双芯光子晶体光纤中的模式干涉

应用全矢量超格子叠加模型分析了双芯光子晶体光纤(PCF)的模式特征.双芯PCF的基模和二次模分别由偏振方向不同的两个偶模和两个奇模组成，讨论了这四个模式的模式电场奇偶性.基于对双芯PCF模式电场奇偶性的认识，讨论了光纤中的矢量模式干涉问题.分析表明，不同偏振态模式的干涉不会引起芯间的功率转移，双芯PCF的芯间功率耦合是由相同偏振模式的干涉引起的.在相同偏振模式干涉过程中，光功率在两个芯子中呈余弦振荡，光纤的两个芯子一般存在约π／2的相位差.还讨论了两个不同偏振方向的耦合系数与波长的关系. 关键词： 光子晶体光纤 双芯光纤 模式 干涉 耦合     

光子晶体光纤色散补偿特性的数值研究

利用矢量有效折射率方法对光子晶体光纤（PCF）的色散补偿特性进行了数值模拟，研究发现通过调节光子晶体光纤包层的空气穴节距或空气穴大小可以灵活地设计光子晶体光纤的色散系数D、色散斜率Dslope以及κ值，可以设计在波长1.55μm附近具有较大绝对值的正常色散和负色散斜率的色散补偿光子晶体光纤，使光通信中的普通单模光纤（G.652）或非零色散位移光纤(G.655)在1.55μm低损耗窗口得到较好的色散补偿.数值模拟和分析表明色散补偿光子晶体光纤的研制具有很大的发展潜力. 关键词： 光子晶体光纤 色散 色散斜率 色散补偿     

长脉冲抽运光子晶体光纤四波混频和超连续谱的理论研究

本文数值研究了长脉冲抽运光子晶体光纤四波混频和超连续谱的产生.依据准连续波近似下的相位匹配条件和能量守恒定律,能够理论上确定在光子晶体光纤正常色散区抽运时,四波混频效应产生的信号光和空闲光波长;以及在光纤反常色散区抽运时,调制不稳现象产生的两个对称旁瓣波长.利用自适应分步傅里叶法,定量地模拟了利用波长为1064 nm的亚纳秒激光器抽运两种不同色散特性的光子晶体光纤四波混频效应和超连续谱的产生,模拟结果与实验结果符合非常好. 关键词： 光子晶体光纤 超连续谱 四波混频 自适应分步傅里叶法     

Fusion splicing small-core photonic crystal fibers and single-mode fibers by controlled air hole collapse

A method based on controlled air hole collapse for low-loss fusion splicing small-core photonic crystal fibers (PCFs) and single-mode fibers (SMFs) was demonstrated. A taper rig was used to control air hole collapse accurately to enlarge the MFDs of PCFs which was then spliced with SMFs using a fusion splicer. An optimum mode field match at the interface of PCF-SMF was achieved and a low-loss with 0.64 dB was obtained from 3.57 dB for a PCF with 4 μm MFD and a SMF with 10.4 μm MFD experimentally.     

Locally-induced permanent birefringence by polymer-stabilization of liquid crystal in cells and photonic crystal fibers

The aim of this work was to induce permanent birefringence both in typical liquid crystal cells and photonic crystal fibers (PCFs) by photo-polymerization. For this purpose three different liquid crystalline materials, namely E7, 5CB, and 6CHBT were combined with a mixture of RM257 monomer and a UV sensitive initiator with the percentage weight less than 10%. Due to the photo-polymerization process it was possible to achieve polymer-stabilized liquid crystal orientation inside LC cells and micro-sized cylindrical glass tubes. In particular, periodic change in spatial molecular orientation was achieved by selective photo-polymerization. Successful results obtained in these simple geometries allowed for the experimental procedure to be repeated in PCFs leading to locally-induced permanent birefringence in PCFs.     

On Guided Waves Created by Line Defects

The propagation of guided waves in photonic crystal fibers (PCFs) is studied. A photonic crystal fiber can be regarded as a perfect two-dimensional photonic crystal (PC) with a line defect along the axial direction. Under the assumption that the background spectrum has gaps, we give a simple condition on the parameters of the medium and of the line defect, which ensures the rise of eigenvalues in a specified subinterval of the given gap of the photonic crystal fiber. Using the modified Combes-Thomas estimates, we prove that the eigenfunctions corresponding to the eigenvalues decay exponentially away from the line defect. The work was supported by the NSFC (10431030) of China. D. Miao is currently visiting Department of Mathematics, Michigan State University, USA.     

