PhD defence of Weijia Li – Advancing Integrated Photonic Designs for High-speed Datacenter Interconnects

Abstract

The demand for high-speed data transmission is escalating with the rapid expansion of services like cloud computing, video streaming, and big data analytics. Traditional electrical interconnects, limited by bandwidth and power consumption bottlenecks, are giving way to optical communications. Integrated photonic devices, offering significant advantages in performance and efficiency, have become critical in addressing these demands. Key to enhancing optical link capacity are multiplexing techniques such as polarization division multiplexing (PDM), wavelength division multiplexing (WDM), and mode division multiplexing (MDM), which necessitate the development of components capable of handling diverse inputs. Additionally, maximizing single-channel transmission capacity in intensity modulation direct detection (IMDD) systems is another vital strategy for capacity enhancement.

This thesis investigates integrated devices and circuits for optical interconnects. The first section develops broadband silicon photonic polarization-insensitive switches essential for PDM transmission. Initial designs employ square-cross-section waveguides that ensure the same thermal optic coefficients (TOCs) for two polarizations, achieving polarization-dependent losses of less than 2 dB across the C-band. Further innovations introduce polarization-mode diversity conversion and mode-insensitive phase shifters, enhancing extinction ratios to over 15 dB across a 100-nm wavelength range while maintaining low insertion losses.

Subsequent research introduces an integrated polarization controller capable of converting any state of polarization (SOP) into the fundamental transverse electric (TE) mode for on-chip modulators, supported by a theoretical analysis of various control algorithms, including Particle Swarm Optimization (PSO), Genetic Algorithm (GA), and Gradient Descent (GD).

The third section presents a colorless, power-efficient silicon photonic switch for WDM, employing ultra-broadband couplers and subwavelength gratings for broadband operation, and mode-looped phase shifters for doubling the device's power efficiency. This switch demonstrates extinction ratios above 10 dB across a wavelength range from 1350 nm to 1675 nm, with a minimal power consumption of 11.1 mW.

The final section demonstrates high-speed transmission using advanced BTO-assisted silicon photonic modulators and InP external modulated lasers (EMLs). A net 300 Gbps/λ transmission is achieved by using the BTO modulator. Furthermore, transmissions using InP EML exceed 200 Gbps over 80 km in the O-band, highlighting the potential of EMLs for metropolitan networks.

Overall, this research not only advances multiplexing technologies but also enhances single-channel capacities, emphasizing the essential role of integrated photonic devices in expanding modern optical networks to meet global data demands.