Compact multimode interference coupler with tapered waveguide geometry

In this paper, a novel MMI coupler, based on general interference, with tapered waveguide geometry has been proposed for reduction of coupling length. The coupling characteristics and power imbalance of the proposed structure are compared with conventional MMI structures by using a mathematical model based on sinusoidal modes. It is seen that the beat length for tapered MMI coupler with angle of taper ∼1.05° is reduced by ∼24% of that of conventional MMI coupler and the coupling characteristics obtained with the mathematical model, match well with those obtained by more sophisticated BPM computer aided design software. The power imbalance for tapered 3 dB MMI coupler is more sensitive to the wavelength than that for conventional 3 dB MMI coupler and variation of power imbalance with fabrication tolerance for both the MMI coupler is almost same.     

多波导定向耦合器耦合特性

利用拉普拉斯变换,将多波导定向耦合器的耦合模方程组变换为三对角线性方程组,并采用追赶法求出了该方程组的通解;进一步,给定初始条件,进行拉普拉斯变换反演,即可严格地求出该耦合模方程组的解。采用该方法研究了5波导定向耦合器光从一个波导入射和等光强从5个波导入射时的耦合特性,求出了耦合模方程组解的解析表达式,给出了每个波导中光功率变化规律曲线。利用该方法,可很方便地求出任意有限数目波导组成的定向耦合器在任意入射条件下耦合模方程组的解,因此对于多波导定向耦合器结构的合理设计,具有非常重要的意义。     

On the radiation mode effects in integrated optical directional couplers

In this work we study the effect of radiation modes generated at the input of a directional coupler on its performance. We focus on the splitting ratio and the coupling length of the coupler. The Beam Propagation Method (BPM) is used for numerical simulation. Numerical results are compared to analytical expectations of the coupled mode theory (CMT). We show that, when the coupler is excited by a single mode fibre, ideal 3 dB couplers could be obtained by a simple adjustment of the exciting fibre. On the other hand, when the coupler has input and output bend structures, its coupling length as well as its splitting ratio are affected by the geometry of this bend.     

L波段小型化同轴-共面波导定向耦合器

设计了一种适用于L波段高功率微波辐射场测量的同轴型定向耦合器。分别选择同轴线和金属底板共面波导（CBCPW）作为主副传输线,减小定向耦合器横截面积;利用小孔耦合理论和微元法对耦合结构进行理论分析,选择连续渐变的菱形耦合缝隙,缩短定向耦合器长度;采用数值模拟的方法对耦合器结构进行优化,并对其性能参数进行实验测试,结果表明, 在1~2 GHz频带范围内耦合度约为47 dB,方向性大于17 dB。     

Coupling and crosstalk characteristics of hybrid silicon plasmonic waveguides

In this paper, the design and analysis of an ultracompact coupler based on a hybrid silicon plasmonic waveguide (HSPW) is proposed and its coupling and crosstalk characteristics have been theoretically investigated for the development of optical interconnects that can be realized using well-established complementary metal-oxide-semiconductor-compatible fabrication techniques. To determine the minimum horizontal separation distance and efficient coupling length for the designed coupler, the symmetric and antisymmetric supermodes are obtained and their characteristics are studied at a wavelength of 1.55 μm. Efficient light coupling is exhibited by the HSPW coupler with 75 % of power transfer between the two HSPWs with ultrashort coupling length of 2.14 μm when the separation distance is 50 nm. Further, it is shown that the crosstalk is significantly reduced with the insertion of metallic strip between the two HSPWs for realizing highly dense integrated plasmonic circuits.     

三轴速调管放大器自激振荡诊断

为满足X波段三轴速调管放大器(TKA)各腔内部自激振荡的诊断需求,设计了一种结构简单紧凑、无需外部密封结构的新型波导耦合器。利用小孔耦合原理和CST仿真软件,针对TKA中常见的自激振荡TE61模式,对该耦合器的耦合特性进行了理论分析和仿真设计。结合PIC方法,建立了利用该耦合器诊断TKA同轴漂移管内传输微波频谱特性的物理模型。通过仿真计算,验证了所设计的耦合器用于TKA自激振荡诊断的有效性。     

Resonant directional coupling of hollow Bragg fibers

Coupling between the lowest-loss TE01 modes of two touching hollow-core Bragg fibers is considered with a view to designing a directional coupler. We have found that for fibers with core radii larger than 10 microm one can identify broad frequency ranges in which intermodal coupling strength exceeds supermode radiation losses by an order of magnitude, thus opening the possibility of building a directional coupler. We attribute such unusually strong intermode coupling both to the resonant effects in the intermirror cavity and a proximity interaction between the leaky modes localized in the mirror.     

