Design and Fabrication of Integrated Optical Waveguides and Sidewall Bragg Gratings
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In this dissertation, a novel design platform with arsenic tri-sulfide (As2S3) on titanium-diffused lithium niobate substrate (Ti:LiNbO3) is introduced to provide physical foundation for integrated optical device applications. LiNbO3 possesses excellent birefringence, electro-optical and acousto-optical effects that enable its high efficiency in nonlinear parametric frequency conversions and flexible tuning capabilities. Secondly, high-quality, low-loss channel waveguide can be made by thin-film metal diffusion or proton exchange with high reproducibility. The mode area size of the channel waveguide is close to single-mode fiber, leading to negligible coupling loss. As2S3 has a large index that provides strong mode confinement and tight bending radii for high integration densities. Both materials exhibit broad transparency: 0.4-5.0 μm for LiNbO3 and 0.63-11.0 μm for As2S3, making it possible to extend their applications to mid-infrared (3-20 μm) regime. On this design platform, a hybrid waveguide structure is optimized for efficient mid-infrared radiation at 4.0-4.9 μm by phase-matched difference frequency generation (DFG). The hybrid waveguide is designed for single mode operation. A normalized power conversion efficiency of 20.52%W^-1cm^-2 is theoretically predicted on a 1 mm-long waveguide pumped at 50 mW, which is the highest efficiency record for LiNbO3. Using a tunable pump at 1.38-1.47 μm or signal at 1.95-2.15 μm, a tuning range at 4.0-4.9 μm is achieved. Such hybrid optical waveguides are feasible for mid-infrared emission with mW powers and sub-nanometer linewidths. Besides, sidewall Bragg gratings in As2S3-Ti:LiNbO3 waveguides are fabricated by electron beam lithography and metal liftoff process. Spectrum measurements are in good agreement with numerical fittings. The measured rejection bandwidth is at 2.4-6.7 nm. Coupling coefficients ranging from 2.5 mm^-1 to 8.9 mm^-1 are obtained by altering the grating depth. A transmission peak with a 3-dB bandwidth of ~0.25 nm is observed on a 432 μm -long phase-shifted grating. Such integrated sidewall gratings are useful for various optical devices including optical filters, switches, modulators, lasers, sensors, and wavelength division multiplexing (WDM). In addition, optical refractive index sensors are designed with phase-shifted sidewall gratings in slot waveguide based on silicon-on-insulator (SOI) platform. The designed optical sensors have a minimum detection limit on the order of 10-6, a linear response and a compact device dimension as small as 11.7 μm offering the capabilities for optical sensor array deployment and lab-on-a-chip integration.
Wang, Xin (2014). Design and Fabrication of Integrated Optical Waveguides and Sidewall Bragg Gratings. Doctoral dissertation, Texas A & M University. Available electronically from