Dual-range pH sensor based on an etched no-core fiber with stimuli-responsive Cu/PVA hydrogel coating

This work presents a dual-range optical fiber pH sensor based on an etched no-core fiber (NCF) coated with a stimuli-responsive Cu-doped polyvinyl alcohol (PVA) hydrogel. The sensor operates via a multimode interference (MMI) utilizing a single-mode–No-core–single-mode (SNS) fiber configuration. This structure is fabricated from a 2 cm-long, 60 µm-diameter NCF, where reducing the diameter enhances the evanescent-field interaction with the surrounding medium. The incorporation of Cu nanoparticles into the PVA matrix improves the swelling dynamics and refractive index modulation, leading to enhanced sensing performance. The sensor exhibits a distinct dual-range behavior, showing a blue shift in acidic conditions (pH 1–7) and a red shift in alkaline conditions (pH 8–14). The maximum sensitivities of -5.35 nm/pH and 5.16 nm/pH were achieved in the acidic and alkaline ranges, with resolutions of 0.00187 and 0.00194 for acid and alkaline, respectively. The proposed sensor offers good repeatability, low hysteresis, and stable operation. The improved performance is attributed to the synergistic effect between Cu doping and hydrogel swelling, combined with enhanced evanescent-field interaction in the etched SNS structure. These results indicate that the proposed sensor is a promising candidate for wide-range and high-sensitivity pH monitoring applications.