【作者】 吕昌贵；

【导师】 崔一平；

【作者基本信息】 东南大学 ， 物理电子学， 2005， 博士

【摘要】 论文首先综述了光纤光栅在WDM高速全光通信和传感领域的应用,总结了目前光纤光栅制作方法以及传输特性理论分析方法的进展,并在现有耦合模理论和实验基础上对光纤布拉格光栅(FBG)的传输特性进行了理论分析,对光栅的制作和应用进行了实验研究。耦合模理论通常被用来分析光纤光栅的传输特性。对于均匀正弦光纤布拉格光栅,耦合模方程组存在解析解。结合分段均匀的传输矩阵法,该方法可以分析各种非均匀的光纤光栅。采用这种方法,我们分析了单模均匀正弦光纤布拉格光栅Fabry-Perot(F-P)腔的传输特性。这种F-P腔的谐振峰只出现在光栅的反射峰内;由于光栅的反射相位因子影响,在光栅的第一对零点带宽内总是存在多个谐振峰;由于光栅反射峰边缘反射率逐渐减小到零,当F-P腔谐振谱线间隔大于光栅3db带宽的一半时,腔就处于单模运转状态。根据光纤光栅和光纤光栅F-P腔的传输特性,论文给出了如何设计F-P腔的结构来实现腔在光栅的中心耦合波长处的单模运转,为线性腔光纤激光器的设计提供理论依据。由于这种单模运转的光纤光栅F-P腔在同一根光纤材料上制作,其光谱形状随温度变化和应变的整体线性漂移特性,使其具有潜在的开发实用前景,如可调的窄带滤波器,带宽比光纤布拉格光栅带宽小几个量级,可调特性与光纤布拉格光栅相同。我们首次将这种单模运转的光纤光栅F-P腔滤波器作为环形腔光纤激光器的内腔选频元件,实现了稳定的单频输出。由于不存在增益介质长度的限制,这种结构的环形腔光纤激光器还具有大功率可调谐输出的潜在优点。多模光纤布拉格光栅正成为一个新的研究热点。一方面,多模光纤易与其他光源耦合,并且相对较小的色散使其适用于光纤通信系统,特别是在接入网应用中。另一方面,多模光纤布拉格光栅较为复杂的光谱特性也为光纤传感提供了更多的选择。我们首次采用耦合模理论分析了多模光纤布拉格光栅的模式耦合及传输特性,模拟了多模光纤布拉格光栅的反射光谱。由于多模光纤中存在传播常数不同的多个传输模式,多模光纤布拉格光栅中模式的相互耦合形成多个反射峰。各个模式携带的能量将影响着这些反射峰的大小,这也使得多模光纤布拉格光栅的反射谱形状依赖于光栅中模式的激发条件。当光纤中存在的模式数量较多时,耦合模方程组因数量太多而不利于求解。针对多模光纤中各个模式的角向分布规律不同,我们因此还提出了适用于具有圆柱对称结构波导中光传输的考虑了角向分布的三维标量时域有限差分法,并模拟了光纤中光的传播。使用此方法分析多模光纤布拉格光栅的工作还没有完善,在此基础上我们还需要能够将反射场与激发场分离开以便分析波导的反射和透射光谱特性。作为实验研究的基础,我们系统研究了光纤布拉格光栅的制作与封装,建立了一套基于相位掩模法的光纤光栅扫描刻写系统,紫外光源采用输出功率约100 mW的连续倍频氩离子激光系统(输出波长244 nm)。在此基础上,我们提出并实现了面向该光栅生产制作过程的计算机逆向程序辅助设计。该逆向设计首先需要在不同条件下刻写的两根光栅的参数作为刻写系统的标准参数,然后根据所需光栅的特性直接给出采用该系统制作光栅的实验刻写条件。在很多应用中,如光纤激光器的稳定单频输出、密集波分复用DWDM的中心波长稳定等,为保证器件性能都要求光纤光栅的光谱特性对环境温度是稳定的。因此我们设计了一种单端温度补偿的封装结构,其结构简单,成本低。另外,我们提出了一种新的利用光栅周期渐变来补偿折射率切趾引起的自致啁啾效应的切趾布拉格光栅刻写方法,这种光栅折射率调制相对于纤芯折射率不存在突变。在光栅刻写过程中总是或多或少地存在着各种随机扰动,我们因此还分析了光栅周期随机涨落和折射率调制横向分布不均匀对布拉格光栅反射特性的影响。最后,我们研究了有机双光子吸收介质中动态光栅的建立,以及因而产生的反向受激瑞利布拉格散射现象。由于存在双光子共振吸收,光致折射率变化得到共振增强,介质的非线性折射率系数n2相对较大。因此我们建议在双光子共振吸收材料制作的光纤上建立动态光纤光栅作为激光器的外腔反射镜来实现全光波长转换器。更多还原

