BROADBAND INTEGRATED OPTICAL DEVICES ON THE SILICON-NITRIDE PLATFORM

Abstract

The silicon nitride (Si3N4) photonic integrated platform is a low-loss and compact optical platform that has many applications. To further increase its wide applicability, we improve the performances of several optical devices in terms of their bandwidths and polarization dependence. For the first part of this thesis, we increase the bandwidths of multi-mode interference coupler (MMI) couplers, Mach-Zehnder interferometers (MZIs) and fiber-to-chip edge couplers, both theoretically and experimentally. In the second part, we demonstrate polarization independent arrayed waveguide gratings (AWGs) using three different methods, each one with its own advantages and disadvantages.We combine sub-wavelength grating (SWG) and MMI to achieve a significantly enhanced bandwidth, compared to a conventional MMI. The optimized SWG MMI exhibits a 1 dB bandwidth of 300 nm for both the insertion loss and power imbalance, which was verified experimentally. We propose a π-phase shift MMI MZI (πPS MMI-MZI) which serves as a broadband nulling interferometer. This new design uses a novel low phase shift error (PSE) broadband taper-sections phase shifter (TSPS). Our simulations predict an extraordinary low PSE, falling below 1°/0.02° in the wavelength range of 1450 nm to 1650nm for two and three-section TSPS respectively. Our experimental results demonstrate a PSE of 1^o within a 190 nm bandwidth for the two section TSPS. We also study the problem of achieving high coupling efficiency in chip-to-fiber edge couplers. With an innovative double-tip coupler design, we demonstrate a high coupling efficiency of 97.1%. The coupling efficiency remains above 90% within the measurement range (1450-1640 nm) for both TE and TM polarizations. In the second part of the thesis, we reduce the polarization dependence of AWGs by managing the light beam’s polarization state in-line (in the fiber system) or on-chip. For the in-line approach, the two orthogonal polarizations are separated using a prism. After polarization control in the fiber, the beams are injected into the chip and processed by a multi-input AWG. The on-chip approach processes the polarization using on-chip integrated optical devices including the polarization splitter and rotator. Finally, we propose a Bragg grating AWG to address the polarization dependence. The AWG is folded by reflecting the power using Bragg gratings placed on the arrayed waveguides. Both TE and TM Bragg gratings are designed so that we can control and align these two polarizations separately. As a result, we match the TE and TM responses and achieve polarization independence.