Photonic integrated circuits on fiber tips for lab-on-fiber sensing

Photonic sensors integrated on the tip of optical fibers offer a compelling platform for compact and remote detection. However, most existing fiber-tip sensors rely on simplified microstructures that provide limited performance as compared to photonic integrated circuits (PICs). Bridging PIC-level functionality and performance with the form factor and deployment flexibility of optical fibers remains a key challenge, primarily due to constraints in fabrication complexity, footprint, and fiber-to-chip optical coupling. We present a fabrication strategy for enabling PICs on the tip of standard multicore fibers, using a transfer-based approach based on wafer-level fabrication. The platform integrates vertically-coupled backside grating couplers (VBGCs) specifically optimized for normal-incidence. These VBGCs maintain robust alignment tolerance, minimal feature sizes consistent with standard fabrication flows, and a compact layout suitable for the 120μm-diameter fiber-tip footprint. Demonstrated devices fabricated on an InP membrane exhibit coupling efficiencies (CE) up to −6.9 dB (20.1%) in the Sband, confirming the efficiency of the normal-incidence backside coupling scheme. PICs integrating ring resonators achieved experimental Q-factors up to 13,000, confirming the low-loss performance achievable within the fiber-tip geometry. This work establishes a practical and scalable route for transferring multifunctional PICs onto fiber facets, providing a versatile platform for future fiber-tip sensors.