Hybrid vertical directional coupling between a long range surface plasmon polariton waveguide and a dielectric waveguide

To investigate light coupling between a long range surface plasmon polariton (LRSPP) waveguide and a conventional integrated optical component, a hybrid vertical directional coupler consisting of a LRSPP waveguides and a dielectric waveguide is investigated and fabricated. In the proposed coupler the dielectric waveguide and LRSPP waveguide are vertically configured for dense integration and strong coupling. The characteristics of the even and odd super-modes of the coupler are also analyzed to design the device. The fabricated device exhibits damped sinusoidal behavior along the coupling length due to propagation loss of the LRSPP waveguide. The maximum power transfer of 86% from the LRSPP waveguide to the dielectric waveguide is achieved at the coupling length of 600 μm. The measured characteristics of the device are in relatively good agreement with a theoretical analysis.     

Ultra-wide bandwidth wavelength selective couplers based on the all solid multi-core Ge-doped fibre

A novel wavelength selective coupler based on the all solid nine-core Ge-doped fibre has been proposed. The wavelength selective coupler is based on the phenomenon of a multi-core coupling. All the cores are made of Ge-doped silica and the index of central core is larger than the outer core. At the fixed fibre length, the different wavelength can be selected. The performances of coupling and propagation characteristics have been numerically investigated by using a full beam propagation method (BPM). Simulation results show that the all solid nine-core Ge-doped fibre can achieve simultaneous shorter coupler length and wideband filtering characteristics. The 0.763 mm and 0.745 mm wavelength selective coupler are proposed to achieve different wavelength division and the bandwidth is up to the 400 nm, and 300 nm, respectively.     

Light propagation in a directional coupler with saturable nonlinearity

The propagation of laser radiation in a nonlinear directional coupler with identical optical waveguides is studied theoretically. Account is taken of refractive index saturation. Bifurcations of the intensity distribution and transmission characteristic are revealed, and respective regions in the parameter plane are delineated. Two points of input intensity self-switching are found. A coupling length vs. normalized input intensity is investigated for different values of the nonlinearity parameter. Distinctive features of the coupler with saturated nonlinearity compared with Kerr-type couplers are highlighted.     

2×6熔融拉锥型单模光纤耦合器的特性分析

以线性耦合模方程为基础,推导了在弱熔、弱耦条件下2×6熔融拉锥型单模光纤耦合器的传播常量、本征模以及各端口的功率分布情况,并利用“熔融拉锥”法研制成功了两层排列结构的2×6单模光纤耦合器,实验得到的功率分布与理论分析结果基本一致。     

BPM Simulation and Comparison of 1 × 2 Directional Waveguide Coupling and Y-Junction Coupling Silicon-on-Insulator Optical Couplers

For developing large area opto-electronic silicon-on-insulator (SOI) devices, the optical coupler is a basic key device. In this article, the authors design and simulate 1 2 2 directional waveguide coupling and Y-branch coupling optical couplers based on Unibond SOI rib waveguides. The beam propagation method (BPM) is used for light propagation analysis. The simulation results and comparisons of the two kinds of optical couplers are reported. The S-bend waveguide for attaching to the two kinds of SOI optical coupler is also analyzed by BPM. We find that the directional coupler has lower power loss, but the Y-junction coupler is more wavelength insensitive with the same device size and splitting angle. The fabrication tolerance analysis shows Y-junction coupler has better fabrication characteristics.     

High‐efficiency single etch step apodized surface grating coupler using subwavelength structure

Grating couplers are key elements enabling the coupling of light between planar waveguide circuits and optical fibers. In this work, it is demonstrated using simulations and experiments that a high coupling efficiency can be achieved for an arbitrary buried oxide thickness by judicious adjustment of the grating radiation angle. The coupler strength is engineered by subwavelength structure, allowing straightforward apodization and single etch step fabrication. The design has been implemented using Fourier‐eigenmode expansion and finite difference time domain methods. The measured coupling loss of a continuously apodized grating is −2.16 dB with a 3 dB bandwidth of 64 nm, therefore opening promising prospects for low‐cost and high‐volume fabrication using 193 nm deep‐ultraviolet lithography. It is also shown by simulations that a coupling loss as low as −0.42 dB is predicted for a modified coupler structure with bottom mirror.     

