Defect Bragg Fiber With Low Loss for Broadband and Zero Dispersion Slow Light

Narrow bandwidth and large group velocity (v_g) dispersion are two fatal limitations of slow light in Bragg fibers. In this paper, by introducing a well-designed defect layer into the cladding of the Bragg fiber, the modal characteristics are modified by the coupling between the core mode and the defect mode. The defect location mainly determines the coupling strength and, thus, exerts a strong influence on v_g and dispersion. The defect thickness mainly determines the resonant wavelength of the defect waveguide and, thus, the wavelength where the modal coupling takes place. Consequently, the two limitations of the slow-light propagation in the Bragg fiber are overcome through proper optimization of the defect parameters. Around 1550 nm, a slow-light bandwidth up to 90 nm is achieved at an average v_g of c/5 (c is the light velocity in a vacuum) under N = 2, whereas an average v_g of c/10 is achieved with a bandwidth of 20 nm under N = 5. On the other hand, the slow-light propagation of v_g = 0.074c with both zero dispersion and zero dispersion slope is achieved, which is able to support applications requiring a subterahertz bandwidth of optical pulse. All of the fiber designs ensure sufficient low losses and good optical field distribution. The results are helpful in developing various Bragg-fiber-based slow-light devices.     

光子晶体光纤色散特性的数值分析

光子晶体光纤日益成为目前研究的热点,但一般只局限于理论上的定性分析,没有进行准确的定量计算.应用平面波法和整胞方法对二维光子晶体光纤进行了全数值模拟,计算了有效折射率、V参数以及波导色散特性.研究了上述参数与光子晶体光纤的结构参量孔距a,相对孔径f等于D/a的关系.显示出光子晶体光纤的无限单模性质和可控色散特性.     

应用等效折射率模型研究光子晶体光纤

应用等效折射率模型研究了光子晶体光纤(PCF)的传播特性.介绍了光子晶体光纤的等效折射率模型.通过求解标量波动方程得到了光子晶体光纤包层基空间填充模的模式折射率,利用阶跃光纤的理论来研究光子晶体光纤的导模特性.应用此模型对不同结构光子晶体光纤包层区的等效折射率与波长的关系进行了讨论.包层区等效折射率与芯子的折射率差随波长的增加而增大,并由此阐述了光子晶体光纤的单模特性.数值分析得到光子晶体光纤的基模的模式折射率,并由此研究了光子晶体光纤的波导色散与结构参量的关系.分析表明,光子晶体光纤的波导色散随空气孔孔距的变化符合Maxwell方程的比例性质.空气孔的相对孔径对波导色散有重要的影响.这些分析表明光子晶体光纤具有可以灵活设计其色散特性的潜在应用前景.     

大模面积色散平坦光子晶体光纤的优化设计

设计了一种正八边形空气孔排列的大模面积色散平坦光子晶体光纤,借助多极法对这种结构的光子晶体光纤的模场面积、有效折射率、色散系数和限制损耗进行了数值模拟.结果表明,正八边形空气孔排列的光子晶体光纤的模场面积较相同空气孔间距和空气填充率的正六边形空气孔光子晶体光纤大,且其色散曲线可以在很宽的波长范围内保持色散平坦并具有较低的色散值.主要分析了当这种光纤的结构参数发生改变时,光纤的限制损耗、有效模面积以及色散特性的变化规律,最终通过选择适当的参数,设计了在1 300～1 650 nm波长范围内色散平坦的大模面积光子晶体光纤.     