掺杂光子晶体光纤自发辐射与掺杂激活杂质的光增益透射谱研究

借助光子晶体中二能级原子的自发辐射理论证明缺陷态的局域场存在的必然性以及局域场基本性质,为研究光子晶体光纤掺杂自发辐射的内在规律提供了理论依据。且将自发辐射理论与数值模拟相结合,在缺陷介质中掺激活杂质时,研究了光子晶体光纤的掺杂局域场特征以及受激辐射增强和透射率大于1现象与光子带隙群速度异常和掺杂层复有效折射率成负的虚部之间的内在关系。由此说明光子晶体光纤的缺陷介质中掺入激活杂质时,光子禁带中能出现品质因子非常高的杂质态,具有很大的态密度,较强的受激辐射放大。     

Bending insensitive large mode area photonic crystal fiber

A novel kind of large mode area photonic crystal fibers (PCFs) are proposed in this paper. In order to achieve large effective mode area, a novel technique is applied to seven missing air hole PCF structures. The modal characteristics of PCF structures such as effective mode area, confinement loss, chromatic dispersion properties with doped cores, are investigated by employing the full vectorial finite element method (FEM). Simulation results demonstrate that effective mode area and confinement losses of fundamental mode simultaneously improved by applying our novel technique to proposed structures. The effects of bending on confinement losses of the proposed PCFs have been thoroughly investigated. Additionally, confinement losses of first higher order modes are presented and the possibility of stripping them in a simple way is discussed.     

基于谐振耦合现象的三芯光子晶体光纤带通滤波器

设计了一种三芯光子晶体光纤带通滤波器结构.利用纤芯间的谐振耦合,实现了滤波.当3个纤芯基模模式有效折射率在同一频率点实现匹配时,将产生谐振现象.通过合理选取光纤结构参数,可以使3个模式满足谐振条件.由于3个纤芯基模只在工作波长处实现有效折射率的匹配,因而纤芯间发生波长的选择性耦合.应用全矢量光束传播法(BPM)分析了这种光纤带通滤波器的性能.结果表明:在1.55 μm工作波长上,光纤耦合长度为22.8 mm;在损耗低于-3 dB前提下,通带带宽为8.9 nm.     

一种晶体光纤基模色散特性的矢量法分析

利用矢量有效折射率方法(矢量法)对光子晶体光纤基模的色散特性进行了数值模拟，并与双正交归一基矢量法以及标量有效折射率方法(标量法)的模拟结果进行了对比.发现所用矢量法的结果与双正交归一基矢量法的结果符合很好，而标量法在低空气填充率f或较高归一化波数A/λ时是一种较好的近似，在空气填充率f较高或归一化波数A/λ较低时，要得到精确的结果必须利用矢量法对光子晶体光纤的特性进行模拟.讨论了光子晶体光纤包层有效折射率与光纤结构的关系. 关键词： 光子晶体光纤 矢量法 有效折射率 色散     

Analytical evaluation of bending loss oscillations in photonic crystal fibers

In this paper we present an analytical formula for bending loss oscillations in photonic crystal fibers (PCFs). We follow the approach originally adopted for conventional double-clad fibers and show that it can be applied to PCFs by substituting the structural parameters of the conventional fiber by their PCF counterparts. We then examine the spectral dependence of the critical bending radius and the position of the first order loss peak as a function of structural parameters of the PCF cladding such as the fill factor and the number of hole rings. Finally, we evaluate the precision of the analytical model by comparing the results to finite element calculations for a selection of PCF geometries.     

Light propagation in highly birefringent photonic liquid crystal fibers

Photonic liquid crystal fibers have already been demonstrated as a promising perspective for creation of new classes of dynamically tunable optical fiber devices. By combining different geometries of photonic crystal fibers with a variety of different liquid crystals it is possible to obtain a new generation of fibers with dynamically tunable properties, e.g., transmission spectra, attenuation or dispersion. In this paper, tunable birefringence in a commercially available highly birefringent Blazephotonics PM-1550-01 photonic crystal fiber selectively filled with a low birefringence liquid crystal has been experimentally demonstrated. Theses experimental results have been compared with simulations based on the multipole method.     

Raman amplification characteristics of As2Se3 photonic crystal fibers

We present the dispersion and Raman amplification characteristics of As2Se3 photonic crystal fibers (PCFs). We compare the gain characteristics with conventional As2Se3 fibers and find that the Raman gain efficiency in PCFs can be improved by a factor of more than 4. This allows us to either use a small length of the fiber or to use the low pump power to attain similar gain characteristics. Numerical simulations reveal that a peak gain of 10 dB can be achieved in a 1.1 m long PCF when it is pumped at 1.5 microm in wavelength with an input power of 500 mW.     

