Power splitting ratio couplers based on MMI structures with high bandwidth and large tolerance using silicon waveguides

We show that it is possible to obtain 2 × 2 couplers based on multimode interference (MMI) structures with nineteen new power-splitting ratios by cascading three or four MMI couplers. The other aim of this study is to use silicon waveguides, that are compatible with the existing CMOS (Complementary Metal-Oxide-Semiconductor) fabrication technology, for designing the proposed devices. The proposed MMI couplers with new power splitting ratios have simple geometries and low losses. These MMI couplers can offer valuable new possibilities for designing MMI waveguide-based photonic integrated circuits such as all-optical interconnects, microring resonators, clock distribution, Mach Zehnder Interferometer based on MMI couplers and other all-optical processing applications. The transfer matrix method (TMM) and modified effective index method (MEIM) along with the support of the 3D Beam Propagation Method (3D BPM) are used to optimize the proposed devices.     

Fabrication of multimode interference waveguides in glass by use of a femtosecond laser

We report on the fabrication of multimode interference (MMI) waveguides that split single-mode light into multimode light in synthesized silica by use of femtosecond laser pulses. The number of modes at the output facet is dependent on both the width and the length of the MMI waveguides. The output spectral responses of the MMI waveguides in the visible region were examined. The fabricated MMI waveguides can be used as compact power splitters with large fan-outs.     

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.     

基于深刻蚀SiO2脊型波导的紧凑型多模干涉功分器

采用深刻蚀SiO2脊型波导,利用其弯曲半径小的优点,设计了一种紧凑型1×2多模干涉功分器.在输入/输出波导与多模干涉区域之间引入了逆向锥形波导,有效地减小了输出波导间距,从而减小了多模干涉区域宽度及其长度,进一步实现了多模干涉器件的小型化(仅为150 μm×20 μm).并采用三维束传播方法对多模干涉区域及输入/输出波导中的光场传输进行了模拟仿真,得到了一组最优参量设计值,从而实现其结构优化.     

Analysis of optical rib self-imaging multimode interference (MMI) waveguide devices using the discrete spectral index method

We have developed a semianalytical approach to study the propagation characteristics of multimode interference (MMI) waveguide structures. The effect of transverse crosssection geometry on self-imaging length and optical power throughput are investigated. We present results confirming that the nonquadratic modal dispersion encountered in stripe-loaded waveguides impairs self-imaging performance. Results indicate that the image degradation is more predominant for asymmetrically fed MMI waveguides.     

Optimal design of multimode interference couplers using an improved self-imaging theory

Self-imaging theory is widely accepted as a good method in designing 1 × N multimode interference (MMI) couplers, but it is also true that self-imaging theory is not suitable for low-contrast structures. An improved self-imaging theory is proposed in this paper for the optimal design of low-contrast 1 × N MMI couplers. The average effective width of MMI waveguide and the average effective propagation constant of MMI waveguide are used as the basis to modify the conventional self-imaging theory. A direct calculation of the average effective width of low-contrast MMIs is presented. We use this approach in the optimal design of a 1 × 4 silica MMI coupler, and the results show that the improved self-imaging theory is more accurate than conventional self-imaging theory for low-contrast structures, the results also show that if the material parameters and the width of an MMI waveguide are fixed, the average effective width of the MMI waveguide will increase with the decrease of the height of the core layer.     

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.     

Steering light into logic patterns with two-dimensional cascaded multimode waveguide

Steering light into logic patterns with two-dimensional cascaded multimode waveguide is demonstrated. By employing the imaging properties of 2D multimode interference (MMI) and partial phase modulation method, the design ideas and the implementing methods of the 2^(2×2) bits type spatial logic steering are discussed; therefore the structure of logical pattern is proposed. Numerical simulation is carried out to verify the design in detail by using the beam propagation method. It is expected to realize logic coders by using the integrated optical methods and exploit their potential applications in the field of optical logic.     

Self-guiding multimode interference threshold switch

We propose a new passive optical thresholding device that combines the principles of multimode interference (MMI) with self-guiding. The multimode region is composed of a nonlinear optical material that will support a self-guided beam (i.e., a material with a positive Kerr nonlinearity). The device operates by switching between the MMI mode of operation and the self-guiding mode of operation, depending on the input light intensity. We describe the basic principles of a self-guiding MMI device, simulate the device, and discuss design issues associated with these optically controlled optical switches.     

多模干涉马赫曾德尔光开关模型

根据N×N多模干涉耦合器的基本原理 ,确定了多模干涉耦合器的结构参数。通过分析多模干涉耦合器的输入光场与其映像间的相位关系 ,提出了模传输矩阵的分析方法 ,并用此方法分析了N×N普通干涉多模干涉耦合器、N×N相移器以及N×N普通干涉多模干涉马赫 曾德尔光开关 ,得到了它们的模场传输方程 ,分析了光开关在光场从任一输入端输入 ,从任一输出端输出时开关的驱动条件。用上面的方法分析了 4× 4光开关的结构及驱动条件     

Two-dimensional overlapping-image multimode interference couplers

The overlapping-imaging effect of one-dimensional (1D) multimode interference (MMI) coupler is widened to study the two-dimensional (2D) MMI coupler. 2D overlapping-image MMI couplers permit uniform and nonuniform 2D power splitting. Analytical formulas are derived for the intensities and phases of the overlapping-images at the end of MMI section. The overlapping-imaging properties in 2D MMI couplers are also concluded. And the guided-mode propagation analysis method is used to confirm the analytical results.     

