一阶模的滤除及在Y分支和多模干涉结构中的应用

Y分支和多模干涉结构广泛应用于平面光波导器件中，在这些器件中，Y分支和多模干涉结构输出对称性具有重要意义。引入Y分支和多模干涉结构输出的不对称主要原因是这两种结构中产生了一阶导模，设计一种新的耦合器结构，在这个结构中．两根单模波导分别放置在一根多模波导两边，并且单模波导的传播常量与多模波导的一阶模的传播常量相同，这种结构可以将一阶导模滤除。在Y分支和多模干涉结构中使用了这种结构，模拟结果表明，采用这种耦合器的Y分支和多模干涉结构具有非常好的输出对称性。     

Ultra-compact multimode interference InGaAsP multiple quantum well modulator

We propose a new structure for an ultra-compact multimode interference (MMI) InGaAsP multiple quantum well modulator. The operating principle is based on restricting the coupling of the self-image produced by the MMI region into a single mode output waveguide. The key is to excite only the even modes within the MMI region, and this is achieved by operating the MMI waveguide under the condition of restricted symmetric interference. By asymmetrically inducing a phase change of along a selected area within the MMI region, mode conversion of all the even modes to odd modes is achieved. Since only the fundamental mode can be coupled to the output waveguide, neither an individual mode, nor any combination of the modes will be coupled, and therefore the injected light is fully attenuated. The modulation characteristics are analyzed using the finite-difference beam propagation method. Extinction ratios as low as –37 dB are demonstrated without electro-absorption effects. For the case of low electro-absorption, which corresponds to a more realistic situation, this value is only increased to –35 dB.     

一种基于多模干涉耦合器的模转换与合并器件的设计

多模波导干涉（MMI）耦合器在集成光路中被用来实现波导模式的转换或合并。对于实际的基于硅绝缘体（SOI）的三维波导，数值设计方法是必需的。采用有限差分波束传输法（FD-BPM）设计了一种66％模式转换、合并器。首先由FD-BPM求出MMI耦合器的输入、输出波导所支持的基模和一阶导模的模场分布，通过数值方法得到MMI耦合器的长度，然后将求得的模场作为输入波场，经过FD-BPM运算后在器件输出面上计算交叉积分得出实际的由基模转换为一阶模的功率百分比。对于算例中采用的SOI波导，器件长度为829μm，65．7％的功率由基模转换为一阶模。     

Stratified waveguide grating coupler for normal fiber incidence

We propose a new stratified waveguide grating coupler (SWGC) to couple light from a fiber at normal incidence into a planar waveguide. SWGCs are designed to operate in the strong coupling regime without intermediate optics between the fiber and the waveguide. Two-dimensional finite-difference time-domain simulation in conjunction with microgenetic algorithm optimization shows that approximately 72% coupling efficiency is possible for fiber (core size of 8.3 microm and delta=0.36%) to slab waveguide (1.2-microm core and delta=3.1%) coupling. We show that the phase-matching and Bragg conditions are simultaneously satisfied through the fundamental leaky mode.     

含间隔层的增益导引-折射率反导引平面波导激光器中高阶模式抑制研究

为了抑制高功率波导激光器中的高阶模式,改善其光束质量,基于增益导引-折射率反导引理论,在对称分层波导结构中添加了一层间隔层,进一步加大了高阶模式与基模之间阈值增益系数的差异,利用这种模式竞争抑制高阶模式.同时从波动方程出发,推导出各模式的本征方程,并给出了基模和高阶模的场分布.引入间隔层之后的高低阶模式损耗计算结果表明,通过合理地选择间隔层参数,可以有效地抑制高阶模式.     

Wave dispersion, growth rates, and mode converter analysis for asheet beam, hybrid-mode Cerenkov amplifier

The hybrid-mode dispersion relation and resonant growth rates are solved for a finite-thickness sheet electron beam propagating through a rectangular guide with a thin dielectric slab. Analytic results for the growth rates, bandwidth and mode competition for an infinite magnetic field in the limit of a dilute beam are then presented. To properly couple to the desired EH₁₀ hybrid amplifier slow wave mode, the coupled waveguide mode equations are solved for a dielectric taper to accomplish the TE₁₀ to EH₁₀ mode transition. A piecewide linear taper which suppresses the competing EH₁₁ mode and other hybrid modes is developed and the overall amplifier system is discussed     

Two-dimensional Hamiltonian description of nonlinear optical waveguides

We present a general method for studying the evolution of a beam propagating in a nonlinear optical waveguide. In the framework of a variational method, and within the Ritz optimization technique, the governing equations describing the evolution of the basic parameters of the beam are reduced to those of a Hamiltonian dynamical system. As a particular case the method yields the guided modes of the device (corresponding to fixed points of the dynamical system). As an example the method is applied to the full two-dimensional analysis of the guided modes of a nonlinear slab directional coupler and of a channel waveguide; numerical experiments are used to validate our approach.     

