Asymmetric light transport in L-shaped and U-shaped photonic crystal waveguides

We present an asymmetric light transport in a modified U-shaped photonic crystal waveguide (PCW). The compact waveguides consist of coupling two L-waveguides in a PC formed by dielectric rods in air arranged in a square lattice. We show that the transmitted power flow depends on the direction of the incident light, the transmission varies between a high and low value upon switching the position of the source beam from the input to the output of the PCW. Computations are performed using the plane wave expansion and finite-difference time-domain methods.     

光子晶体平面波导与脊波导高效耦合技术的研究

利用一维变周期谐振腔阵列和非线性缓变边界，可以实现光波从脊波导到光子晶体平面波导 （PCW）的高效耦合.基于平面波展开法（PWE）和时域有限差分法（FDTD），深入分析和讨 论了普通脊波导、2D-PCW结构和本征模以及工作模式、缓变边界形状等对耦合效率的影响， 从而得出光波从脊波导到2D-PCW、再返回脊波导的统一图景.指出考虑模式转换和采用缓变 边界条件可以极大提高PCW与脊波导间的耦合效率.对PC-PW边界采用线性和非线性缓变结构 进行了仿真，讨论了边界缓变程度对耦合效率的影响.结果表明，采用模式耦合和PC-PW余弦 缓变边界时的耦合效率在较宽的带宽内超过了95%. 关键词： 光子晶体波导 脊波导 PWE FDTD 耦合边界     

Tunable optical switch based on two orthogonal line defect waveguides

The transmission characteristics of a 2-dimensional (2D) square lattice photonic crystal waveguide (PCW) with an additional perpendicular line defect are investigated by the plane wave expansion method (PWM) and the finite difference time domain (FDTD) method. From simulation results and theoretical analysis, it is found that changing the refractive index of the dielectric columns in the additional line defect can effectively adjust the light transmission through the PCW. The working mechanism can be boiled down to either the excitation of a single defect mode or the contra-directional coupling of the two defect modes. And this mechanism can be used to design other tunable optical devices.     

The directional emission sensitivity of photonic crystal waveguides to air hole removal

To obtain highly directional light output from photonic crystal waveguides (PCWs), the emission characteristics of the narrow-width waveguide structures are investigated by tailoring the geometry of the exit sides. The local structural deformations in the form of air hole removal from the triangular-lattice photonic crystal (PC) show the effectiveness of the previously proposed approach that was implemented by us for another type of PC. The spatial broadening of the beam is greatly suppressed. With the modified waveguide exits, highly directional emissions with small side lobes are achieved. The frequency dependency of the directional emissions is evaluated. We show that the divergence angles of the beams depend linearly on the wavelength for a regular type of PCW but the modified PCW exits have local minima with respect to wavelength in terms of the divergence angle. The present work may prove to be helpful in the design of couplers and edge-emitting lasers and in the implementation of free-space optical communications.     

基于波导间能量耦合效应的光子晶体频段选择与能量分束器

基于波导间能量耦合效应的光子晶体功率分束器具有结构紧凑、带宽较宽、弯曲损耗低、分光角度大和不受外界电磁场干扰等优点.本文利用时域有限差分方法,理论研究了二维三角晶格光子晶体耦合波导的功率分束特性,设计得出了一种能够在宽频谱范围内针对不同频率区间实现不同分光比的功能器件.在此基础上通过改变耦合区介质柱形状以及输出分支波导与能量耦合波导的连接位置,最终针对三个相邻频率范围内的入射光信号,较好地实现了三均分、二均分、单一输出通道这3种能量分配输出模式.该功能器件具有透过率对比度高、结构紧凑等特性,对于发展全光功能器件在大规模全光复杂集成领域内的实际应用具有一定的促进作用.     

Design of an augmented reality display based on polarization grating

A new optical system for an augmented reality(AR) display is proposed in this paper.The optical system mainly includes a ray deflector, coupling input grating, optical waveguide, and coupling output grating.Both the ray deflector and the coupling input grating are designed based on the diffraction characteristics of the polarization grating, and the coupling output grating is the Bragg reflection grating.Compared with other AR schemes, this AR optical system not only reduces the number of projections from two to one, but also improves the efficiency of light coupling into the optical waveguides.The energy loss is reduced by utilizing the single-order diffraction characteristics of the polarization grating in its coupling input structure.The light deflector uses the polarization selectivity of the polarization grating and the characteristics of the rotating light of the twisted nematic liquid crystal layer to realize beam deflection.The working principle of the optical system is experimentally and theoretically demonstrated.     

