Efficient Power Splitters and Couplers in Silicon Nanophotonics
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There has been a recent explosion of interest in the optical properties of silicon in the near infrared regime, and in particular using Silicon-on-Insulator (SOI) as a platform for integrated optics. Due to the high refractive index contrast between silicon and silicon dioxide, the optical mode for typical waveguides is tightly confined in a submicron region, creating a number of advantages and disadvantages. On the one hand, the small mode volume enables compact layout, thus high device density. However, the high index contrast also raises the requirements for fabrication resolution and uniformity. For example, a simple silicon waveguide Y-branch usually has over 1 dB insertion loss because the required sharp corners will not resolve in a typical CMOS process. Another challenge is efficient coupling between the very small modes in silicon waveguides, and the larger modes often found in other material systems. Here I demonstrate the solution to a number of key optical design problems in silicon photonics. I report on a novel Y-branch with insertion loss of 0.13 dB and minimum feature size of 0.2 μm, followed by a discussion on cascaded directional couplers that are insensitive to evanescent coupling coefficient variations. I also report on a new approach to achieving edge coupling on the silicon platform, which utilizes thin waveguides with an expanded mode size. This approach can be used to couple a silicon waveguide efficiently to an external laser cavity in a III-V material, and so may play an important role in the development of an on-chip laser. This is one of the first proposals in the literature to address the gain-chip/silicon waveguide coupling problem. An external cavity laser using the new Y-branch and thin waveguide coupler is proposed.
- Electrical engineering