Variational analysis of Er:Ti:LiNbO3 waveguide lasers based on Gaussian/two-half Gaussian mode field distribution

Starting with two dimensional, scalar wave equation, a variational equation was established for the fundamental TE and TM modes guided in Ti:LiNbO₃ waveguides on the basis of assuming a symmetric Gaussian mode field function in the width direction and two-half Gaussian trial functions in the depth direction. The controllable waveguide fabrication parameters, including channel width, diffusion temperature, initial Ti-strip thickness and diffusion time, dependent of fundamental mode size, effective pump area, coupling efficiency between pump and laser modes, and the coupling loss between a Ti:LiNbO₃ waveguide and a fiber were numerically calculated for Z-cut Er:Ti:LiNbO₃ channel waveguide lasers at three possible emission wavelengths 1532,1563 and 1576 nm and two possible pump wavelengths 1480 and 980 nm. The calculated results were compared with those of Gaussian/Hermite–Gaussian mode field distribution in detail.     

Optimal design for a flat-top AWG demultiplexer by using a fast calculation method based on a Gaussian beam approximation

Passband broadening of an AWG (array waveguide grating) demultiplexer with an MMI (multimode interference) coupler connected at the end of a tapered input waveguide is considered. An explicit formula based on the field propagation of an approximate Hermit-Gaussian beam is used to calculate quickly and reliably the spectral response of the AWG demultiplexer. The widths of the input waveguide, the output waveguides and the MMI coupler are optimized. The optimal design is verified with the experimental measurement.     

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.     

Generation of millimeter wave Gaussian beams and their propagation in a dielectric waveguide

Linearly polarized fundamental mode Gaussian beams were generated and coupled to a hollow circular oversized dielectric waveguide by placing the waist of the beam at the guide entrance. The transmission properties of the waveguide were characterized as a function of frequency for a variety of coupling conditions. These conditions included changes in the input beam waist radius, angle of incidence, and displacement perpendicular and parallel to the guide axis. It has been found that: 1.) power transmission is maximized when the waist of the input beam is centered at the guide input, injected normally, and has a radius of 0.43 times the waveguide radius, 2.) power transmission decreases rapidly with increasing angle of incidence and the rate of that loss increases with frequency, 3.) the waveguide preserves the linear polarization of the input beam, 4.) power transmission in the fundamental waveguide mode is not greatly affected by moderate displacements in the input beam position, and 5.) upon exit from the waveguide the launched EH₁₁ mode propagates as a fundamental mode Gaussian beam in the quasi-far field. The results compare favorably to the transmission theory of Belland and Crenn and approximately to the near and far field mode pattern theory of Degnan.     

对称单负介质包层平面波导的模式特征

系统研究了负μ材料(MNG)作为包层的对称三层平板波导的传输特性. 研究发现,这种波导既支持快波的传播,又支持TE₀,TE₁模式的慢波传输. 其模式特性不同于左手介质波导和传统介质波导,导模存在的模折射率范围要比它们的大. MNG波导的TE₀快波模缺失,且TE_m模(m>1)的传播常数大于TM_m模的传播常数. TE_m模具有双模特征,且与波导的结构参数密切相关,导致波导中的净能流出现负值. 关键词： 单负介质 平板波导 快波与慢波 传播常数     

Volume grating preferential-order focusing waveguide coupler

A volume grating focusing waveguide coupler has been designed, fabricated, and tested. The volume grating is designed to couple a 633-nm wave guided in a single-mode polyimide waveguide preferentially into the air cover. The beam is outcoupled normally to the surface of the waveguide and focused to a line 25 mm above the waveguide. The slanted-fringe volume grating is holographically recorded by the interference of two coherent 364-nm ultraviolet waves focused with a cylindrical lens to produce the chirped grating. The 1-mm-long volume grating coupler exhibits a preferential-coupling ratio of 0.98, a coupling efficiency of 95%, and an almost diffraction-limited focal line with a full width at half-maximum of 10.49mum.     

Dispersionless slow light by photonic crystal slab waveguide with innermost elliptical air holes

We report a low-loss photonic crystal slab waveguide formed by deforming the innermost circle air holes in the conventional photonic crystal slab waveguide into elliptical ones. We obtain the photonic bands and group index of guided modes in this photonic crystal waveguide by guided-mode expansion method and investigate the dependence of photonic bands and group index of guided modes on the parameters of the innermost elliptical air holes. The group velocity and group velocity dispersion of this waveguide strongly depend on the innermost elliptical air holes. Photonic crystal slab waveguide with the optimum innermost elliptical air holes possesses a wider single mode region below the light line, in which light can easily propagate without intrinsic loss. At the same time, the guided mode supported by this waveguide has nearly constant group velocity and vanishing group velocity dispersion in a 3-5 nm bandwidth.     