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。     

A study of multimode interference power splitter with a truncated-and-raised interference region

Multimode interference (MMI) power splitters with various truncated-and-raised interference regions near the input edge are proposed. Simulation results show that ridge width decreasing is more effective for the reduction of non-uniformity and device length, whereas the ridge height increasing is more advantageous for the increase of the total transmitted power. Based on these results, MMI power splitter with a truncated-and-raised interference region is proposed. Simulation results show that the proposed power splitter can have both negligible non-uniformity and larger transmitted power.     

A 1 × 2 optical switch using one multimode interference region

A 1 × 2 optical switch using only one multimode interference (MMI) region is designed and demonstrated in GaAs/AlGaAs. This design makes a single MMI region works as MMI coupler using paired interference at “off” state and symmetric interference at “on” state. By injecting a current of 110 mA, the measured on/off ratio and crosstalk are 23 dB and 33 dB, respectively in the demonstrated device with GaAs/GaAlAs.     

The 650-nm variable optical attenuator based on polymer/silica hybrid waveguide

Visible light variable optical attenuators(VOA) are essential devices in the application of channel power regulation and equalization in wavelength-division multiplexing cross-connect nodes in plastic optical fiber(POF) transmission systems.In this paper, a polymer/silica hybrid waveguide thermo–optic attenuator based on multimode interference(MMI) coupler is designed and fabricated to operate at 650 nm. The single-mode transmission condition, MMI coupler, and transition taper dimensions are optimized through the beam propagation method. Thermal analysis based on material properties provides the optimized heater placement angle. The fabricated VOA presents an attenuation of 26.5 dB with a 21-mW electrical input power at 650 nm. The rise time and fall time are 51.99 and 192 μs, respectively. The time–stability measurement results prove its working reliability.     

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.     

新型1×N多模干涉分束器的三维设计与模拟

相比于传统的1×N对称型多模干涉（Mult-Mode Interference,MMI）分束器设计,提出了一种新型埋入式弱限制光波导分束器件.它的干涉区及输入输出波导采用倒锥形式,器件尺寸减小,且不均匀性与附加损耗也减小.以1×4的对称型MMI分束器为例,当只对干涉区采用倒锥形结构后,在TE偏振中心波长为1.55 μm时,器件长度减小了500 μm,均匀性增加了0.131 dB,而附加损耗仅增加了0.02 dB,波长响应较传统设计增加了40 nm.在此基础上,又在输入输出臂上也各增加倒锥形结构后,相比于传统设计附加损耗减小了0.02 dB,均匀性增加了0.139 dB,器件长度减小了500 μm.改进后的器件具有优越的容差性.器件采用掺氟型聚合物材料进行优化设计,通过在合理范围内偏离输出波导位置,使输出光强达到最大值.     

平顶型蚀刻衍射光栅波分复用器优化设计

本文对利用多模干涉(MMI)结构实现解复用器蚀刻衍射光栅(EDG)频谱平坦化的设计方法进行了改进.通过使用预展宽输入结构把带间串扰降到很低值;通过使用抛物线型多模干涉区,把带通纹波减小到接近于零的同时,大大增加了1dB带宽,最终得到了平坦化效果最佳的设计方法.作为例子,最后给出了两种不同频带间隔下的最优设计尺寸.     

High-performance 90° hybrid based on a silicon-on-insulator multimode interference coupler

We propose a multimode interference coupler (MMI) design for high-index-contrast technologies based on a shallowly etched multimode region, which is, for the first time to our knowledge, directly coupled to deeply etched input and output waveguides. This reduces the phase errors associated with the high-index contrast, while still allowing for a very compact layout. Using this structure, we fabricate a 2 × 4 MMI operating as a 90° hybrid, with a footprint of only 0.65 mm × 0.53 mm, including all the structures necessary to couple light to a fiber array. We experimentally demonstrate a common mode rejection ratio better than -20 dBe and phase errors better than ±5° in a ~50 nm bandwidth.     

Crosstalk comparison of lattice-form optical interleaver with different coupler structures

Lattice circuit made from a cascade of couplers and delay-lines is a popular approach for optical interleaver based on planar lightwave circuit (PLC) technology. Different coupler structures can be employed in the lattice circuit, including 1-stage directional couplers (DCs), 4-stage DCs, and 2-stage multimode interference (MMI) couplers. We fabricated optical interleavers with above three coupler structures, respectively. The experimental results prove that the latter two coupler structures can help to reduce crosstalk, which meets the simulation results well.