【Abstract】 The applications of fiber gratings in WDM high-speed all-optical communication system and sensing system are firstly introduced in this dissertation. Then the current fabrication methods of fiber gratings and the theories to analyse their transmission characteristics are briefly summarized. Based upon the coupled-mode theory and experiments, the spectra of fiber Bragg gratings (FBGs) are analysed theoretically and the fabrication and application of FBGs are explored experimentally here.The coupled-mode theory is usually used to analyse mode’s coupling in waveguides. There exists analytical solution to the coupled-mode equations for the case of a uniform FBG. Combined with piecewise-uniform approach, it can be used to analyse any kind of nonuniform FBG. By this method, the spectral characteristics of a Fabry-Perot (F-P) cavity composed of two single-mode uniform sinusoidal FBGs are analysed. The resonances of this type of F-P cavity only exist inside the reflection band of its FBGs. Due to the phase factor of the reflective coefficient, there is always more than one resonance inside the reflection band between the first zeros on either side of the maximum reflectivity. When the resonance space of the F-P cavity is larger than the FWHM bandwidth of its FBGs reflection, the cavity operates on the state with single resonance. Knowing the characteristics of the F-P cavity and its FBGs, we present how to design a F-P cavity with the single resonance at the FBG’s Bragg wavelength, which provides theoretical guidance for the design of fiber lasers with linear cavity.The FBG F-P cavity with single resonance is fabricated in one fiber, which makes the whole resonant spectrum shift linearly with respect to temperature and strain. That the bandwidth of the resonance is much smaller than that of FBG leads to the application of narrow band-pass filters (NBPF). Using this type of NBPF as an intra-cavity frequency-selective component, stable single frequency Er-doped fiber ring laser can be achieved. Because of no limitation of gain medium length, this type of fiber ring laser has advantages of potentially large output power and wavelength tunability.Multimode fiber Bragg gratrings (MMFBGs) are attracting more and more attentions. On one hand, multimode fiber is easier to couple with laser sources for its large core size, and its relatively small dispersion makes it suitable for optical fiber communication system, especially for the access network. On the other hand, the complicated MMFBG spectrum provides more selections for the fiber sensing system. We analyze to our knowledge for the first time the transmission characteristics of a MMFBG theoretically by using the coupled-mode theory and considering two modes coupling. The reflection spectra are simulated here. The results of the calculation show that MMFBGs have multiple reflection peaks due to the coupling between the same modes in counter-propagating direction and the coupling between the adjacent modes incounter-propagating direction, and the spectra depend on excitation conditions of the bounded modes, such as mode power and mode number.We also explore other theoretical methods to analyze the propagation of light in MMFBGs and present a three-dimensional scalar FDTD method for the waveguide with circular symmetry since it’s difficult to solve the coupled-mode equations when there are many modes coupled in MMFBGs. Light propagation in a fiber is simulated by using this scalar FDTD method. However, this method needs to be improved to be able to separate the reflection field from the excitation field in order to obtain the transmission and reflection characteristics.To do further experimental research, we study systematically the fabrication and encapsulation of FBGs, and build a set of scanning system of fabrication with phase mask method. The UV source is a double-frequency Ar+ laser system with output power of 100 mW (244 nm). To guide the fabrication of FBGs, an assistant converse program facing directly the process of fabrication is presented. This program needs the parameters of two FBGs fabricated by the system as the standard parameters of the system, then provides the fabrication conditions to fabricate a FBG which we want. In many applications, the Bragg wavelengthes of FBGs need to be insensitive to the environments. A new type of encapsulation of fiber gratings with single-end compensation for temperature effect with simple structure and low cost is thus proposed. Also we present a new method for the fabrication of the apodised FBGs without self-induced chirp. There is no abrupt change of refractive index between the grating and fiber core for the apodised FBG fabricated by this method. Relating to the random factors during the fabrication of FBGs, we analyze the influence of the random fluctuation and transverse asymmetry of the refractive index modulation.In the last chapter, dynamic grating formed in a two-photon absorbing organic dye solution and its induced backward stimulated Rayleigh Bragg scattering are studied here. Due to the enhancement of the two-photon resonant absorption, the medium has large nonlinear coefficient of refractive index. Based upon these studies, we proposed to form a dynamic grating in a fiber with two-photon resonant absorption as a feedback component of lasers to achieve a new type of all-optical wavelength convertor.更多还原