Realization of a low loss thin-film lithium niobate edge coupler based on staircase structure

Thin film lithium niobate (TFLN) has been proven to be a promising platform to realize high-performance integrated photonic devices. However, due to the large mode field mismatch between fibers and TFLN waveguides, low-loss edge couplers are necessary for practical TFLN devices. In this paper, we experimentally demonstrated a low-loss TFLN edge coupler based on our previously proposed staircase structure. The edge coupler has a minimum feature size of 3.0 μm and exhibits a high tolerance to lithographic overlay misalignment, ensuring its feasibility for fabrication using a contact aligner. Special etching mask patterns and etching processes were developed to efficiently fabricate the desired staircase structure. Test results show that a fiber-to-chip coupling loss of 1.5 dB/facet for TE polarized light is achieved at 1560 nm wavelength, and the polarization-dependent loss (PDL) is lower than 0.5 dB. To the best of our knowledge, it is the first low-loss TFLN edge coupler fabricated with a contact aligner, which paves the way for low-cost fabrication of practical TFLN devices.