一种新型高双折射光子晶体光纤的特性研究

设计了一种新型结构的光子晶体光纤,建立了对应的数学模型并采用全矢量有限元法对该结构的模场强度、有效折射率、双折射、色散特性和限制损耗进行了分析。研究表明,该光纤在1 550nm处可以获得高达7.66×10-3的双折射和低至12ps/(nm·km)的色散值,同时在800～1 600nm波长范围内,始终保持1.498×10-6 dB/m以下的极低限制损耗,可用于制造极低色散值的保偏光纤。     

C波段色散补偿光子晶体光纤的研究和设计

设计了一种新颖结构的双层芯色散补偿光子晶体光纤。此光纤在整个C波段具有高负色散特性。通过合理选取双层芯光纤的外层芯层数,同时优化孔间距和空气孔直径,设计的光纤在C波段的色散值在-520ps/(km.nm)和-390ps/(km.nm)之间近似线性变化,残余有效色散系数近似为零,相关色散斜率(RDS)在0.0032nm-1的色散补偿光纤,其RDS值与标准单模光纤匹配,有效模场面积优于常规色散补偿光纤,可以对其长度30倍以上、用于宽带传输的标准单模光纤进行良好的色散和色散斜率补偿。     

Effect of the structural parameters of photonic crystal fibers on propagation characteristics

Using a full-vector finite-difference time-domain (FDTD) method, this article explores the propagation characteristics of photonic crystal fiber (PCF) theoretically. The dependence of structural parameters on the effective index of the fundamental guided mode, effective index of the fundamental cladding mode, mode field diameter, confinement loss, effective mode area, and chromatic dispersion in PCF have been studied, respectively. The research presents a reference for designing of PCF with a specific purpose.     

高折射率纤芯光子晶体光纤的特性研究

在二氧化硅中掺入适量GeO2可增大折射率，用其作为光子晶体光纤的纤芯时易于将光场捕获在纤芯中，形成稳定的传输模式。本文通过有限差分法数值解亥姆霍兹方程，研究了空气孔呈三角形典型结构排列的光子晶体光纤的特性．当纤芯及空气孔的大小都相同时，纤芯掺杂比例越高，光子晶体光纤的有效折射率就越高，色散则会向负向增长。此外，在这种高折射纤芯的光子晶体光纤中，当纤芯的大小及折射率均固定仅增大周围空气孔时，光子晶体光纤的色散增大，有效折射率趋于降低，模场有效面积也趋于减小。     

具有混合形状空气孔的OAM传输光纤设计研究

本文提出了一种新型轨道角动量模(OAM)传输的光子晶体光纤(PCF)结构,该光纤由最中心空气孔、高折射率环形层和外包层构成,其中外包层由一圈椭圆形空气孔和多圈呈周期排列的扇形空气孔共同组成,无需通过额外的掺杂,就可以使中心空气孔和外包层空气孔之间形成一个用来传输OAM光束的等效高折射率环形区。通过对该PCF的传输特性进行仿真分析,发现在1.55μm处,各模式的限制损耗维持在10^-6 dB/m-10^-10 dB/m,在C波段的色散值均维持在310 ps/(nm×km)以下,HE2,1模在1.55μm-1.6μm波段内的色散变化为3.1 ps/(nm×km)。在1.55μm处最大的Δnneff能够达到4.83×10^-3,大的有效折射率差可有效地抑制了模式间的简并,避免HE模和EH模耦合成LP模。验证了该光纤具有大带宽、小而平坦色散、大有效折射率差、低限制损耗和低非线性系数等优良性能。在1.25μm-2.0μm波段内共可以支持34个OAM模式的有效传输,每个模态都可作为独立的信道传递信息,适合用于大量数据的传输,大幅提高了光通信的系统容量和频谱效率。     

Dual-frequency electric-magnetic-electric microstrip leaky-mode antenna of a single fan beam

