| dc.description.abstract | Silicon photonics technology has a wide range of applications, including filters, detectors, modulators, nonlinear optics, etc., due to its high bandwidth, low loss, and compatibility with complementary metal oxide semiconductor processes. In these applications, the coupling between the bus waveguide and the resonator plays an important role, which can provide a strong extinction ratio and high sensitivity for linear components in the critical coupling regime, and also provide better conversion efficiency for nonlinear components in the over coupling regime. In the past, there are several methods to optimize the coupling, including gap distance reduction or elongation of the coupling region, but these methods require better process control and complicated waveguide designs. Therefore, the flexibility in waveguide design is limited. In this thesis, the coupling of the silicon nitride micro-ring resonator is investigated with the design of the tapered waveguide in the coupling region, and the effect of the tapered waveguide with different geometric dimensions is discussed. Coupling enhancement can be identified with coupling of a straight bus.
Firstly, the tapered waveguides with different dimensions are simulated by the finitedifference-time-domain (FDTD) method, the energy in the resonator is studied, and the
changes in coupling strength of different tapered waveguides are investigated. We utilize the design of the single-side tapered waveguide to achieve coupling up to 347 times higher strength in comparing to that without a tapered design.
Then, the author introduces the fabrication process of the device. In this paper, a silicon nitride layer deposited by low pressure chemical vapor deposition is used as the
waveguide core, and an i-line stepper lithography and an electron beam writing system are both utilized for the lithography process. After optimizing the process, the quality factor of both systems can be achieved with the order of 10^5. Because of the proposed design of the tapered waveguide, the micro-ring resonator with a gap of 0.4 μm complies the available (minimum) resolution of the stepper lithography, mass production with strong coupling can be achieved by the i-line stepper without relying on the costly
electronic beam writing system. In terms of measurements, the optical transmission spectra at 1550 nm are analyzed. The coupling strength of the fabricated resonators with
i-line stepper and the electron beam writing system can be compared with or without the design of the tapered waveguide, showing an enhancement by 7 times and 15 times,
respectively.
The work in this thesis provides a flexible design method, which can effectively improve the coupling between the bus waveguide and the resonator using a single-side tapered waveguide. In addition, it provides critical coupling without the need of a narrow gap width (< 400 nm) at a moderate quality factor. This design paves the way for the
mass-productive and cost-effective process of microresonator fabrication through the iline stepper lithography.
| en_US |