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 $50 ~{\rm mm} \times 7$ 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 $\sim 23$ and $\sim 1.5$ dB. The device has an electrical wavelength-tuning sensitivity of $\sim 0.2$ nm/V and a temperature sensitivity of $\sim 0.1$ nm/°C. Our design approach can significantly improve the quality of LN devices that require waveguide bends.