This paper presents a dual-frequency electric-magnetic-electric (EME) microstrip exhibiting two leaky-wave regions of similar radiation characteristics like the microstrip EH/sub 1/ mode. The EME microstrip incorporates a photonic bandgap (PBG) structure, which is a two-dimensional array consisting of unit cell made of coupled coils connected by a via. The PBG structure employed in the EME prototype conducts at dc and shows the first stopband between 8.8-12.4 GHz, thus rendering the so-called magnetic surface. The EME microstrip is essentially made by substituting the PBG cells for the metal strip of a conventional microstrip. The finite-element method (FEM) analyses of the PBG structure show that the first and second modes are TM-like and TEM-like, respectively. The latter is leaky between 12.4-12.9 GHz and is found to be responsible for the second leaky region of the EME microstrip. The dispersion characteristics of the EME microstrip are obtained by two theoretical methods, namely, the matrix-pencil method and the FEM. Both show excellent agreement in the two leaky regions. Furthermore, the measured far-field radiation patterns of the two leaky regions also validate the dispersion curves. The first leaky region is of EH/sub 1/ type and between 5.05-5.45 GHz. The second leaky region radiates a frequency-scanning fan beam between 11.95-13.0 GHz, similar to those of the EH/sub 1/ mode. Detailed modal current analyses show even and odd symmetry along longitudinal and transverse plane of EME microstrip, respectively, further confirming the two leaky regions behave like the well-known EH/sub 1/ leaky mode. The proposed EME microstrip enriches the modal characteristics of the conventional, uniform microstrip and is thus a manifestation of application of PBG structure for new guiding device.     

降低光子晶体谐振腔Q值的方法分析

该文在分析计算金属光子晶体的正三角形晶格TE模式的色散特性、全局带隙分布图的基础上,针对金属光子晶体结构谐振腔Q值较高的问题,对降低光子晶体谐振腔Q值的方法进行了分析和设计。采用加载介质柱的混合结构和介质微扰两种方法分别对谐振腔的Q值进行有效的控制,并分析了两种方法对谐振腔模式选择性的影响。结果表明,两种方法都能在不改变谐振腔模式选择性和场分布的基础上有效降低Q值,而介质微扰的方法还同时清除了与TE04竞争的两种杂模,提高了模式选择性。     

锥形光子晶体光纤的基模特性分析

本文研究了锥形光子晶体光纤(TPCF)的基模演化特性,分析了包层空气填充率对其特性的影响.数值计算结果表明TPCF比常规锥形光纤更能束缚模场;在相同的拉锥条件下,包层空气填充率越大的TPCF,其模场能保持芯模的形式传输更长的距离并且其芯区能量密度也越大.而TPCF的有效面积沿锥体纵向先缓慢减小,后急剧增大,最后又减小,随着空气填充率的增大,有效面积的最大值逐渐向锥尾方向移动.同时,对色散研究表明,在整个锥体中,色散值先负向增大,后正向增大;在空气填充率较小时,其色散曲线沿锥体纵向变化平缓,而空气填充率较大时,色散曲线起伏明显;并且随着空气填充率的减小,色散曲线的最低点移向初始拉锥端.     

环形结构塑料光子晶体光纤色散特性分析

利用有限差分光束传播法模拟了光波在环形结构塑料光子晶体光纤内的传输情况,讨论了基模有效折射率与光纤结构的关系,并分析了色散特性随孔间距、孔大小变化的一般规律,发现塑料光子晶体光纤的总色散主要来自材料色散,尤其在短波长范围内材料色散更是居于主导地位,但随波长的增大波导色散逐渐增大,可以设计零色散波长位于可见光波段的塑料光子晶体光纤,此光纤也能很好地应用到非线性领域.     

空心布拉格光纤及其在传感应用中的研究进展

基于一维光子带隙(PBG)效应导波机制的空心布拉格光纤(HC-BFs)具有灵活方便的带隙调控能力和优良的宽带低损耗传输特性,作为一类独特的光子带隙型空心光纤,在光波传输、色散管理与控制、光纤型器件和传感检测等领域表现出巨大的应用潜力.从光纤波导结构设计与导波模式传输特性、包层材料构成与光纤制备工艺,以及基于气态或液态被测物质填充中空纤芯的痕量气体检测和生化传感中的应用等方面对HC-BF研究的发展历程和新进展进行了综合评述.     

