Rod type photonic crystal optical line defect waveguides with optical modulations

This paper reports the design and principles of two dimensional rod-type photonic crystal (PhC) line defect waveguides for bandgap based optical waveguiding, static modulation and high speed dynamic optical switchings. Experiments were carried out for both high aspect ratio and slab type configurations. The differences in waveguiding mechanisms for the two configurations resulting from the presence of bottom cladding systems, without out-of-plane symmetries are compared for their advantages and disadvantages. In particular, the designs of non-top-clad optical waveguides of layout sizes within micrometers and operational frequencies centered at the optical communication wavelength of 1550 nm, were investigated for the feasibility of large scale integration by batch fabrication process techniques – such as sub-micrometer optical lithography etc. Based on such techniques, specifically designed dispersions of line defect PhC waveguides within a missing row of PhC rods were accompanied by optical testing structures of suitable coupling modes. Optical measurements of waveguiding coefficients were therefore enabled for the different configurations, together with further static and dynamic modulations. PACS 42.70.Qs     

折叠矩形槽波导行波管输入输出过渡波导

为了将槽波导端与标准矩形波导相连接,设计了一种适用于折叠槽波导结构行波管的输入输出过渡波导, 可将其视为槽宽渐变的双槽加载矩形波导。利用电磁仿真软件CST微波工作室对该结构进行仿真计算,讨论了各个结构参数对其性能的影响。对比了直线渐变、抛物线渐变和指数渐变3种槽宽渐变规律在W波段对其传输特性及损耗特性的影响。研究结果表明:指数渐变结构的驻波系数小于1.15的带宽比其他两种结构都要宽,且在90~99 GHz、驻波系数小于1.25时,该结构的整体长度也远小于另外两种结构,能够实现良好的过渡效果; 而直线渐变结构的损耗在90~97 GHz为最低。     

Sensitivity improvement of broadband electro-optic polymer-based optical phase modulator using 1D and 2D photonic crystal structures

This Letter introduces the design and simulation of a microstrip-line-based electro-optic(EO) polymer optical phase modulator(PM) that is further enhanced by the addition of photonic crystal(PhC) structures that are in close proximity to the optical core. The slow-wave PhC structure is designed for two different material configurations and placed in the modulator as a superstrate to the optical core; simulation results are depicted for both1 D and 2D PhC structures. The PM characteristics are modeled using a combination of the finite element method and the optical beam propagation method in both the RF and optical domains, respectively.The phase-shift simulation results show a factor of 1.7 increase in an effective EO coefficient(120 pm/V) while maintaining a broadband bandwidth of 40 GHz.     

Fabrication and demonstration of square lattice two-dimensional rod-type photonic bandgap crystal optical intersections

This paper reports fabrication and demonstration of optical intersections in two-dimensional (2D) rod-type photonic crystal (PhC) structures. High resolution and aspect ratio 2D square lattice PhC waveguide intersections were designed and fabricated for application at the optical communication wavelengths centered at 1550 nm. In the silicon processing front, challenges resolved to overcome issues of drastically reduced process windows caused by the dense PhC rods arrays with critical dimensions (CDs) reduced to only a few hundred nanometers were addressed not only in terms of critical process flow design but also in the development of each processing module. In the lithographic process of deep ultraviolet laser system working at 248 nm, PhC rods of sub-lithographic wavelength CDs (115 nm in radii) were realized in high resolution, even near periphery regions where proximity errors were prone. In the deep etching module, stringent requirements on etch angle control and low sidewall scallops (undulations arising from time multiplexed etch and passivation actions) were satisfied, to prevent catastrophic etch failures, and enable optical quality facets. The successfully fabricated PhCs were also monolithically integrated with large scale optical testing fiber grooves that enabled macro optical fiber assisted coupling to the micro scale PhC devices. In the optical experiments, the transmission and crosstalk properties for the PhC intersection devices with different rod radii at the center of the PhC optical waveguides crossings were measured with repeatability. The properties of the PhC intersections were therefore optimized and verified to correspond well with first principle finite difference time domain simulations.     

Dispersion-based beam splitter in photonic crystals

A novel implementation of a dispersion-based beam splitter in a photonic crystal (PhC) is proposed. The beam splitter consists of two periodic structures: a nonchannel dispersion-guiding region and a splitting structure operating inside the photonic bandgap. The dispersion-guiding PhC structure is used to route the optical wave by exploiting the dispersion properties of the lattice. An arbitrary power ratio between the output beams can be achieved by varying the parameters of the splitting structure. Within the studied range of splitting structures, high output power was observed and verified experimentally.     

Interferometer-type structures for guided atoms

We experimentally demonstrate interferometer-type guiding structures for neutral atoms based on dipole potentials created by microfabricated optical systems. As a central element we use an array of atom waveguides being formed by focusing a red-detuned laser beam with an array of cylindrical microlenses. Combining two of these arrays, we realize X-shaped beam splitters and more complex systems like the geometries for Mach-Zehnder and Michelson-type interferometers for atoms.     

