A Highly Sensitive pH Sensor Based on Tapered Single-Mode Optical Fiber by Calcium Alginate Hydrogel Operating at Dispersion Turning Point

A highly sensitive pH sensor based on a tapered single-mode optical fiber (SMF) working at the dispersion turning point (DTP) is proposed, with calcium alginate hydrogel coated on its surface. The tapered SMF is prepared using a two-step tapering method, and the pH sensor is fabricated by coating the surface of the tapered SMF with calcium alginate hydrogel. Experimental results demonstrate that within the pH range of 7.03–8.45, the sensor exhibits the highest sensitivity at the DTP, with a sensitivity of 115.6 nm/pH. Furthermore, this sensor boasts benefits, including excellent repeatability and rapid response time. Compared with other sensors, the proposed pH sensor demonstrates superior pH sensitivity, simple structure, and low cost, making it highly promising for applications in seawater monitoring and biomedical fields.