Directly laser-written integrated photonics devices including diffractive optical elements

Femtosecond laser-written integrated devices involving Fresnel Zone Plates (FZPs) and waveguide arrays are demonstrated as built-in optical couplers. These structures were fabricated in borosilicate glass using a direct laser writing technique. The optical properties of these integrated photonic structures were investigated using CW lasers and high-resolution CCDs. For a single FZP coupled to a single waveguide, the overall coupling efficiency was 9%. A multiplexed optical coupler composed of three FZP layers was demonstrated to couple three waveguides simultaneously in a waveguide array. Structures of this type can be used as platforms for multichannel waveguide coupling elements or as microfluidic sensors that require higher light collecting efficiency.     

Pure Reflection and Refraction of a Surface Polariton by a Matched Waveguide Structure

We study the fundamental problem of reflection and refraction of a surface polariton as it strikes the interface between two waveguide structures. By making the two waveguide structures match to each other, coupling of surface polaritons to radiating modes is cancelled, and the reflected and transmitted waves consist of only surface modes. The reflection and transmission coefficients are calculated, and negative refraction of surface polaritons is demonstrated. Finite-difference time-domain numerical simulations are also performed to verify the analytical results. As one of the applications of the matched waveguide structure, a square corner reflector for phase-retardation-free reflection of surface polaritons is proposed.     

Enhanced beaming of light from a photonic crystal waveguide via a self-collimation photonic crystal

A simple method to control the light transmission from a photonic crystal waveguide (PCW) is proposed. The self-collimation (SC) effect in PC is utilized to depress the diffractive behavior of the light beam emitting out from the PCW. The finite difference in time domain (FDTD) simulation results show that choosing appropriate self-collimation frequency and the space between the PCW and the SC-PC can achieve a very good collimating beam in the outgoing side.     

全腔输出相对论磁控管输出模式转换结构的理论设计和数值模拟

采用全腔输出结构后,相对论磁控管径向尺寸显著减小,轴向长度也有较大幅度的缩短.但是,由于输出结构为三个相对独立的扇形波导,实际应用时,一般需要对微波输出模式进行转换.针对全腔输出相对论磁控管,本文研究了两种输出模式转换结构并利用三维全电磁粒子模拟程序对其进行了研究.首先研究了将三个扇形波导角向增宽从而渐变或者突变为一个同轴波导的情况,研究结果表明,两种情况下输出微波功率均大于采用传统三个独立扇形波导输出时的90%,输出模式主要是TEM模.其次研究了输出区由三个扇形输出波导分别变换为三个截面大小与之接近的矩形输出波导的可行性,研究结果表明,注入扇形波导中的TE₁₁模式几乎全部转换为矩形波导中的TE₁₀模式.实际应用时,可根据需要选择上述输出模式转换结构.     

Numerical Analysis of Coupling between Two-Dimensional Conventional Dielectric Waveguides and Photonic Crystal Waveguide

A problem of coupling between conventional dielectric waveguides and a two-dimensional photonic crystal waveguide is analyzed, using the Fourier series expansion method combined with the absorbing boundary condition of a perfectly matched layer (PML). A novel structure for the input and output ends of the photonic crystal waveguide is proposed to increase the coupling efficiency. Numerical examples are demonstrated and discussed to validate the effectiveness of the proposed structure.     

慢光在光子晶体弯折波导中的高透射传播

研究了慢光模式在SOI(silicon-on-insulator)材料光子晶体线缺陷弯折波导中的传输特性. 通过优化波导弯折处的结构参数,慢光模式在光子晶体60°与120°弯折波导中的透射率提高10倍以上,归一化透射率分别达到80%和60%以上. 为了进一步减慢光速,设计了新颖的高Q值耦合腔弯折波导结构,在归一化透射率达到75%的基础上,光波群速度低至c/170(c为真空光速). 研究结果对于增强光子晶体的慢光效应,提高光子晶体慢光器件的微型化和集成化都有一定的积 关键词： 光子晶体 慢光 弯折波导 透射率     

基于时域有限差分法分析等离子波导和介质波导耦合

使用Rsoft软件中的时域有限差分模块Fullwave分析二维介质波导和等离子波导耦合特性,利用软件仿真耦合结构并自动计算出光在介质波导和等离子波导中传输的耦合效率,进而测绘耦合效率随波导尺寸和光波长的变化曲线图,发现MDM导波结构的缝隙宽度和光通信质量密切相关,在确定尺寸下,传输损耗随传输距离成指数衰减.根据分析得到的耦合效率变化规律发现介质波导和等离子波导间距最佳点都应设为15 nm,进而优化波导的几何结构参数后,可以将耦合效率提高到83%.     

Propagation properties of elastic waves in semi-infinite phononic crystals and related waveguides

The transmission and reflection coefficients of two-dimensional semi-infinite solid-solid phononic crystal systems and fluid-fluid phononic waveguide structures have been investigated. The numerical results show that the transmission spectra for longitudinally and transversally polarized incident waves are different, and the spectra of the transmission and reflection coefficients of the semi-infinite system agree well with the band structure. The numerical results show that when a guided wave incident, localized modes are excited, and different polarities have different coupling efficiencies with the incident guided wave. At the same time, far from the cutoff frequency, the guided wave couples out of semi-infinite waveguide highly efficiently.     

