Relaxed fabrication tolerance for self-imaging photonic crystal waveguide splitters using a tapered multimode interference region

The power imbalance between different waveguide outputs is compensated by manipulating the dispersion of the guided propagation in the multimode interference (MMI) region. This is attainable using a tapered region at the beginning of the MMI region that has been verified through simulation and experiment. From this, the fabrication tolerance for the diameters of holes in a tapered 1×3 photonic crystal waveguide (PhCW) splitter is relaxed up to a range of at least 27 nm. The output power is well-balanced to within 1 dB. The effective bandwidth of the splitters shifts only around 13 nm, for a reduction of 10 nm in the diameter of the PhCW holes. The optimized component is an outstanding ultracompact 1×3 splitter for the photonic integrated circuit (PIC).     

Multimode interference effect in plasmonic subwavelength waveguides and an ultra-compact power splitter

The real part of the effective index is enhanced in a metal-insulator-metal (MIM) surface plasmon waveguide when the thickness of the insulator is quite small and this mechanism can be utilized to achieve some ultra-compact photonic devices. The mode properties of an index-guided multimode plasmonic waveguide of subwavelength size are analyzed and the multimode interference (MMI) effect is investigated. A novel ultra-compact MMI power splitter, with the transverse size less than 1/3 of the free space wavelength, is proposed.     

采用三维波束传输法的多模干涉耦合器成像位置的数值计算

多模干涉(MMI)耦合器需要精确定位成像位置,以便器件的设计制作。针对强限制和弱限制的三维多模波导干涉耦合器,采用三维交替方向隐式有限差分光束传输法(BPM),数值计算得出多模波导长度、输入波导和输出波导位置。首先通过对对称干涉多模干涉耦合器的数值分析求得多模干涉耦合器的等效宽度Weq及最低二阶模之间的拍长Lc,然后将这些参量结合光束传输法直接用于器件设计。计算显示该方法得到的成像位置和导模传输分析法(MPA)的理论预测比较接近,但Weq和Lc却是由光束传输法计算得到的,导模传输分析法理论只能在得到Weq和Lc的前提下才能得到成像位置。该方法直接针对三维波导进行,没有采用基于等效折射率方法的从三维波导到二维波导的简化处理,并且也没有采用导模传输分析法所采用的近似,保证了计算精度,对于实际多模干涉器件的设计制作可起参考作用。     

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。     

Enhanced third harmonic reflection and diffraction in Silicon on Insulator photonic waveguides

Enhanced third harmonic (TH) generation from Silicon-On-Insulator (SOI) planar waveguides as well as SOI photonic crystal (PhC) slabs is studied in different angular configurations, both in the visible and infrared energy ranges. In the SOI planar waveguide, the multilayer structure causes the optical properties such as TH reflection to be different from those of bulk silicon samples. This behavior is well reproduced by calculations of TH reflectance.Measurements of third-harmonic reflection and diffraction from one-dimensional PhC slabs etched in the SOI waveguide are also reported. The angular positions of TH peaks at various diffraction orders agree well with those calculated from a nonlinear grating equation. Both reflection and diffraction processes contribute to enhanced TH generation efficiency in the PhC slabs.TH reflectance measurements performed on PhC slabs in the near infrared show a resonant interaction between the incident beam and the photonic structure, dependent on the angle of incidence. This leads to a nonlinear conversion efficiency which is strongly enhanced with respect to that of the SOI waveguide, due to the excitation of strong local fields associated with the presence of photonic modes in the PhC slab.     

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.     

基于自映像的光子晶体波导分束器的设计

通过研究2维正方晶格光子晶体波导多模干涉的自映像效应,优化设计了一种新型1×2光子晶体波导分束器,采用时域有限差分法对其传输特性进行模拟分析。设计过程中,根据多模干涉耦合区中周期出现的双重像的位置确定两个单模输出波导的位置,通过在分束器输入和输出耦合区的连结处设置介质柱,改变输入和输出耦合区中的模场分布,实现模式匹配,从而明显减小分束器的反射损耗。计算结果表明:设置的介质柱归一化半径分别为0.08和0.177时,对于波长为1.55μm的入射光,该分束器的透射率可高达93%。     

A Flattened AWG Demultiplexer with Low Chromatic Dispersion

A design method is introduced to obtain a flat-top arrayed-waveguide grating (AWG) demultiplexer with low chromatic dispersion. A multimode interference (MMI) section is connected at the end of the input waveguide, and a tapered waveguide is connected at the entrance of each output waveguide of the AWG demultiplexer. The design procedure is presented. A design example is given and shown to have a much better performance than the conventional flat-top design using only an MMI section. The insertion loss of the designed AWG demultiplexer is also reduced.     

掩埋式多模干涉型分束器的反射性能

通过束传播方法(BPM)模拟了SiO2基掩埋式波导结构多模干涉(MMI)型分束器的反射性能,模拟结果表明,MMI工作在分束模式时存在最优的多模干涉长度实现最大输出和最小反射,而在合束模式下实现最大输出时反射也达到最大,这是由自映象原理决定的。SiO2基掩埋式波导结构MMI分束器对反射具有良好的抑制作用,其最大反射功率为-60dB。分析表明,多模干涉区末端的界面反射率决定了器件的反射强弱,SiO2基掩埋式波导的界面反射率非常低,这是其低反射的原因。     

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.     

