Control Systems for Silicon Photonic Microring Devices
The continuing growth of microelectronics in speed, scale, and complexity has led to a looming bandwidth bottleneck for traditional electronic interconnects. This has precipitated the penetration of optical interconnects to smaller, more localized scales, in such applications as data centers, supercomputers, and access networks. For this next generation of optical interconnects, the silicon photonic platform has received wide attention for its ability to manifest, more economical, high-performance photonics. The high index contrast and CMOS compatibility of the silicon platform give the potential to intimately integrate small footprint, power-efficient, high-bandwidth photonic interconnects with existing high-performance CMOS microelectronics. Within the silicon photonic platform, traditional photonic elements can be manifested with smaller footprint and higher energy-efficiency. Additionally, the high index contrast allows the successful implementation of silicon microring-based devices, which push the limits on achievable footprint and energy-efficiency metrics. While laboratory demonstrations have testified to their capabilities as powerful modulators, switches, and filters, the commercial implementation of microring-based devices is impeded by their susceptibility to fabrication tolerances and their inherent temperature sensitivity. This work develops and demonstrates methods to resolve the aforementioned sensitivities of microring-based devices. Specifically, the use of integrated heaters to thermally tune and lock microring resonators to laser wavelengths, and the underlying control systems to enable such functionality. The first developed method utilizes power monitoring to show the successful thermal stabilization of a microring modulator under conditions that would normally render it inoperational. In a later demonstration, the photodetector used for power monitoring is co-integrated with the microring modulator, again demonstrating thermal stabilization of a microring modulator and validating the use of defect-enhanced silicon photodiodes for on-chip control systems. Secondly, a generalized method is developed that uses dithering signals to generate anti-symmetric error signals for use in stabilizing microring resonators. A control system utilizing a dithering signal is shown to successfully wavelength lock and thermally stabilize a microring resonator. Characterizations are performed on the robustness and speed of the wavelength locking process when using dithering signals. An FPGA implementation of the control system is used to scale to a WDM microring demultiplexer, demonstrating the simultaneous wavelength locking of multiple microring resonators. Additionally, the dithering technique is adopted to create control systems for microring-based switches, which have traditionally posed a challenging problem due to their multi-state configurations. The aforementioned control systems are rigorously tested for applications with high speed data and analyzed for power efficiency and scalability to show that they can successfully scale to commercial implementations and be the enabling factor in the commercial deployment of microring-based devices.