Pd-coated elastooptic fiber optic Bragg grating sensors for multiplexed hydrogen sensing

We report a new type of optical hydrogen sensor with a fiber optic Bragg grating (FBG) coated with palladium thin film. The sensing mechanism in this device is based on mechanical stress that is induced in the palladium coating when it absorbs hydrogen. The stress in the palladium coating stretches and shifts the Bragg wavelength of the FBG. Using FBGs with different wavelengths many such hydrogen sensors can be multiplexed on a single optical fiber. Here multiplexing two sensors is demonstrated. Moreover, hydrogen and thermal sensitivities of the sensors were calculated using a simple elastic model. Additionally, to quantify the amount of stress in the palladium film as a function of hydrogen concentration, a novel and very sensitive method was devised and used to detect deflections in a Pd-coated cantilever using an evanescent microwave probe, This stress was in the range of 5.26-8.59 X 10(-7) Pa for H-2 concentrations of 0.5-1.4% at room temperature, which is about three times larger than that found in the bulk palladium for the same range of H-2 concentrations. (C) 1999 Elsevier Science S.A. All rights reserved.