Large negative dispersion in dual-concentric-core photonic crystal fiber with hybrid cladding structure based on complete leaky mode coupling

Considering the optical stability of solution, the sugar-solution is infused into the outer core ring of dual-concentric-core photonic crystal fiber (DCCPCF). The influences of structure parameters and solution concentration on the phase and loss matching are comprehensively analyzed. By choosing the appropriate outer core mode to completely couple with the inner core fundamental mode, the large negative dispersion PCF around 1.55 μm is designed, which has the dispersion value of − 39,500 ps/km/nm as well as bandwidth of 7.4 nm and effective mode area of 28.3 μm². The designed PCF with hybrid cladding structure can effectively compensate the positive dispersion of conventional single mode fiber, and suppress the system perturbation caused by a series of nonlinear effects. Considering the mode field mismatching between the DCCPCF and the tapered fiber, the calculated connection loss around 1.55 μm is below 3 dB. In addition, the equivalent propagation constants of two leaky modes are deduced from the coupled-mode theory, and the complete mode coupling case can be well predicted by comparing the real and imaginary parts of propagation constants.     

新型缓变锥型PCF结构接口耦合损耗特性

提出一种新型的缓变锥型光纤接口器件。该接口器件是一个长度为150mm左右的缓变锥型光子晶体光纤。利用有限元方法对模场直径差别较大的不同光纤进行拼接时的损耗问题进行研究。研究结果表明:在模场直径分别为9μm和10.4μm的光纤之间加入锥型光纤后可以有效降低耦合损耗,并且在不同波长下耦合损耗都维持在一个较低水平。因此,该接口器件能够实现光纤拼接时物理结构的过渡和不同模场直径的转换,从而使拼接损耗降到最低。     

Loss measurement of plasmonic modes in planar metal-insulator-metal waveguides by an attenuated total reflection method

We report experimental excitation and characterization of surface plasmon modes in planar metal-insulator-metal (MIM) waveguides. Our approach is based on determining the width of the reflection angular spectrum in the attenuated total reflection (ATR) configuration. Owing to its transverse character, the ATR configuration provides a more straightforward and simpler way to determine the loss of plasmonic modes in MIM structures, compared to using tapered end couplers with multiple waveguide samples or scanning near-field optical microscopy. In this Letter, two waveguide structures with Au claddings and 50/200 nm SiO(2) cores are investigated. The propagation lengths measured at λ = 1.55 μm are 5.7 and 18 μm, respectively, in agreement with the theoretical predictions.     

Two dimensional silicon waveguide chirped grating couplers for vertical optical fibers

Shallow etched two dimensional gratings for coupling light between silicon-on-insulator nanophotonic waveguides and vertical optical fibers were designed and experimentally characterized. We show that the large second order back reflection could be suppressed effectively by applying a linear chirp in the grating period for two dimensional grating couplers. The total coupling efficiency from an optical fiber to two orthogonal silicon waveguides is independent of the input polarization. An almost linear relationship between the average effective index of the grating region and the fill factor for 1D grating is obtained and verified. We also show that the average effective index for the two dimensional grating is similar to the one dimensional grating with the same fill factor in the light propagation direction, when the fill factor of the etched holes was larger than 0.5.     

Simple low-loss waveguide bends using ARROW effect

A new concept for reducing bend loss in dielectric planar waveguides is presented. It is based on the introduction of a set of antiresonant reflecting optical waveguides (ARROWs) on the outside of the bent core and defined in the same fabrication step as the main bend. It has been ascertained by simulation that the bending loss can be significantly reduced. The method is compatible with other bend-lossreduction strategies, as well as with different waveguiding structures, such as rib and buried waveguides and fibers.This revised version was published online in August 2005 with a corrected cover date.     

Influence of depressed-index outer ring on evanescent tunneling loss in tapered double-cladding fibers

A tapered fiber with a depressed-index outer ring is fabricated and dispersion engineered to generate a widely tunable (1250-1650 nm) fundamental-mode leakage loss with a high cutoff slope (-1.2 dB/nm) and a high attenuation for stop band (>50 dB) by modification of both waveguide and material dispersions. The higher cutoff slope is achieved with a larger cross angle between the two refractive index dispersion curves of the tapered fiber and surrounding optical liquids through the use of depressed-index outer ring structures in double-cladding fibers.     

Perturbation analysis of finitely clad optical waveguides and couplers

In calculating the modal characteristics of a finitely clad optical waveguide, if the wavelength is much shorter than the core of the waveguide, then the cladding of the waveguide can be assumed to be infinite. On the other hand, if the wavelength is much longer than the core, the core can be ignored. These two approximations, although simple and accurate in the two respective extreme cases, do not provide adequate information in the transition region, where many tapered devices are operated. To overcome this a novel perturbation method is devised, in which these two approximations are combined in such a way that a result that is accurate in all cases can be obtained. Analytical expressions based on this method for describing the dispersion characteristics of finitely clad slab waveguides and circular step-index fibres are derived and compared with other conventional approximations. The combined use of this method and the coupled-mode theory for the analysis of optical couplers is also described.     

