Miniature pH optical fiber sensor based on waist-enlarged bitaper and mode excitation

A polyvinyl alcohol/poly-acrylic acid (PVA/PAA) hydrogel coated optical fiber interferometer was been evaluated as a pH sensor. The interferometer, which comprises a waist-enlarged bitaper, a round tip and a tantalum pentoxide layer displays high refractive index (RI) sensitivity. With the hydrogel coated, the sensor's optical interference fringes shifted with exposure to varying pH solutions. High pH sensitivity of 1.58 nm/pH was achieved for pH 2.48-6.47. The miniature sensor demonstrates little hysteresis, high repeatability and low cross-sensitivity to temperature. A rising/falling time of 10/11 s for the sensor time response was acquired, respectively.