Topology Optimized Components for Mode- and Wavelength Division Multiplexing

This thesis deals with the topic of passive integrated nanophotonic devices realized in silicon on insulator material. The project has been concerned with all the steps of the process: Design, fabrication and characterization. The focus has been on using the inverse design method topology optimization.The method has been employed extensively to obtain various nanophotonic components. One of the outcomes of the project has been the development of a solid understanding of the benefits and limitations of topology optimization. This thesis presents numerous designs that have been characterized both through simulations and experiments. Among these are converters and (de-)multiplexers for mode division multiplexing, both realized with a record small footprint. Wavelength multiplexing devices were used as a basis for investigating the correlation between structure sizes and performance. Fortunately a larger footprint does not always give rise to better performance, however allowing for smaller feature sizes will. The design of compact tapers was commenced. Difficulties were met when working with very wide waveguides but methods for overcoming these were suggested. A novel form of cladding modulated Bragg gratings, utilizing continuous rails to modify the refractive index and cause the reflections, has also been proposed and experimentally verified as part of this project. This work has contributed additional components to the toolbox of devices necessary for integrated photonics. It has been shown that topology optimization is a strong method for creating extremely compact devices, the small features do however mean that they are not yet possible to fabricate on a large scale. Complex device functionalities can be obtained. Building on previous work of simpler structures it is comparatively easy to remake the new designs and then increase complexity without much impact on the footprint. The benefit of inverse design tools, like topology optimization, is that they lead to structures without geometrical constraints and which are independent of the designer. This project has however shown, that the best results are obtained when iterating on the optimized structures and providing the tool with well-chosen starting point structures.