Bandgap properties of Kagomé photonic crystal fibers

Photonic crystal fibers (PCFs) can guide light by the photonic bandgap (PBG) effect created by the periodically arranged air holes in the cladding. In this paper, the bandgap properties of Kagomé photonic crystal fibers (KPCFs) are investigated in detail. First, the bandgap properties of PCFs based on the basic Kagomé lattice are analyzed and compared with the PBGs of PCFs based on honeycomb and triangular lattices. We highlight the similarities between KPCFs and honeycomb PCFs in their PBGs, both having air-guiding regions only at very large air filling fractions (AIFs), whereas the PBGs of triangular PCFs can have large air-guiding regions at smaller AIFs due to the difference in the gap structure. In the second half of this paper, we show how the PBGs of KPCFs can be modified by introducing an extra air hole into the vacant space of the original lattice. In particular, KPCFs with medium-sized air holes can be designed to guide air by introducing extra air holes of a larger size. The air-guiding regions of KPCFs with very large air holes can also be greatly extended by the extra air holes. These air-guiding regions occur at higher normalized frequencies, resulting in larger air hole pitches favorable for fabrication. PACS 42.70.Qs; 42.25.Bs; 42.81.Qb     

Differential toolbox to shape dispersion behavior in photonic crystal fibers

We present an analytical procedure to compute the first derivatives of the propagation constants with respect to several structural parameters in photonic crystal fibers (PCFs). From them we can easily evaluate the same derivatives of other directly related magnitudes. The above derivatives provide the trend of the magnitude at issue, which allows us to take advantage of a gradient-based algorithm to shape the properties of the guiding structure. In this way we implement an optimization process to carry out real inverse design in PCFs. We focus our attention on designing PCFs with a specific chromatic dispersion behavior. Likewise, the same approach makes it possible to analyze their fabrication tolerances.     

Splice-free interfacing of photonic crystal fibers

We report a new method for making low-loss interfaces between conventional single-mode fibers and photonic crystal fibers (PCFs). Adapted from the fabrication of PCF preforms from stacked tubes and rods, this method avoids the need for splicing and is versatile enough to interface to virtually any type of index-guiding silica PCF. We illustrate the method by forming interfaces to two problematic types of PCF, highly nonlinear and multicore. In particular, we believe this to be the first method capable of individually coupling light into and out of all the cores of a fiber with multiple closely spaced cores, without input or output cross talk.     

全矢量有限元模型及其在光波导中的应用

为了研究光波导和光子晶体光纤的模式特性和传输特性,从矢量波动方程出发,推导出了各向异性介质中场微分方程复数泛函表达式,利用棱边/节点混合元离散了该泛函,加入了各向异性介质匹配层边界条件,得到关于传播常量的广义特征值方程.以矩形波导为例,对各向异性介质匹配层边界条件的吸收特性进行了研究,得到了基模以及几个高阶模的场分布、色散曲线和损耗曲线.结果表明该方法可靠有效.对正六边形晶格光子晶体光纤进行了分析.数据表明：光纤有效折射率随空气孔直径或波长的增大而减小,但与空气孔圈数无关;光纤限制损耗(confinement loss)随波长增大近似成指数增大,而增加空气孔直径或者空气孔圈数则可使之显著降低.     

Highly birefringent extruded elliptical-hole photonic crystal fibers with single defect and double defects

Highly birefringent elliptical-hole photonic crystal fibers （PCFs） with single defect and double defects are proposed, which are supposed to be achieved by extruding normal circular-hole PCFs based on a triangular-lattice photonic crystal structure. Comparative research on the birefringence and the confinement loss of the proposed PCFs with single defect and double defects is presented. Simulation results show that the proposed PCFs with single defect and double defects can be with high birefringence （even up to the order of 10^-2）. The confinement loss increases when the ellipticity of the air hole of the PCFs increases, which nevertheless can be overcome bv increasing the ring number or the air holes in the fiber cladding.     

光子晶体光纤接续损耗的理论分析

基于超格子构造法,采用全矢量模型计算了光子晶体光纤的模场半径,由此出发理论分析了光子晶体光纤与普通单模光纤之间接续损耗分别受横向偏移、轴向倾斜以及模场不匹配的影响,给出了光子晶体光纤在部分常用结构参量区域{Λ,d/Λ}内与SMF-28接续损耗的理论值,讨论了光子晶体光纤各结构参量与接续损耗之间的关系。并简要分析了不同结构光子晶体光纤之间的接续损耗。结果表明,接续损耗对横向偏移和轴向倾斜都非常敏感;孔距是决定接续损耗大小最主要的因素;与普通单模光纤接续,当光子晶体光纤的孔距比该单模光纤纤芯半径大一些时,接续损耗比较小;两种不同结构光子晶体光纤之间的接续损耗大小最主要取决于它们孔距的差异。     

八边形光子晶体光纤色散补偿特性分析

利用多极法对八边形光子晶体光纤的色散补偿特性进行数值模拟,分析了结构参数变化对色散补偿特性的影响;计算了具有相同参数的六边形结构光子晶体光纤的色散系数和非线性系数;研究表明八边形光子晶体光纤比六边形结构的光子晶体光纤的大负色散特性明显提高,非色散系数低,更有利于进行色散补偿.因此,本文设计了一种新型的八边形色散补偿光纤,在λ=1.55μm时色散值为-1434.9ps·nm^-1·km^-1,色散斜率为-4.6338ps·nm^-2· 关键词： 光子晶体光纤 多极法 色散斜率 色散补偿