硅光子技术直接利用成熟的微电子技术实现光子器件的低成本部署，除在数据通信领域成功商用外，在激光雷达、生物传感及光计算等领域也展现出巨大前景。耦合器是这些芯片与光纤等外界光路的接口。为了适配这些新兴应用的多变需求，新型耦合器等光子集成器件亟待开发。光栅耦合器加工简单，放置灵活，耦合对准容差大，支持晶圆级测试。然而，该器件对偏振及波长敏感，制约了应用。此外，若该器件除耦合外还能实现复用功能，则将助力许多场景下硅光子芯片的应用，包括光纤到户服务及空分复用系统等。本论文研究硅光子多功能光栅耦合复用器，以解决上述性能限制、满足应用需求。本论文主要包括以下内容： 首先，分析光栅耦合器的基本工作原理，介绍多种提高性能指标的设计方法，总结器件仿真及制备流程、测试方法等。 其次，针对无源光网络等应用需求，研究双波段光栅耦合复用器。提出一种三端口偏振分集的双波段光栅耦合复用器，可在高效耦合上行波长的同时实现下行波长的偏振不相关接收，实测O波段和S/C波段耦合效率分别为–3.8 dB和–5.4 dB。在双波段耦合的基础上，为降低光纤封装难度，利用双刻蚀光栅独特的角度色散特性，实现光纤的完全垂直放置；为降低加工成本，利用亚波长光栅有效折射率调控，提出仅需单次刻蚀的光栅耦合分束器。 再次，针对空分复用光纤通信的需求，研究芯片的少模光纤接口。通过拓扑优化的逆向设计，实现长度仅为10 μm的超紧凑模斑大小转换器，实测插入损耗最低为–1.02 dB。结合经优化的非均匀光栅，可实现芯片对不同高阶光纤模式的耦合。 然后，探索新型氮化硅-硅多功能光栅耦合复用器。设计层间耦合器并提出多种新型双层器件，包括双波段光栅耦合分束器，偏振分束光栅耦合器，以及支持四种光纤模式的复用器。 最后，提出一种紧凑型光子角动量产生器。在不同波导模式输入下，通过各层衍射模场的组合，可实现携带±1及±2级次光子角动量的涡旋光的产生。
Silicon photonics leveraging matured microelectronics manufacturing technologies is an emerging platform for low-cost and high-performance optical devices and systems. It can lower the cost of transceivers in telecom and datacom systems, while it has also shown increasing potential for light detection and ranging, biosensing, as well as optical computing. Couplers are bridges between the chip and fiber or other bulk optics systems. For these emerging applications, novel couplers that best suits different application scenarios are desirable. Grating couplers have advantages of easy fabrication, versatile placement on-chip, high alignment tolerance and wafer-level testing capability. Currently, their wavelength and polarization sensitivities are limiting factors for various emerging applications. Besides, for certain applications such as fiber-to-the-home services and space-division multiplexing systems, it is also desirable that grating couplers not only serve as couplers, but also achieve multiplexing functionalities. This dissertation will focus on the research of multifunctional grating couplers for fiber-chip bridging and multiplexing. First, we briefly introduce basic operation principles for grating couplers, summarize design methods to enhance their performance metrics, and overview the workflow for simulation and fabrication, as well as measurement techniques. Secondly, we demonstrate dual-band-multiplexing grating couplers for passive optical network applications. We propose a three-port grating coupler that achieves dual-band multiplexing and downstream signal reception with polarization diversity, with experimentally measured coupling efficiency of –3.8 dB at O-band and –5.4 dB at S/C-band. We utilize the unique diffraction angle dispersion of dual-etch grating coupler to achieve dual-band coupling and perfectly vertical fiber placement, which facilitates fiber packaging. We engineer the effective refractive index of subwavelength grating to design dual-band grating coupler and diplexer with only one etching step, which lowers the cost of manufacturing. Thirdly, we demonstrate compact silicon photonic few-mode fiber interface for space-division multiplexing fiber communication system. By adopting inverse design based on topology optimization, an ultra-compact dual-mode mode-size converter with a total length of 10 μm is designed, with experimentally measured minimum insertion loss of –1.0 dB. It is further connected with an optimized non-uniform grating coupler that generates and multiplexes different spatial modes in a few-mode fiber with inputs of different waveguide modes. Fourthly, we investigate multifunctional silicon-nitride-on-silicon grating coupler and multiplexer. We design a novel trident interlayer transition coupler and propose various dual-layer couplers, including a dual-band splitting grating coupler, a polarization-splitting grating coupler, as well as a few-mode fiber mode-multiplexing circuit for four distinct spatial modes. Lastly, we propose a compact orbital angular momentum generator. It combines multiple diffraction fields from different input modes at different layers, in order to generate optical vortex beam carrying orbital angular momentum with topological charges of ±1 and ±2.
