Evanescent wave optical-fiber sensing (temperature, relative humidity, and pH sensors)

Sensitive and versatile evanescent wave-sensing systems featuring polished optical fiber-based sensor designs with low-cost light sources have been developed for temperature, relative humidity, and pH measurements. The work herein contained describes the fabrication of three types of sensors based on standard silica, single-mode fibers previously subjected to a lateral polishing of the cladding. Temperature sensing through oils whose refractive index varied linearly with temperature showed applicability with up to 5 dB/°C for a 5° range. Polyvinyl alcohol films on the fibers showed almost 10-dB linear variation from 70% to 90% relative humidity. Sol-gel trapped dyes as thin films on the polished surface were capable of performing 15-dB output variation (although not linearly) for pH ranging from 2 to 11.