Synergistic mode-field pre-expansion and geometric compression in hetero-structured microfibers for ultrasensitive glucose sensing.

The inherent conflict between tight optical confinement for transmission and evanescent field delocalization for sensing restricts the detection limit of traditional fiber-optic biosensors, particularly for small biomolecules. Herein, we introduce a synergistic dual-stage evanescent-field enhancement mechanism realized using a tapered spliced long-period fiber grating (TS-LPFG) for ultrasensitive glucose monitoring. The architecture utilizes hetero-structured fiber as a mode-field pre-expander and adiabatic tapering as a geometric compressor to force optical energy toward the interface. This synergistic design achieves a theoretical penetration depth of 560 nm, ensuring spatial overlap with the glucose oxidase (GOD) layer. Experimental results demonstrate an ultra-high sensitivity of 5.06 nm/(mg/mL) within a 0-3.0 mg/mL range, with an exceptional response time of 0.25 s. Meanwhile, the sensor exhibits low thermal crosstalk of 0.029 mg/mL/°C and high specificity against common interferents. This work provides a theoretical model for spatially matched biosensing, offering a robust platform for high-fidelity detection of trace analytes.