Silicon self-focusing fiber–chip grating couplers with millimeter-long working distance

Optical packaging remains a major bottleneck for large-scale silicon photonics, as conventional input/output coupling interfaces demand precise fiber-chip alignment and minimal working distances. Self-imaging focusing grating couplers can relax these constraints by enabling coupling over hundreds of micrometers, but current implementations typically rely on external lenses, multi-step lithography, or low-index-contrast platforms. In this work, we experimentally demonstrate lens-free long-distance focusing surface grating couplers on a commercial silicon-on-insulator (SOI) platform using a single full-depth process compatible with deep-UV lithography. The design exploits double-period subwavelength gratings to achieve weak radiation strengths while independently controlling the emission angle, allowing the required apodization profiles for efficient large-aperture focusing. Fiber-chip coupling is achieved at working distances up to 1 mm, with efficiencies of -1.85 dB (simulated) and -3.7 dB (measured) in the absence of bottom reflectors. These results constitute the longest working distance yet reported for SOI grating couplers and pave the way for robust, non-contact optical packaging suitable for standard CMOS processes.