Fiber-optic Bragg grating sensors for hydrogen gas sensing

We report a new type of optical hydrogen sensor with a fiber optic Bragg grating (FBG) coated with palladium. The sensing mechanism in this device is based on the 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. Operation of two multiplexing sensors is demonstrated. Moreover, hydrogen and thermal sensitivities of the sensors were measured and calculated using a simple elastic model. Moreover, to quantify the amount of stress in the palladium film as a function of hydrogen concentration, a novel and very sensistive method was devised and used to detect deflections in a Pd-coated cantilever using an evanescent microwave probe. This stress uas in the range of 5.26-8.59x10(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.