Optical waveguides fabricated by nickel indiffusion on lithium niobate are reviewed. In particular, the fabrication process, index change versus concentration, wavelength dispersion, process-dependent polarization, propagation loss, and electro-optic modulation of the nickel indiffusion waveguide are discussed. To improve the confinement of single-ordinary polarization waveguide fabricated by nickel indiffusion, a novel waveguide made by zinc and nickel indiffusion is presented for the first time. Though the measured propagation loss of the waveguide is larger, the measured output power contours are found more symmetric. Moreover, several TE-TM mode splitters using one or more nickel indiffusion waveguides for the complete mode sorting effect are discussed.
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