Flat-topped beam output from a double-clad rectangular dielectric waveguide laser with a high-index inner cladding

A novel method to produce a flat-topped laser beam by using a double-clad rectangular waveguide laser with high-index inner cladding is presented. The waveguide dispersion equation for cosine mode was deduced, the condition for the flattened mode was given out, relative gains for guided modes were calculated numerically and analyzed. Results indicate that a gain advantage for the flattened mode is clear, a flat-topped laser beam can be achieved when the optical confinement factor, the gain intensity and the output coupler are chosen suitably.     

左手材料平板对傍轴高斯光束聚焦特性分析

在傍轴近似下,推导出了左手材料平板聚焦系统的传输矩阵,并利用ABCD定律得到了高斯光束在左手材料中和经过平板透镜聚焦后的传输公式。高斯光束在左手材料内部和像空间的传输公式的研究表明:像高斯光束和物高斯光束束腰大小一致,即左手材料平板透镜实际上对高斯光束没有聚焦作用。研究同时表明左手材料平板对高斯光束的聚焦与几何光学成像规律完全一致,而不存在一般透镜聚焦时的焦移效应。     

InGaAs/InAlAs多量子阱脊形波导及定向耦合器光波特性准矢量分析

提出了一种基于级数展开的三维准矢量束传播法(SE-QV-BPM)用以分析由InGaAs/InAlAs多量子阱构成的脊形光波导及定向耦合器.结果表明，刻蚀深度相同时，TM模比TE模在水平方向限制强，且TM模的模场在波导角上出现畸变；波导间距相同时，定向耦合器TM波的耦合长度大于TE波的耦合长度，对偏振态敏感.分析获得了浅/深刻脊形光波导承载的准矢量TE/TM基模、定向耦合器承载的TE/TM偶/奇模的模场分布及其有效折射率，模拟了光场在定向耦合器中的传输演变情况.另外，SE-QV-BPM导出矩阵小，计算效率高 关键词： 级数展开 准矢量束传播法 多量子阱 InGaAs/InAlAs     

Analysis of Radiation Phenomena in a Wedge Ended Dielectric Slab Waveguide

Radiation phenomena observed in a wedge shape ended dielectric slab waveguide are analyzed using mode matching technique. The case of transverse electric polarization (TE) being parallel to dielectric slab waveguide is assumed. In order to describe the fields in the wedge region, a stack of dielectric plates is assumed and in each layer the fields are expanded in terms of the mixed spectrum of guided and radiated modes. A similar expansion is used in the constant thickness slab waveguide while in free space medium a continuous-radiation mode expansion is used. Then a mode matching approach is applied, incorporating the orthogonality properties of mixed spectrum modes, in order to compute the wave fields inside the dielectric slab waveguide and wedge medium. Mode matching is achieved by discretizing the continuous radiation mode spectrum leading into a numerically stable solution provided a sufficient large number of points are used to convert integrals into finite summations. Numerical computations are carried out for various wedge geometries and shapes including linear and exponential profiles.     

一种多模干涉耦合器的性能模拟

为减小多模干涉（Multimode Interference ,MMI）耦合器件的尺寸,提出一种多模波导宽度为指数型变化的Taper结构.理论分析了多模波导长度与该Taper结构的参量之间的关系.与已被用于减小MMI器件尺寸的抛物线型Taper结构相比,该结构可进一步减小器件的尺寸.就几种波导结构参量下的MMI耦合器,利用宽角光束传播法进行了数值模拟.结果表明,指数型MMI耦合器的性能与抛物线型MMI耦合器的性能类似.指数型Taper结构可以用于MMI器件以减小该类器件的尺寸.     

Guided modes in asymmetric graphene waveguides

An asymmetric quantum well in graphene can act as a slab waveguide for electron waves in a manner analogous to the electromagnetic waves in dielectrics. Guided modes and the probability current density are analyzed in the graphene electron waveguide induced by asymmetric electrostatic potential. The modes in an asymmetric graphene waveguide include guided modes, “cover modes”, “substrate modes” and “radiation modes”. The conditions for a guided mode are quantified. It is found that the fundamental mode is absent when both the Klein tunneling and classical motion are present. The confinement of electrons for lower order mode is stronger than for higher order mode. We hope that these characteristics in asymmetric graphene waveguide can provide potential applications in graphene-based waveguide devices.