Optical fiber evanescent hydrogen sensor based on palladium nanoparticles coated Bragg gratings

A new optical fiber evanescent wave hydrogen sensor coated with palladium nanoparticles, based on the intensity evaluation of the back reflected fiber Bragg grating signal, is proposed in this paper. The sensor is based on etched single-mode fibers in order to enable the evanescent field interaction of the guided light of the sensing part of the fiber. This interaction is enhanced by a Bragg grating structure. For specific detection of hydrogen, the surface of this sensory part is coated with palladium nanoparticles. Due to the presence of hydrogen in ambient atmosphere, the optical absorption coefficient of the palladium is decreasing, resulting in increasing intensity changes of the Bragg reflection signal. At room temperature the dynamic working range, starting from the limit of detection (0.3%) up to 5%, addresses almost all applications, where early alarm management is needed for security. For higher concentrations, the sensor's signal remains almost constant. For higher temperatures the maximum signal of the sensor decreases linearly with temperature, which restricts our sensor to ambient temperatures of less than 65 °C for 4% hydrogen in nitrogen. The small size of the palladium nanoparticles (8 nm on average), compared to bulk layers of palladium, enables our sensor to be applied in air without permanent degradation due to oxidation even after several months. Further, the sensor has a short response and recovery time of less than 90 and 140 s, respectively, and shows no hysteresis, which are very important properties for practical usage. The possibility to define individual reflection wavelengths of the etched FBG enables this type of sensor to be spectrally addressed in hydrogen sensor networks.