性、可控的色散特性、高非线性、光子带隙特性等。具有这些特性的微结构光纤在通信、生物医学、传感探测、超短脉冲激光等领域都有极大的应用前景。空芯带隙型微结构光纤作为一种真正以光子带隙为导光原理的微结构光纤,其在传输超短脉冲激光上有着其它光纤不可比拟的优势,这在生物医学等领域中具有潜在的应用价值。本文从理论和实验上对空芯带隙型微结构光纤进行了研究。通过数值计算对空芯带隙型微结构光纤的特性进行了理论模拟。首先利用平面波展开法计算了空芯带隙型微结构光纤包层的光子带隙,分析了在这类光纤中存在空气导模的条件及其与晶格常数的关系,并与全固态微结构光纤的带隙特性进行了比较。然后利用有限元方法对空芯带隙型微结构光纤的有效模式折射率和色散特性进行了理论模拟。理论模拟为分析实验中所用光纤的带隙和色散特性奠定了基础。实验上研究了超短脉冲激光在一种大孔径空芯带隙型微结构光纤中的传输特性。实验测量得到了该光纤的透射谱及在800nm处的损耗。研究了光纤输出光谱特性与入射激光脉冲的中心波长、平均功率以及光纤长度的关系。实验结果表明该光纤能在频谱上几乎无失真的短距离传输纳焦量级的飞秒激光。结合理论计算的光纤色散特性分析得到了引起超短激光脉冲输出频谱变化的因素:经光纤传输后输出光谱主要受自相位调制效应影响;随着输入光功率的增大,由于自陡峭效应使频谱蓝侧边沿发生展宽;高阶色散导致了输出光谱的不对称性。此外,实验发现亚纳焦量级的飞秒激光脉冲能在空芯带隙型微结构光纤中不同的次芯上传输并产生多重频率转换。在零色散点为690nm的次芯中产生了450nm至1000nm的超连续激光谱。在零色散点为630nm的次芯中获得了源于四波混频效应的550nm附近的反斯托克斯光。据我们所知,本文工作首次证实和明确提出了空芯带隙型微结构光纤能同时实现超短脉冲激光传输和多重频率转换。这种既可以产生超连续光谱作为宽带光源,又可以传输超短脉冲激光的特性在生物光子等领域具有潜在的应用价值。更多还原

【Abstract】 A new type of fiber, known as microstructure fiber, has emerged in the past several years. These fibers are characterized by wavelength-scale microstructures distributing regularly or randomly in the cladding region running along the entire fiber lengths, which have resulted in some unusual properties unattainable for conventional optical fibers, such as endless single mode, flexible dispersion, high nonlinear and photonic band gap. Therefore, microstructure fibers have great potential in the field of information communication, biomedicine, transducer and ultrashort pulse laser technology. Hollow core photonic band gap microstructure fiber is one kind of microstructure fibers which guide light by photonic band gap. However, this fiber has preponderance on delivery ultrashort pulse laser. which enable it in the field of biomedicine. The propagating properties of microstructure fibers are investigated theoretically and experimentally in the present dissertation.Hollow core photonic band gap microstructure fiber is simulated by using of theoretical method. Firstly, the photonic band gaps of microstructure fiber are calculated by using of full-vectorial plane-wave expansion method. The photonic band gaps of different crystal lattice constant and difference between hollow core and all solid PBG fibers are analysed. Secondly, we used the finite element method to simulate the effective mode index and dispersion properties of hollow-core photonic band gap microstructure fibers.Propagating of femtosecond laser pulse in a new structure hollow-core microstructure fiber is investigated. The transmission spectra and loss of microstructure fiber are measured on experiment. We researched the relations between the spectrum of output pulses through fiber core and laser central wavelength, laser power and fiber length. It is demonstrated that nanojoule femtosecond laser pulses coupled into the air core propagate with nearly preserved spectral profiles through the shorter length fiber. Combining to the property of dispersion, we give analysis as follows: The spectrum of output pulses is mainly affected by Four-wave Mixing; the role of self-steepening and higher order dispersion is more highlighted as the increment of average power. The secondary cores between air holes in the cladding of this fiber can be served as waveguide channels. These channels allow multiple frequency conversion for sub-nanojoule femtosecond laser pulses. Supercontinuum spectra with specific wavelength bands can be produced in some channels. A broadband continuum spectra extending with from 450nm to 1000nm generated with subnaojoule femtosecond laser pulses coupled into a secondary core with zero GVD wavelength of 690nm. A four wave mixing process with anti-Stokes emission around 540-560nm took place in one channel with zero GVD wavelength of 630nm. The results show that the microstructure fiber has potential applications in some domains where both frequency conversion and delivery of femtosecond laser pulses are required.更多还原