Silicon-based Multiplexing Devices With Digital Meta-structure

Silicon photonics integration platform has unique advantages of small size,hign integration,low price and optical communication characteristics of large bandwidth,ultra-high speed,low energy consumption,which has been widely researched and applied in optical communication and interconnection systems.At present,the working-bandwidth improvement of silicon photonic modulators and detectors has been far from meeting the communication capacity requirements.On-chip multiplexing technology is a common method to improve communication capacity.As the key functional device of on-chip multiplexing system,the performance parameters such as bandwidth,loss and crosstalk of single（de）multiplexing device cannot be an obstacle that limits the performance of the entire multiplexing system.In addition,the size of the multiplexing device should be as small as possible to improve the integration of the system.Therefore,higher performance（large bandwidth,low loss and low crosstalk）and smaller size are an important research direction for on-chip multiplexing devices.In recent years,Inverse-design method has provided a new design idea for the research of silicon-based photonic integrated devices.Inverse design automatically searches the entire design region with arbitrary shapes,which greatly expands the design parameter freedom and enables optical-field manipulation at the nanoscale,showing great potential to simultaneously realize small size and high-performance devices.This paper mainly focuses on the research of silicon photonic devices with digital metastructure for on-chip multi-dimensional multiplexing.Focusing on the size,crosstalk,loss and other problems of on-chip wavelength/mode/polarization multiplexing devices,a digital meta-structure based on inverse-design method is proposed.The thesis realizes the miniaturization and high-performance research of several key silicon-based multiplexing devices.Main research contents and conclusions of this paper are as follows:Firstly,a digital meta-structure using inverse-design method is proposed to solve the limited design freedom of conventional silicon waveguide.according to the theoretical basis of the optical waveguide,a waveguide model is established,the variation law of the light propagation constant with the waveguide structure is analyzed and summarized.Then,a digital meta-structure model is bult and improved,the optical field response of the highly complex disordered digital meta-structure is explored by combining the theoretical and numerical simulation.Furthermore,we demonstrate the accurate manipulation of the optical field phase,wavefront,polarization,amplitude dimensions based on the digital meta-structure.This provides theoretical guidance for the design of high-integration and high-performance silicon-based photonic devices.Based on this research result,this thesis designs and demonstrates a series of ultra-small and high-performance silicon-based multiplexing devices based on the digital meta-structure.In terms of wavelength multiplexing,ultra-small coarse wavelength demultiplexer and dense wavelength division multiplexer are realized based on the digital metalens with large phase gradient and high dispersion.The digital meta-waveguide structure has the ability to precisely control intensity and phase distribution of the light field.The optimization of the digital meta-structure using inverse-design method can realize on-chip meta-lenses with a large phase gradient of 5.7πrad/μm.A highly dispersive digital metalens with continuous and huge phase gradient is designed and realized.The thickness of the metalens is only a wavelength scale（～1.55μm）,the width is only 25.4μm,and the focal length is 19.5μm,allowing the beam occur 45°bending and focusing through the metalens.This metalens can realize wavelength demultiplexing function.Through the folded distribution of multiple metalens,the beam can be continuously bnet in a small size,and the dispersion effect can be accumulated to achieve more effective wavelength splitting.Thus,an 8-channel coarse wavelength demultiplexer with a small area of only 48×24μm² is realized by optimizing the distribution of the two-layer folded metalens.Furthermore,a 32-channel dense wavelength demultiplexer based on five-layer folded metalens is realized.The wavelength channel spacing is 1 nm.The footprint size is only 100×100μm².In terms of mode multiplexing,focusing on the size,loss,inter-mode crosstalk of on-chip mode division multiplexing devices and photonic circuits,multi-mode waveguide bending,waveguide crossing and mode multiplexer with the digital meta-structure are realized using inverse design method.The device can support TE₀,TE₁ and TE₂ modes simultaneously.The insertion loss of devices in the 80 nm bandwidth range is less than 1 d B,the inter-mode crosstalk is less than-20 d B,and the device size is only a few microns,which is an order of magnitude smaller than conventional devices.Based on these devices,arbitrarily routed mode-division multiplexed photonic circuits for dense integration are demonstrated.And the experiment proves that the three-mode multiplexed high-speed signal（3×112 Gbit/s）can be transmitted for the two mode-division multiplexing photonic circuits.In addition,the fabrication error and the compatibility with the silicon photonic oundry’s design rule of the digital meta-structure are simulated and analyzed,which enhances the device manufacturability.In terms of polarization multiplexing,the optimization design is mainly aimed at the characteristics of size,bandwidth,dual-polarization and hybrid-multiplexing.The polarization splitter-rotator is one of the key components to realize on-chip polarization multiplexing system.A polarization separation rotator with a length of only 7.92μm is proposed based on the digital meta-structure.The device exhibits good performance over a large bandwidth over 100 nm and±10 nm fabrication tolerance.Then,an ultra-compact polarization multiplexing system based on polarization-insensitive crossing is demonstrated,which further reveals the powerful manipulation capability of optical field waves using digital meta-structure.Since the polarization separation rotator only utilizes two fundamental orthogonal polarization states,it can only double the system capacity.Further,an ultra-compact dual-polarization and multi-mode mode multiplexer and power splitter are designed and experimentally characterized to increase mode channels.The designed device area is only 6.8×6μm² and 6×5.25μm².And the thsis demonstrates a dual-polarization four-channel hybrid multiplexing photonic circuit,which has a very compact integration size and can realize hybrid polarization and mode multiplexing simultaneously.This ultra-small size and high-performance device helps to improve the integration density of polarization and mode division multiplexing systems in photonic integrated circuits.