基于宽度抛物线型和渐变孔径的超高Q低V 一维光子晶体纳米梁腔的设计

设计了基于宽度抛物线型和孔径渐变的一维(1-D)光子晶体 (PhC)纳米梁腔.通过FDTD的计算模拟, 设计的这种纳米梁腔可以实现超高Q值1.8× 10⁷, 同时拥有超小模体积V～ 0.04(λ/n)³, 在继承抛物线型和渐变孔径型纳米梁腔高Q的基础上,进一步降低了模体积V. 我们设计的这种腔具有紧凑, 低制造工艺要求以及高Q/V 值等优点,在未来应有广泛应用.     

Investigation of the efficient coupling between a highly elliptical Gaussian profile output from a laser diode and a single mode fiber using a hyperbolic-shaped microlens

A tapered hyperbolic-shaped microlens has been improved for efficient the coupling of the output from a laser diode into an optical fiber. A tapered hemispherical-end fiber microlens is also evaluated for comparison purposes. The Fresnel diffraction theory is used to evaluate laser to fiber coupling using microlenses. Experimental results demonstrate that, for an elliptical Gaussian laser diode with an aspect ratio of 1:1.5, the coupling efficiencies for the hyperbolic-shaped microlenses and the hemispherical-end fiber microlenses are 87% and 62%, respectively. Simulation results show excellent agreement with the measurements. The research results illustrate that a hyperbolic-shaped microlens has a much higher coupling efficiency than a hemispherical-end microlens due to efficient mode matching and phase matching.     

Analysis of coupling between two parallel dielectric waveguides

The coupling between two parallel dielectric waveguides with a finite coupling length is investigated. The ends of the waveguides are tapered in order to reduce the scattering losses. It is shown that the taper sections can be utilized to achieve an effective coupling which is insensitive to the coupling length, thus, providing a much greater tolerance for the design and manufacturing of coupling devices.     

硅基脊型波导器件过渡区损耗及偏振效应

在绝缘层上的硅材料上制作了四种具有不同输入输出结构的星形耦合器并进行了测试,对脊形波导与平板波导相互过渡时过渡区的损耗问题进行了研究和讨论,计算得到在所使用的材料参量下利用锥形结构可以得到1dB左右的最小损耗,这一损耗是由于脊型波导与平板波导间的模式失配造成的。以文献中的实验数据为出发点,分析了脊形波导的偏振问题,并通过对脊型波导器件层厚度、脊高、脊宽进行优化设计,得到了不同偏振模式的有效折射率差仅为10^-5量级的单模脊型波导结构,这样的偏振效应在器件设计中可以忽略。     

基于二维六方氮化硼材料的光子晶体非对称传输异质结构设计

二维六方氮化硼(hexagonal boron nitride,hBN)材料在产生光学稳定的超亮量子单光子光源领域有着潜在应用,有望用于量子计算和信息处理平台,已成为研究热点.而光学非对称传输设备是集成量子计算芯片中的关键器件之一.本文从理论上提出了一种基于hBN材料光子晶体异质结构的纳米光子学非对称光传输器件.运用平面波展开法研究了光子晶体的能带结构与等频特性,从理论上分析了hBN异质结构中可见光波非对称传输的可行性.同时,采用时域有限差分方法研究了可见光波段异质结构的晶格常数和半径对透射光谱的影响.研究结果显示,该结构实现了在610—684 nm波长范围内TE偏振光的非对称传输,在652 nm波长处正向透射率达到0.65,反向透射率为0.006,非对称传输透射对比度高达0.98.本文提出的结构模型为基于hBN的新型纳米光子器件设计提供了新的可能性,可用于不同功能光学器件的集成设计.     

光纤纤芯及包层模有效折射率计算及仿真

光纤结构设计、模间色散求解、光纤光栅模式耦合等问题的研究, 都需要对光纤纤芯及包层模的有效折射率进行精确计算. 本文以光纤三层结构模型为基础, 结合该模型下的模式本征方程, 使用截弦法求解了纤芯模有效折射率, 并将计算结果与COMSOL软件模拟的对应纤芯模的传输光场进行对比, 验证了计算结果正确.使用区间遍历算法对包层模有效折射率进行了求解, 与已有的传统方法相比, 该方法可以有效防止求解过程中根的遗漏、避免特征方程产生的奇点, 并能保证模式的正规性.本文采用Mathematica软件对求解过程进行仿真, 获得了纤芯模和包层模有效折射率与波长关系曲线. 关键词： 光纤传输模式 有效折射率 截弦法 区间遍历算法     

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.