A Toolbox of Optimized Silicon Photonics Devices

Abstract

Silicon photonics has emerged as a disruptive technology to address the exponentially growing demand for bandwidth in optical interconnects and data communications. The silicon material system supports large wafers, high device yield and performance uniformity, leveraging the half-century of investment by the electronics industry. Silicon is transparent in the 1.3 - 1.6 µm wavelength range, and has higher refractive index than its oxide, enabling submicron waveguides. On the other hand, the high index contrast between silicon and oxide, 3.4 to 1.4, making silicon photonic devices very sensitive to geometry variations commonly seen in complementary metal-oxide-semiconductor (CMOS) fabrication. I demonstrate a design approach using coupled particle swarm optimization (PSO) and finite difference time domain (FDTD) method that produces highly compact, efficient and robust passive devices. Silicon also has a high thermo-optic coefficient, 1.8×10-4 K-1, making resonant devices, which are critical for wavelength filtering and multiplexing, sensitive to temperature perturbations that are common in practical environments. A universal stabilization approach based on bandgap temperature sensor and active feedback control is presented. In addition to passive devices, active device such as modulators, photodetectors, and lasers are needed for practical photonics integrated circuits (PICs). Electro-optic modulation in silicon can be achieved by free carrier plasma effect, typically implemented as reverse biased p-n junction overlapping with the waveguide mode. However lasing or photo detection is inherently not available in silicon, and is only possible by integrating other optically active materials. Germanium is the preferred absorber because of its CMOS compatibility, but germanium processing has received far less attention than silicon so far, with active ongoing research trying to understand its implantation, dopant diffusion, and metal contact alloying properties. Hence a photodetector that does not require doping or metallization of germanium is highly desirable. I propose and demonstrate a germanium-on-silicon photodetector using only intrinsic germanium and no metal-germanium contact, which significantly simplify the process flow. The detector shows 1.14 A/W responsivity, over 40 GHz 3 dB bandwidth, and less than 1 µA dark current. I also demonstrate a hybrid-integrated laser based on Sagnac loop mirror and micro-ring wavelength filter with 44 dB side mode suppression ratio, 1.2 MHz line-width, and 4.8 mW on-chip output power. Compared to distributed Bragg grating based cavity, which require ultra-fine feature size, the Sagnac loop mirror is simple to fabricate, and provides accurately controlled transmittance (and reflectivity) with negligible excess loss. The toolbox of devices presented in this thesis includes, in summary: i. Highly efficient, compact, and robust Y-junctions and waveguide crossings, as well as an universal PSO-FDTD design methodology; ii. A floating germanium photodetector that is free from doping in Ge and direct Ge-metal contact, with high responsivity, high bandwidth and low dark current; iii. A Sagnac loop mirror and micro-ring based on-chip cavity configuration for laser integration; iv. A bandgap temperature sensor for resonance thermal stabilization. These address the unique challenges of building integrated photonics devices using CMOS-compatible material system, and, bring the vision of large-scale photonic integration on silicon an important step forward to practical application.