Dual-parameter pH and temperature optical fiber sensor based on core-offset structure with smart hydrogel coating

The core-offset structure exhibits a theoretical RI sensitivity of 17714.286 nm/RIU. • The maximum sensitivity is 42.713 nm/pH with a correlation coefficient of 0.998. • Temperature sensitivity is 1.970 nm/°C, with a cross-sensitivity of 0.046 pH/°C. • Dual-wavelength demodulation enables simultaneous pH and temperature measurement. A hydrogel-coated core-offset fiber pH sensor is proposed and demonstrated for sensing the acidic environments with a pH-range of 2 –6. The fiber structure consists of an 800 μm single mode fiber core-offset fused between two sections of 2 mm no-core fiber with an offset of 62.5 μm. The core-offset structure is demonstrated to have a sensitivity of 17714.286 nm/RIU through finite element simulation. The pH sensing is realized by encapsulating a smart hydrogel at the core-offset structure, whose refractive index varies with pH. Within the pH range of 2–6, a cubic polynomial fitting yields a correlation coefficient of 0.999. The sensor shows a linear operating region from pH 3 to 5 with an ultra-high sensitivity of 42.713 nm/pH and a correlation coefficient of 0.998. Additionally, temperature response experiments reveal a sensitivity of up to 1.970 nm/°C, with a corresponding temperature cross-sensitivity of approximately 0.046 pH/°C. By employing a dual-wavelength demodulation method, simultaneous pH and temperature measurement is achieved, effectively eliminating temperature cross-sensitivity. Meanwhile, the experimental for consistence, stability, response time and repeatability are all carried out. It is indicated that the proposed hydrogel-coated core-offset sensor has excellent performance and is expected to have potential applications from environmental to industrial to biomedical.