Fast Calculation of Cavity-Mode Characteristics of Photonic Crystal Cavities

A fast approach based on effective index perturbation method is proposed to evaluate the intrinsic characteristics of photonic-crystal-slab-based microcavity with two-dimensional finite-difference time-domain (2-D FDTD) technique. For two widely used single defect structures, less than 2% computational error was obtained in calculating the defect mode frequencies. Accurate prediction of cavity modal properties and resonant peak frequencies is feasible based on 2-D FDTD simulation by adjusting the effective index to match the dielectric band edge for donor-like defect mode. The correlation between the modified effective index and the cavity (lasing) mode with the highest quality factor$Q$offers an efficient tool in the design of defect mode based photonic crystal microcavities.     

二维光子晶体的能带及慢光特性研究

通过二维方形与二维三角晶格光子晶体光纤能带的对比,选取二维三角晶格作为研究对象。介电常数的周期性在光子晶体中可以形成特殊的光子禁带,进而控制光子的运动状态。基于此,提出了一种单线缺陷光子晶体波导结构,通过分析其含有线缺陷时的能带结构及其色散关系、对群速度的影响等,使其有效慢光区域的GVD位于105～106量级。通过动态...     

全内反射光子晶体光纤

从模式、色散、偏振和非线性等出发，对全内反射光子晶体光纤（TIR-PCF）进行了分析和讨论。运用有效折射率和有效归一化频率对TIR-PCF包层在短波区的色散特性进行了理论分析，解释了其零色散点兰移的特性，并与常规光纤进行了比较。     

微结构光纤的有限元分析计算法

利用有限元法对零色散点在780nm的均匀微结构光纤进行经过简化的矢量波动方程模拟计算，获得了所需要的模场分布、有效折射率、色散、有效模面积等参数，并与实验数据相参照验证了这种方法的准确性和精度。进而将这种计算方法用于非均匀微结构光纤的模拟计算，不仅获得了很好的结果，与其他方法相比还具有更快的计算速度，计算所得到的结果对将来设计和拉制微结构光纤很有帮助，并且这种方法在设计不规则的微结构光纤方面具有很好的优势。     

Periodic bandgap and effective dielectric materials in electromagnetics: characterization and applications in nanocavities and waveguides

The main objectives of this paper are to characterize and develop insight into the performance of photonic bandgap (PBG) periodic dielectric materials and to integrate the results into some novel applications. A powerful computational engine utilizing the finite-difference time-domain technique with periodic boundary conditions/perfectly matched layers integrated with Prony's method is applied to provide an in-depth look at the physics of PBG/periodic bandgap structures. Next, the results are incorporated into two classes of applications in the areas of nanocavity lasers and guidance of electromagnetic (EM) waves in sharp bends. A two-dimensional PBG structure with finite thickness is presented to strongly localize the EM waves in three directions and design a high-Q nanocavity laser. It is shown that the periodic PBG/total internal reflections remarkably trap the EM waves inside the defect region. The effect of the number of periodic cells and defect's dielectric constant on the Q of structure is investigated. It has been found that a seven-layer PBG with a dielectric impurity defect can be used in the design of a laser with a Q as high as 1050. Additionally, potential applications of the PBG structures for guiding the EM waves in sharp bends, namely, 90/spl deg/ and 60/spl deg/ channels are demonstrated. It is shown that shaping the bend by introducing small holes can noticeably improve the guidance of the waves at the bends and channel the EM waves with great efficiency. A comparative study between PBG and effective dielectric materials in controlling the EM waves is also provided and it is observed that the novel characteristics of the PBG cannot be modeled using the effective material for the frequencies within the bandgap.     

单模色散位移光纤的高斯近似理论

本文基于高斯近似建立了一种研究单模色散位移光纤的理论模型。可以直接从实测的色散位移光纤本身或其预制棒的径向折射率分布计算出光纤的色散、模场直径（ＭＦＤ）和非线性效应有效截面积等参数。理论计算和用仪表实测的光纤传输参数相比较得到了满意的结果。