Reflective mercury ion and temperature sensor based on a functionalized no-core fiber combined with a fiber Bragg grating

A reflective fiber optic sensor is proposed and experimentally validated, which enables simultaneous measurement of temperature and mercury ion (Hg2+) concentration. The sensor consists of a fiber Bragg grating (FBG) and a functionalized no-core fiber (NCF). The chitosan (CS)/poly acrylic acid (PAA), as Hg2+ sensitive film, is assembled on the surface of the NCF by layer-by-layer self-assembly method. Besides, the silver film is coated on the end of NCF to ensure the reflection of light. This reflective fiber structure forms a fiber modal interferometer. When the sensor is immersed in the test solution, the Hg2+ adsorption of sensitive film leads to change in its refractive index, and then the interference spectrum of the fiber modal interferometer will shift accordingly, which is the measurement principle of Hg2+ sensor. Meanwhile, when the surrounding temperature changes, the interference fringe of the fiber modal interferometer and the Bragg wavelength of FBG will shift simultaneously. Therefore, the Hg2+ concentration and temperature can be simultaneously measured by using dual-wavelength matrix method, and then the cross sensitivity problem of temperature and Hg2+ concentration can also be solved. The experimental results show that the detection sensitivity of Hg2+ concentration is 0.0823 nm/μM in the range of 0 μM-100 μM and 0.0178 nm/μM in the range of 100 μM-500 μM, respectively. The temperature sensitivity is 0.0147 nm/°C in the range of 20 °C-50 °C.