Dual-Sensitive Fiber Optic Humidity and Temperature Sensor Based on a Hydrogel Diaphragm in a Concave Etched Structure

This study presents a fiber-optic Fabry-Perot humidity sensor based on a concave-etched graded-index (GRIN) fiber and a thin composite hydrogel diaphragm. The concave cavity is made using chemical acid etching and a thin layer of chitosan-tetraethyl orthosilicate (Chi:TEOS) hydrogel. The hydrogel is placed inside the cavity using capillary forces to form a Fabry-Perot interferometer. The swelling behavior of the hydrogel in response to humidity variations leads to changes in the cavity length, resulting in a wavelength shift of the interference spectrum. Additionally, the hydrogel film is covalently bonded to the fiber surface through air plasma treatment, ensuring structural stability and reliable integration. The sensor demonstrated a sensitivity of 1.4 pm/%RH in the range from 20% to 62% RH. Additionally, the interference spectrum showed responses to temperature changes, with a measured temperature sensitivity of approximately 38.2 pm/°C. Fourier domain analysis also demonstrated a dual-parameter sensing capability, allowing for the simultaneous estimation of temperature and humidity from spectral features. It exhibited a robust architecture suitable for harsh environments due to the protective nature of the embedded fiber structure. This design introduces a mechanically resilient and optically responsive platform for compact humidity sensing applications.