Abstract
A spot-size converter is proposed based on lithium niobate on insulator (LNOI) waveguide. The spot-size converter is composed of the adiabatic taper and the low refractive index cladding. The evanescent wave coupling method is used to complete the conversion between fiber mode field and waveguide mode field, and the high-efficiency edge coupling between the fiber and LNOI waveguide can be realized. The parameters of the spot-size converter are designed and optimized, and the performance of the whole device is simulated. The total length of the spot-size converter is only 500 μm. An ultra-high numerical aperture (UHNA) fiber with mode field diameter (MFD) of 3.2 μm is applied to couple with LNOI waveguide. The coupling loss is 0.4 dB for TE mode and 0.47 dB for TM mode at the wavelength of 1550 nm. It can cover the whole C-band well. The proposed spot-size converter is expected to be used in high-density monolithic integrated optical system and gives a favorable reference for on-chip light source coupling.
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