Gap-controlled photonic coupling in femtosecond laser-written waveguide interconnections

Physical gaps between collinear waveguide segments often appear in femtosecond laser direct-written (DLW) photonic systems due to fabrication tolerances, alignment limitations, or modular assembly approaches. As photonic integration advances toward three-dimensional architectures, it becomes increasingly important to maintain efficient light coupling across such discontinuities. In this work, we present a systematic experimental and numerical study of light coupling efficiency across micron-scale gaps between DLW-fabricated collinear waveguides, combining simulations and experiments to assess how gap length, the quality of the waveguide end facets, and mode field influence transmission. These results establish practical design guidelines for minimizing insertion loss in interconnect structures, and we further demonstrate a compact photonic platform formed by assembling two independent glass chips separated by a fixed gap and observe distinct coupling behaviors by introducing different materials into the gap region. These results provide further insight into light transmission across discontinuous photonic structures.