熔锥平面型三光纤耦合特性分析

以线性耦合波方程为基础，分析了具有不同耦合强度的平面型三光纤耦合系统的模场特性和功率耦合特性，并与常规的平面型三光纤耦合系统的耦合特性作了比较，给出的耦合器谱响应实验结果与理论结果较为一致。     

上海光源150 MeV直线加速器次谐波腔耦合器的设计与调试

通过将次谐波腔体等效为RLC并联回路,求得了射频功率馈入腔体的耦合度公式,分析了上海光源直线加速器次谐波腔输入耦合器的设计要点,通过3维电磁计算软件CST的数值模拟方法对耦合器结构尺寸、耦合环的深度与方位、耦合度进行了模拟计算。介绍了对次谐波腔耦合器进行小信号测试的过程、腔体安装及在线调试的方法。     

An accurate analysis for two-mode interferometer based Mach-Zehnder interferometers interleaver

By using a more accurate analysis, we investigate the effect of the mode coupling and radiation loss of a two-mode interferometer (TMI) which is employed in the design of Mach-Zehnder interferometer interleaver by considering different branching angles and index contrasts. Based on our study, we find that the transmission characteristics of a TMI in the interleaver design cannot always be treated as a simple zero gap directional coupler (DC), called pure model. Under certain conditions, its characteristics should be treated as a zero gap DC combined with two variable gap DCs, and correspondently called integrated model. The conditions under which a TMI is treated as pure model or integrated model are defined in terms of branching angle and index contrast. Our findings are confirmed with the experimental results.     

熔锥型微纳光纤耦合器的理论与实验研究

针对熔融拉锥系统制得的熔锥型微纳光纤耦合器,选择适当的连续函数描述其光场分布,采用归一化的三角分布和矩形分布的加权叠加,以及高斯分布和三角分布的加权叠加实现了模场沿耦合器区域的连续变化;利用局部模式耦合理论推导出腰区及腰区附近锥形区的耦合系数计算公式,并得到微纳光纤耦合器输出光功率随拉伸长度的变化曲线.计算结果表明,随着拉伸长度的增加,光能量在两臂中来回交替耦合的程度变小并且呈现包络样,直至腰区耦合功能消失.通过实时监测拉制微纳光纤耦合器的输出光功率,得到火焰扫描宽度以及氢气流量对双纤失去耦合效应拉伸长度的影响:火焰扫描宽度(均匀腰区)越宽,拉伸长度临界值越大;氢气流量(熔融度)越大,拉伸长度临界值越小.实验结果显示,当光纤耦合器腰区直径达到1.6μm时,耦合功能消失,两输出端口光功率相同且恒定,微纳光纤耦合器具备稳定的光学传输特性.     

1 × 4 coupler based on all solid five-core photonic crystal fibers

An improved 1 × 4 coupler based on all solid multi-core photonic crystal fiber is proposed and analyzed. The expressions to calculate the coupling length and the coupling efficiency are deduced based on the coupled-mode equations firstly. Then a full-vector finite element method (FEM) is used to calculate the coupling length and the coupling efficiency. Next, the propagation characteristics and the performances of the coupler are analyzed through using a full vector beam propagation method (BPM). Research shows that the results derived by FEM agree with that by BPM. The coupling length of the coupler is 4.1 mm at λ = 1.55 μm. A maximum coupling efficiency of 24.96% can be obtained. The coupling ratio is more than 22.5% over a wavelength range of 100 nm. The polarization-dependent loss at λ = 1.55 μm is equal to 0.73 dB. Finally, the influences of the micro-variation of structure parameters and the material refractive index on the working performances of the coupler are investigated.     

紧凑型菱形缝隙V波段过模圆波导选模耦合器

设计了一种用于测量V波段过模高功率毫米波源在线功率的选模耦合器。利用小孔耦合理论和微元法对菱形耦合缝隙进行了理论分析，采用两组相同的菱形耦合缝隙实现选模功能，缩短了耦合器长度，增加了工作带宽。采用数值模拟的方法对耦合器结构进行优化设计，模拟结果表明:在60 GHz频点处，TM01模耦合度约为50 dB，TM02模耦合度仅为75 dB，对TM02模的抑制度为25 dB；在500 MHz带宽内对TM02模的抑制度大于15 dB，1 GHz带宽内的抑制度大于10 dB，可满足工作于TM01模的V波段高功率毫米波测试需求。同时针对工作于高次模TM0n（n=2,3,4,）的器件设计了可抑制TM01模的耦合器，为高次模工作的高功率毫米波器件的在线功率测量和模式诊断提供了技术方案。