Integrated optic waveguide bragg grating devices in silicon-on-insulator and their applications : design, analysis and fabrication

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PhD; 2023; Electrical communication engineering

Integrated Waveguide Bragg Gratings are on-chip counterparts of Fiber Bragg Gratings. The working principles are similar, and functionalities could be explained with Coupled Mode Theory. Periodic index contrast required for Bragg reflection is achieved through physical corrugation of the waveguide walls. Being an 1D photonic crystal, such an Integrated Optic component is necessary for numerous applications involving wavelength selective functionality. Due to the large index contrast and tight mode confinement of sub-micron waveguides, realization of narrow bandwidth Bragg Gratings on chip remains a difficult task. In this thesis, we study the integration of Bragg gratings on different Silicon Photonic waveguide configurations and engineer the structures to design devices for communication and sensing applications. The work covers intuitive conceptualizations, novel designs, and also materials and methods of fabrication and characterization of few devices. Firstly, Bragg Gratings on conventional Photonic Wire Waveguides are designed and analyzed. Further, shallowly etched ribs, deeply etched ribs (DER) and slot waveguides are optimized for the incorporation of sidewall corrugations. Wavelength filters with narrow bandwidth, smaller device footprint, lesser complexity and higher fabrication tolerance are essential for on-chip Wavelength Division Multiplexing (WDM) applications. As part of this work, we design novel sub-micron, DER waveguide to achieve ultra narrow bandwidth gratings. The perturbations applied on thin slab sidewall eliminates the requirement of narrow corrugation width, which is otherwise possible only with high resolution lithography systems. 3-dB bandwidths as narrow as 0.5 nm at 1550 nm, desirable for DWDM are achieved. Further we analyze DER grating designed on Silicon Nitride platform and model its surface adsorption sensing capability for electrostatic assembly of Poly Electrolyte Multilayers (PEM). Following this, we focus on novel Silicon on Insulator (SOI) slot waveguide geometries. A dual slot waveguide is optimized in terms of induced geometrical asymmetry, to tailor the dispersion parameters. We observe that Bragg gratings on vertical slot waveguides exhibit a stop-band closure behavior. This phenomenon is utilized to bring down the transmission bandwidth of slot gratings, which is then used to design a refractive index sensor. In comparison to conventional vertical slots, loaded slots are found to have better confinement factor and are optimized to achieve polarization independent behavior. Further we implement Bragg gratings on sidewalls of loaded slots to obtain narrow bandwidths.