Miniature pH Optical Fiber Sensor Based on Fabry–Perot Interferometer

A polyvinyl alcohol (PVA)/poly-acrylic acid (PAA) hydrogel coated fiber-optic intrinsic Fabry-Perot interferometer (IFPI) has been demonstrated as a pH sensor. The proposed Fabry-Perot pH interferometric sensor consists of a section of hollow-core photonic crystal fiber (HCPCF) sandwiched between the lead-in single mode fiber (SMF) and sensing SMF. The principle of the proposed sensor is based on the swelling effect of PVA/PAA hydrogel sensing film at the tip of sensing SMF, which induces optical path modulation when exposed to varying pH solutions. The pH sensor exhibits a relatively high sensitivity of 11 nm/pH ranging from the value of 4.1 to 6.9 with good repeatability and stability.