A non-uniform segmented grating coupler with a metal reflective layer is designed by finite difference time domain simulation. The grating coupler is on a 400 nm thick z-cut lithium niobate thin film waveguide. A metal reflective layer is added to reduce the leakage to the substrate, and a non-uniform grating structure is constructed to improve the mode mismatch. The non-uniform grating coupler consists of three segmented grating regions, with a gap between two adjacent grating regions. The grating periods of these three grating regions are not completely the same, namely Λ1, Λ2, and Λ3, Λ1 is shorter than Λ2 and Λ3. According to grating coupling principle, the key structural parameters of the grating coupler such as the gap between the grating regions, the grating period, the duty cycle are discussed with respect to the grating coupling efficiency. Through FDTD simulation calculation, the coupling efficiency under theoretical simulation (TM mode, at 1550 nm) can reach nearly 76%. The proposed non-uniform segmented grating coupler is expected to be fabricated and applied to the input-output coupling of waveguide devices.
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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