Lithium-Niobate Mach-Zehnder Interferometer With Enhanced Index Contrast by SiO2 Film

An electro-optic tunable lithium-niobate (LN) waveguide Mach-Zehnder interferometer (MZI) is designed and fabricated with the annealed proton-exchange process. The MZI consists of a straight waveguide arm and a bent waveguide arm with a SiO₂ film deposited on the two sides. The residual stress generated by the SiO₂ film can enhance the index contrast of the bent waveguide arm and thus reduce the bending loss of the MZI. A comparison of several experimental devices fabricated on the same chip size of 50 mm x 7 mm confirms significant reduction of the insertion loss by incorporating the SiO₂ film. For our best sample, which functions as a tunable comb filter with a channel spacing of 0.75 nm in the C-band, the insertion losses of the two output ports are 13.5 and 6 dB, respectively, and the corresponding maximum extinction ratios are ~23 and ~1.5 dB. The device has an electrical wavelength-tuning sensitivity of -.0.2 nm/V and a temperature sensitivity of ~0.1 nm/°C. Our design approach can significantly improve the quality of LN devices that require waveguide bends.