Design and Fabrication of Controllable Zinc-and-Nickel-Codiffused and Gallium-Diffused Lithium Niobate Optical Polarization Splitters

In this thesis, a controllable polarization splitter with Zn and Ni co-diffusion optical waveguide and Ga diffusion optical waveguide is successfully fabricated on Z-cut LiNbO3 substrate. The basic structure of the proposed polarization splitter is an asymmetrical Y-branch with the advantage of high fabrication tolerance. Both TE and TM modes are supported in the input waveguide fabricated by Zn and Ni co-diffusion. For electrooptic modulation, a pair of interdigitated electrodes is placed on both sides of the input waveguide. In the output section, only TE modes are supported in the straight waveguide fabricated by Zn and Ni co-diffusion, whereas only TM modes are supported in the bent waveguide fabricated by Ga diffusion.
As only one diffusion step is needed, instead of two or more diffusion steps used in previous works, the device fabrication process can be greatly simplified.
The measured extinction ratios are 24.2dB for the TE mode and 20.8dB for the TM mode, which are good enough for practical applications. In addition, using the property of electro-optic effect in LiNbO3 crystal and design of finger-type electrodes can achieve 37.5% mode conversion efficiency in the condition of input light with 1.55 μm wavelength and 90V drive voltage. The output power ratio between two polarization components can be electrically controlled.