Abstract
Silicon-based microring devices are attracting much research attention due to their small footprint, versatility in various applications, and compatibility with the existing IC process for mass production. Hence, there is a strong demand for the nanofabrication and analysis method of high-index-contrast and ultra-compact microring resonator. The objectives of this dissertation are to utilize electron beam lithography, focusing on nanofabrication technology, to develop high quality factor and ultra-compact microring resonator on the SOI substrate with a large free spectrum range (FSR) and narrow pass-band for DWDM application. The configuration of single waveguide coupled microring resonator and couple mode theory in a time-domain method are utilized to extract ring resonator loss and coupling coefficient. These two parameters are important for designing the multi-ring-based optical switch devices, which the optical switch is a basic element in DWDM system. We use a quasi-3D-FDTD method, a two dimensional Finite-difference time-domain method (FDTD) plus an effective index method (EIM), to analyze the transmittance spectrum of single waveguide-coupled microring resonator, and to verify the experimental results. By analyzing the transmittance spectrum, we find that the structure of waveguide-coupled microring resonator induces the radiation loss in the coupling region. As the gap width shrinks, the radiation loss increases rapidly. The gap width dependent optical loss gives us new insights into waveguide coupled resonator-based devices. With the simulation results of a silicon microring resonator with a radius of 2.75 μm and gap width of 100 nm, the coupling coefficient is just 4%, but the optical ring loss achieves 0.0625dB/circumference. That seriously affects the design and performance of multi-ring-based devices. By employing the effect of narrow gap-inducing radiation loss, as the gap width is large enough for weak coupling condition, it can obtain the intrinsic loss of a microring resonator; this is an effective and accurate method to estimate the optical loss of a waveguide-coupled ring resonator. Furthermore, we find that the Fano resonance phenomenon creates a little asymmetry on the transmission spectrum of ring. This could be due to multiple modes interacting with a single eigen-mode ring resonator for different relative phases, for example, slot mode and higher order mode brought from the neighbor bus waveguide and ring resonator. To avoid the narrow gap width inducing radiation loss, we use the configuration of a tapered waveguide embedded, and a conformal racetrack to enhance the coupling efficiency and reduce radiation loss. By changing the pattern segmented method and electron beam writing strategies to reduce the sidewall roughness on the waveguide, we demonstrated a high quality, ultra-compact microring filter with waveguide width of 0.5μm, radius of 1.75μm, FWHM of 0.143nm and quality factor of 10,938 at TE-like mode. We show that single waveguide coupled silicon microring resonator provides the radiation loss due to narrow gap width. For example, the silicon microring resonator with radius of 2.75μm and waveguide width of 0.5μm has the intrinsic loss of 0.01382 dB/circumference. At gap width of 200 nm, the optical ring loss is 0.03455 dB/circumference and radiation loss is 0.2 dB/circumference. Through IME’s sample, we can see that the fabrication variation on the performance of silicon microring resonator involves the drifting of resonance wavelength in standard deviation of 0.4 nm and group velocity in standard deviation of 2×10-3.