Graded-Index Lens Based Edge Coupler With Low-Loss, Broad Bandwidth for Efficient Coupling Between Silicon Waveguide and Standard Single-Mode Fiber

Efficient fiber-to-chip coupling is crucial for achieving high-performance silicon photonic chips. While lensed fiber to chip coupling has been extensively researched and proven excellent, it does have limitations such as strict alignment tolerance and high cost. Therefore, the recommended approach is the standard single-mode fiber (SMF)-to-chip coupling. One potential solution for SMF-to-chip coupling involves an edge coupler based on a graded-index (GRIN) lens; however, this method often requires a GRIN lens with numerous layers, making fabrication extremely challenging. In this study, we propose and experimentally validate an edge coupler based on a GRIN lens assisted by a complementary taper structure. This new type of composite structure demonstrates low coupling loss, broad bandwidth, and polarization insensitivity, enabling efficient SMF-to-chip coupling. Our design simplifies the fabrication process by significantly reducing the number of layers in the GRIN lens while achieving lower coupling losses compared to conventional GRIN lens-based edge couplers. Experimental results demonstrate that at 1550 nm, the measured TE and TM mode coupling losses are 1.21 dB and 1.78 dB respectively, while maintaining the 1 dB bandwidth exceeding 90 nm.