Recently,silicon-based photonic integrated circuits (PICs) have attracted considerable interest due to the advantages of high index contrast and complementary metal oxide semiconductor compatible process.Furthermore,to meet the ever-growing bandwidth requirements for data center and supercomputing,several multiplexing on-chip technologies by using silicon PICs are proposed.Among them,the mode division multiplexing (MDM) is widely recognized to be important,where mode-order converters (MOCs) are fundamental building blocks.In addition,slot waveguides can efficiently confine the light in low-index regions,thus forming various kinds of novel photonic devices.In this paper,a compact 1 × 2 multimode interference (MMI) mode-order converter (MOC) for silicon-based slot nanowires is proposed,where straight waveguides,as the input/output channels,are connected to a quadratic-tapered multimode waveguide via linear-tapered waveguides.A full-vectorial finite-difference frequency-domain method is used to analyze the modal characteristics of the used silicon-based vertical slot waveguides;from this,quasi-TM mode is chosen as an input optical signal since its field distribution is strongly confined in the slot,i.e.,the electric field strength is greatly enhanced in the vertical slot,and with the increase of the width of the slot waveguide,it can support higher-order quasi-TM modes.Compared with the beating length of rectangular MMI structure,the beating length of quadratic-tapered MMI structure can be effectively reduced with transmission loss lowering.From the imaging position of the guided-mode in MMI region via self-imagining effect,the length of quadratic-tapered MMI structure can be determined accurately where first-order and fundamental quasi-TM modes are outputs,respectively,from wider and narrower channels.A three-dimensional finite-difference time-domain method is utilized to assess the performance of the proposed MOC,where the insertion loss and crosstalk are analyzed in detail.The results show that an MOC with an MMI section of 3 × 5 μm2 is achieved to be an insertion loss and a crosstalk of ～0.35 dB and ～-16.9 dB,respectively,at a wavelength of 1.55 μm by carefully optimizing the key structural parameters.Moreover,the fabrication deviation of the proposed device is also analyzed in detail and the performance is evaluated,where insertion loss and the contrast are considered.To demonstrate the transmission characteristics of the proposed MOC,the evolution of the excited fundamental quasi-TM mode through the MOC is also presented.Numerical results show that the presented MOC realizes the desired function,converting the fundamental quasi-TM mode into first-order one with reasonable performance.We remark that the present MOC has a good potential application in MDM system to improve the capacity of the silicon-based on-chip transmission system.%提出了一种基于硅基槽式纳米线结构的紧凑式1×2多模干涉器型模阶数转换器,其中输入/输出通道为槽式直波导,经线性锥形过渡器连接居于中心的二次锥形槽式多模波导.采用全矢量频域有限差分法详细分析了垂直槽波导的模式特性,选取电场主分量Ey得到增强的quasi-TM模作为转换器的光信号模式.对比分析了矩形结构与二次锥形结构中的周期自镜像效应,发现二次锥形结构尺寸更短、损耗更低的特点.根据自镜像效应中一阶模成像位置设计多模干涉区域长度,经线性锥形过渡器从较宽输出端口输出一阶模,从较窄输出端口输出基模,从而实现模阶数转换功能.采用三维有限时域差分法详细分析了该转换器的光波传输特性,详细讨论了器件关键结构参数的制作容差.参数优化结果表明,该转换器的多模干涉区域的尺寸为3×5μm2时,在1.55 μm工作波长下,quasi-TM基模在输出quasi-TM一阶模端口的插入损耗约为0.35 dB,输出波导间的串扰约为-16.9 dB.另外,给出了输入模场主分量在器件中的传输演变情况.
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