An optical fiber Fabry-Perot interferometer (FPI) coated with polyvinyl alcohol/poly-acrylic acid (PVA/PAA) hydrogel is proposed for pH value measurement. The FPI is formed by splicing a section of hollow-core photonic crystal fiber (HCPCF) between two standard single mode fibers. Its optical path different is modulated by the coated PVA/PAA hydrogel sensing film through swelling effect which depends on pH value of the tested solution. High sensitivity up to 11 nm/pH is achieved within the pH value range between pH 4.1 and pH 6.9.
