Inverse Taper Edge Coupler Based on LiNbO₃ Thin Film

The modal field size of lithium niobate on insulator (LNOI)-based waveguides and single-mode fibers has a huge difference, and the direct coupling efficiency is low due to the modal field mismatch. For the edge coupling between the LNOI waveguide and ultra-high numerical aperture (UHNA) optical fiber, an inverse-taper edge coupler was optimized based on finite difference time domain (FDTD) method. The simulation realizes the efficient coupling of lithium niobate rectangular waveguide and UHNA fiber. The designed inverse taper coupler was fabricated by electron beam lithography, and inductively coupled plasma etching. The test system of edge coupling was established to evaluate the performance of inverse taper coupler. The test results show that at the wavelength of 1 550 nm, the mode field diameters of the near-field output spot of the inverse taper coupler in the horizontal and vertical directions are 3.3 μm and 3.6 μm, respectively. The 3 dB alignment tolerance with the UHNA fiber in horizontal and vertical directions are 3.0 μm and 3.2 μm, respectively. The insertion loss of single end reaches 6.24 dB/facet, and this verifies that the designed inverse taper reduces the direct coupling loss between the LNOI waveguide and single-mode fiber effectively, and has the fine coupling performance. This study provides a high-coupling-efficiency optical interface scheme for the coupling between high-capacity, high-rate integrated photonic devices and optical fibers.