Examination Committee

Prof Chi Keung TANG, CSE/HKUST (Chairperson)
Prof Andrew W O POON, ECE/HKUST (Thesis Supervisor)
Prof Marc SOREL, School of Engineering, University of Glasgow (External Examiner)
Prof Nelson Sze-Chun CHAN, Department of Electronic Engineering, City University of Hong Kong (External Examiner)
Prof Kei May LAU, ECE/HKUST
Prof Hoi Sing KWOK, ECE/HKUST
Prof Kam Sing WONG, PHYS/HKUST
 

Abstract

Silicon photonics has gained a significant advancement over the past two decades. In this thesis, we experimentally study silicon sub-bandgap photodetectors in the telecommunications wavelengths of 1550 nm in silicon waveguides and microring resonators using surface-state absorption (SSA) and defect-state absorption (DSA). We integrate such photodetectors into silicon microring resonators as a photomonitor to actively stabilize the microring resonances for emergent optical switch fabrics applications in optical interconnects for datacenter applications.
 
For characterizing SSA, we have proposed and demonstrated a modulated photocurrent-transmission spectroscopy method to measure the linear SSA coefficient from the photocurrent extracted from a PIN-diode-integrated silicon waveguide, while referencing with the nonlinear two-photon-absorption coefficient measured from the modulated transmission spectrum. Our experiments reveal a responsivity of ~1.4 mA/W/mm from an air-clad waveguide photodetector upon -1 V, indicating an SSA coefficient of 0.013 ~ 0.017 mm-1.
 
We have designed and integrated such PIN photodetectors in silicon microrings. The microring is integrated with an air-clad photodetector and an electrically isolated silica-clad photodetector as a control. Our experiments reveal a responsivity of 0.9 ~ 2.9 mA/W/mm upon -1 V from the air-clad photodetector, and of only 0.13 ~ 0.19 mA/W/mm from the control photodetectors. Likewise, we integrated ion-implantation-induced DSA-based PIN photodetectors in microrings. Our experiments reveal a responsivity of 7.0 ~ 65.0 mA/W/mm upon -1 V using B-, P-, and Ar-ion implantations.

In order to obtain actively stabilized silicon microrings, we utilize the SSA/DSA-based photodetectors to monitor the spectral alignment between the microring resonance and a carrier wavelength. We use feedback-controlled integrated electro-optic and thermo-optic tuners, along with a threshold-detection/slope-detection method for actively stabilizing the microring. We demonstrated a reduced transmission power variation of < 2.5 dB over a 14oC on-stage temperature modulation. We observe open eye-diagrams with a 30Gb/s data transmission through an actively stabilized microring array.