Photonic bands, gap maps, and intrinsic losses in three-component 2D photonic crystal slabs

We obtain the photonic bands and intrinsic losses for the triangular lattice three-component two- dimensional （2D） photonic crystal （PhC） slabs by expanding the electromagnetic field on the basis of waveguide modes of an effective homogeneous waveguide. The introduction of the third component into the 2D PhC slabs influences the photonic band structure and the intrinsic losses of the system. We examine the dependences of the band gap width and gap edge position on the interlayer dielectric constant and interlayer thickness. It is found that the gap edges shift to lower frequencies and the intrinsic losses of each band decrease with the increasing interlayer thickness or dielectric constant. During the design of the real PhC system, the effect of unintentional native oxide surface layer on the optical properties of 2D PhC slabs has to be taken into consideration. At the same time, intentional oxidization of macroporous PhC structure can be utilized to optimize the design.     

The study of electro-optical sensor based on slotted photonic crystal waveguide

A compact and sensitive electro-optical sensor based on slotted photonic crystal waveguide (S-PhCW) is demonstrated. The electro-optical sensor can be realized in photonic crystal (PhC) slabs of silicon in Silicon-on-Insulator (SOI). Nonlinear optical polymer is used as infiltration. By applying three-dimensional finite difference time domain (3D-FDTD), the sensitivity and quality factor of electro-optical sensor with different slotted waveguide width are calculated. In addition, sensitivity and the optical properties such as transmission spectrum and field distributions are compared between electro-optical sensor based on line defect photonic crystal waveguide (W1-PhCW) and that based on slotted photonic crystal waveguide (S-PhCW). Simulation results demonstrate that, compared with electro-optical sensor based on line defect photonic crystal waveguide, the sensitivity and quality factor is improved by 30 times and 6.6 times respectively in sensor based on slotted photonic crystal waveguide. Besides, the proposed PhC sensor devices have the advantage of a compact structure with the potential for monolithic integration with optical-to-electrical on-chip conversion and detection.     

基于多模干涉耦合器的阵列波导光栅设计研究

研究了基于多模干涉(MMI)耦合器的阵列波导光栅(AWG).通过模式传输分析方法,分析了多模干涉耦合器及阵列波导;并给出在硅基底上的二氧化硅波导上四通道100 GHz-AWG普通结构和紧凑结构的设计结果.     

Hybrid photonic crystal 1.31/1.55 μm wavelength division multiplexer based on coupled line defect channels

The objective of this paper is to investigate the implementation of a hybrid photonic crystal (PhC) 1.31/1.55 μm wavelength division multiplexer (WDM) and wavelength channel interleaver with channel spacing of roughly 0.8 nm between the operating wavelengths of 1.54-1.56 μm. It is based on 1-D photonic crystal (PhC) structure connected with an output 2-D PhC structure. The power transfer efficiency of the hybrid PhC WDM at 1.31 μm and 1.55 μm were computed by eigen-mode expansion (EME) method to be about 88% at both the wavelengths. The extinction ratios obtained for the 1.31 μm and 1.55 μm wavelengths are − 25.8 dB and − 22.9 dB respectively.     

Analysis of single-mode condition and high-order modes discrimination in photonic crystal VCSEL

A new technique for analyzing the single-mode condition and mode discrimination of high-order modes in photonic crystal VCSEL is proposed and reported in this paper. The technique proposed is a semi-empirical approach that uses analytical analysis based on experimental data. For that purpose, single- and multimode photonic crystal VCSEL devices are fabricated and characterized for comparison of device performance. The mode width for first- and second-order mode of PhC VCSEL is computed for analyzing the mode behavior. In order to verify the findings, the fabricated single- and multimode devices are analyzed using finite difference frequency domain technique for the purpose of showing the loss as a function of mode order for the PhC VCSEL. The results show close agreement between the computed and experiment findings, justifying the use of the proposed semi-empirical technique for analyzing single-mode condition and of high-order modes discrimination in photonic crystal VCSEL.     

Numerical characterization of nanopillar photonic crystal waveguides and directional couplers

We numerically characterize a novel type of a photonic crystal waveguide, which consists of several rows of periodically arranged dielectric cylinders. In such a nanopillar photonic crystal waveguide, light confinement is due to the total internal reflection. A nanopillar waveguide is a multimode waveguide, where the number of modes is equal to the number of rows building the waveguide. The strong coupling between individual waveguides leads to the proposal of an ultrashort directional coupler based on nanopillar waveguides. We present a systematic analysis of the dispersion and transmission efficiency of nanopillar photonic crystal waveguides and directional couplers. Plane wave expansion and finite difference time domain methods were used to characterize numerically nanopillar photonic crystal structures both in two- and three-dimensional spaces.     

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.     

Focusing effect of a graded index photonic crystal lens

We present a new graded-index photonic crystal (GRIN PhC) structure which can be made by laser interference lithography (LIL). It can be applied as miniaturized focusing lens. Numerical simulations are carried out on the structures with different widths, lengths, and modulations. In the qualitative analysis of the focal length, focusing of a GRIN PhC was proved to originate from both the graded sizes of the rods and multimode interference. Multimode interference dominates the focusing effect when a GRIN PhC has a small length or a small difference in the rod sizes. Intensity distributions in different cases were given to support the conclusion.     

Efficient terahertz coupling lens based on planar photonic crystals on silicon on insulator

We present a promising coupling device, namely, a terahertz (THz) planar photonic crystal (PhC) lens based on the effective refractive-index contrast between the PhC and the surrounding unpatterned area. Three-dimensional finite-difference time-domain calculations show a 90% power transfer from a 100-microm silicon waveguide to a 10-microm waveguide, and 45% coupling efficiency is confirmed experimentally. These results demonstrate the utility of the PhC lens as an effective approach to coupling into PhC THz circuits.