Full-Range Ultrasensitive Fiber-Optic Hydrogen Detection via Twin Vernier Amplification and Lightweight CNN.

Reliable hydrogen monitoring requires sensors that combine high sensitivity, wide dynamic range, and resistance to environmental interference. Here, we report a monolithically integrated dual-Fabry-Pérot interferometer (dual-FPI) fiber-optic hydrogen sensor that couples twin Vernier spectral amplification with a lightweight dual-branch fusion convolutional neural network (DBF-CNN). The sensor incorporates a polydimethylsiloxane (PDMS)-filled temperature reference cavity and a PDMS/Pd-WO3 hydrogen-sensitive cavity on a single fiber, enabling intrinsic temperature compensation and humidity-insensitive detection without external reference devices. Digital twin Vernier amplification provides an approximately 10-fold sensitivity enhancement, giving a sensitivity of -62.989 nm/% and a limit of detection of 5.33 ppm. To resolve the wavelength ambiguity imposed by the free spectral range (FSR), we combine physics-guided spectral data augmentation with a lightweight DBF-CNN regression model to achieve end-to-end 0-100% volume fraction hydrogen concentration inversion, with an R2 of 0.999443 and a root mean square error (RMSE) of 0.682%. This integrated sensing strategy simultaneously addresses sensitivity, detection range, and environmental cross-sensitivity, offering a compact approach for fiber-optic hydrogen safety monitoring.