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UBC Theses and Dissertations

Spiral Bragg gratings for TM mode silicon photonics Chen , Zhitian


In this thesis, we demonstrate long transverse magnetic (TM) Bragg gratings wrapped compactly using spiral waveguides on the silicon-on-insulator (SOI) platform. We developed three types of TM spiral Bragg grating waveguides (SBGWs) including uniform spiral Bragg grating (U-SBGWs), phaseshifted spiral Bragg grating (P-SBGWs), and chirped spiral Bragg gratings (C-SBGWs). Our spiral waveguides are space-efficient, requiring an area of only 189 x 189 μm² to accommodate 1 cm long Bragg grating waveguides and, thus, are less susceptible to fabrication non-uniformities. Due to these factors, the TM U-SBGWs are able to successfully obtain narrow bandwidths and high extinction ratios (ERs), as narrow as 0.09 nm and as large as 52 dB respectively. Also, the TM P-SBGWs can obtain sharp resonance peaks with high quality factors of 78790. Finally, we demonstrate the TM C-SBGWs, which exhibit group delays that are linear functions of the wavelengths over their passbands. Traditionally, due to the large coupling coefficients and the flexibility for achieving desired spectral characteristics, short Bragg grating waveguides for transverse electric (TE) modes on the SOI platform have been developed for applications in optical communication and sensing systems. In contrast, TM modes Bragg gratings on SOI platform have small coupling coefficients and, therefore, the grating lengths need to be much longer than TE mode devices, in order to obtain large ERs. However, such TM mode Bragg gratings can achieve very narrow bandwidths. Creating long gratings in regular straight waveguides suffer from the fabrication non-uniformity effects caused by the wafer thickness. As is shown here, spiral-shaped waveguides can be used to increase the grating length, while still being made using little on chip real estate, thus reducing the effects of fabrication non-uniformity.

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