Characterizations and Applications of Zinc and Nickel Co-Diffused Optical Waveguides on Lithium Niobate

In this dissertation, optical waveguides and devices made by Zn and Ni co-diffusion on Z-cut lithium niobate (ZNI:LiNbO3) are systematically investigated from their fabrication characteristics, (co-)diffusion behaviors, refractive index models, to unique applications for the first time.
In the waveguide fabrication process, the required diffusion temperature is no higher than 950 Celsius degree, and thus free of lithium out-diffusion problem. The adhesion of deposited Zn layer to substrate is significantly improved by the presence of Ni. A two-step thermal cycle is programmed in the diffusion process to enhance oxidation of Zn and yield stable waveguide conditions. To study the diffusion behaviors, secondary ion mass spectrometer (SIMS) and electron probe x-ray microanalyzer (EPMA), along with a FDTD-based diffusion simulator, are used to build and verify quantitative diffusion models for Ni, Zn, and ZNI. The diffusion parameters are successfully retrieved, and major effects of co-diffusion are also discussed.
With the quantitative diffusion models, prism-coupler measurement and IWKB methods are combined to construct refractive index models for related diffusants. Experimental results show that index change induced by unit concentration of Ni is higher than that of Zn. Different polarizations exhibit different trends in refractive index models so that the fabrication-dependent polarizations can be achieved. The index profiles of ZNI:LiNbO3 waveguides can be derived by co-diffusion model and refractive index models of individual diffusants. It is also found that the optical confinement of ZNI waveguides is better than Ni-indiffused ones because the diffusivity of Zn is lower than that of Ni.
For practical applications, ZNI:LiNbO3 waveguides supporting single polarization are used as waveguide polarizers with polarization extinction ratios of 24dB and 29dB for extraordinary and ordinary waves, respectively, without any special design of buffer layer or waveguide structure. In addition, tunable polarization splitters, in which the output power ratio between two polarization components can be electrically tuned, are proposed and demonstrated with ZNI:LiNbO3 waveguides, leveraging both the fabrication-dependent polarization property and EO effect. Experiment results show polarization splitting ratio of 18dB and 23dB for extraordinary and ordinary waves, respectively, with mode conversion efficiency up to 60%. Advantages of applying ZNI:LiNbO3 waveguides to integrated optical applications are also addressed in this dissertation.