Optical fiber Bragg grating hydrogen sensor based on evanescent-field interaction with palladium thin-film transducer

Fiber Bragg grating (FBG) sensors in single-mode optical fibers are widely applied for measurement of temperature and strain. If exposing FBG sensors to an external analyte by planar side-polishing technique of the fiber, evanescent-field interaction yields a Bragg wavelength shift also by changing the refractive index of the analyte. Deposition of sensor-specific transducer layers on the side-polished fiber can specify this spectrally encoding and network-capable optochemical fiber Bragg grating refractometry to the monitoring of specific substances, absorbed gases and vapors. In this paper, the sensor principle is demonstrated for the example of a hydrogen gas sensor based on a palladium thin-film transducer. Hydrogen in 0.1–4% volume concentration range can be monitored by the spectral shift of the Bragg wavelength, which is caused by the decreasing complex refractive index of Pd with increasing absorption of hydrogen.