阵列波导光栅梳状带通滤波器的传输效率

在光波导模场分布高斯近似条件下,根据星形光波导耦合器的耦合特性,推导出了基于累加运算和卷积运算近似表达的阵列波导光栅梳状带通滤波器光谱响应效率的函数表达式。给出了阵列波导光栅梳状带通滤波器光谱响应效率曲线的半最大值全宽度和阵列波导光栅梳状带通滤波器的通道中心波长的光谱响应度与器件参数的关系。在输入信号光谱分布高斯近似条件下,给出了阵列波导光栅梳状带通滤波器信号通道传输效率的计算表达式和输入信号光谱宽度对阵列波导光栅梳状带通滤波器信号通道输出特性的影响。给出了物理意义明确的函数表达式,它们可为快速分析阵列波导光栅梳状带通滤波器的特性提供理论基础。     

基于硅纳米线波导的两级光子晶体缩束器

鉴于在微观领域光波的缩束对实现光电集成的重要意义,提出了基于硅纳米线波导的两级光子晶体缩束器。其中一级压缩基于W5型和W1型光子晶体波导间的高效耦合。二级压缩则由宽为0.1μm,长为3.06μm的纳米线波导和W1型光子晶体波导构成,通过二者的高效耦合实现光束压缩。当W1型光子晶体波导和纳米线波导间介质柱的半径为0.04μm时,对于1550nm波长的电磁波,缩束器的通光效率可达93.4%,压缩比为16.08,出射光束半峰全宽仅为0.148μm。     

Photonic Crystal Power-splitter Based on Mode Splitting of Directional Coupling Waveguides

In the paper, a novel power-splitting scheme based on two dimensional photonic crystal (2D PhC) is proposed. The structure can be divided into three sections, including the input waveguide, coupling region, and output region, and the latter two sections consist of two parallel waveguides placed in proximity. The operation principle of the splitter is that only one of the super-modes splitting from the directional coupler can propagate through coupling region in the frequency range of interest. The radius of air holes next to the guiding region in coupling region is increased to avoid the acute back reflection at the entrance to the input waveguide induced by the modes mismatch between the input waveguide and coupling region. While in output region, the radius of these corresponding air holes is also increased so that the two splitting super-modes have same propagation constants to avoid the coupling between the two output waveguides. Moreover, as the length of coupling region is varied, its influence on the splitting performance is discussed, and it is verified that the relationship between the splitter length and bandwidth has a trade-off.     

组合型TM01-TE11弯形圆波导模式转换器研究

 研究了组合型TM₀₁-TE₁₁弯形圆波导模式转换器（两弯曲段中间加一段直圆波导），在保持其输出与输入端口轴线平行的前提下，分别从理论推导、数值计算、软件模拟三个方面对此结构进行了分析，模拟结果与数值计算结果吻合得很好。结果表明：计及功率损耗，频率为3.75 GHz，铜波导内径为9.0 cm，轴向间距为10.57 m时，模式转换器总的功率转换效率为93.8%。     

Polarization-independent grating couplers for silicon-on-insulator nanophotonic waveguides

We propose the use of subwavelength structures in a waveguide grating to achieve polarization-independent coupling of light between an optical fiber and a silicon-on-insulator (SOI) optical waveguide. The subwavelength structure allows the mode effective indices of the TE and TM modes in the grating section to be precisely engineered. We calculate that coupling efficiency of over 64% is possible using the proposed design for polarization-independent coupling between single-mode optical fibers and SOI nanophotonic waveguides.     

数值模拟二维间隙表面等离子波导传输特性

利用表面等离子激元的新颖特性,设计了二维间隙表面等离子波导.以这种结构为基础通过变形和组合形成90°直角弯曲波导、T型光功率分配器和光开光,采用时域有限差分法研究了它们的传输特性.结果表明:不同于介质光波导的弯曲损耗来自于辐射泄漏,90°直角弯曲间隙表面等离子波导的能量损耗主要来自于金属中的欧姆热损耗.在间隙达到40 nm以上后,当直行段的长度适当时,弯曲段的透射率较相同长度的直波导的透射率要大.T型光功率分配器在两输出波导的间隙宽度比达到0.6及以上时,不同于传统介质波导的分光原则,能量主要沿等效折射率较小的输出臂流出.当两输入光的相位反相时,T型光开关处于输出截止的状态,当两输入光的相位同相时,T型光开关处于输出导通的状态.所有波导间隙均小于衍射极限,实现了超衍射极限传播,可用于未来了超大规模集成光路中.