All Fibre based Hydrogen Sensing using Palladium coated Long Period Gratings

All optical hydrogen detection techniques are required to provide a safe and potentially compact monitoring system for use with existing and future hydrogen technologies. Reversible chemochromic changes induced in a palladium thin film, following hydrogen absorption, provide a mechanism for an optical detection of hydrogen.
Long period gratings [LPGs], written into standard communications type optical fibres exhibit absorption bands with the spectral positions being sensitive to changes in the refractive index contrast at the cladding – free space (or sensor layer) interface.
Optical parameters such as the refractive index of a thin, (~40nm) palladium coating deposited onto the outside of a fibre at the location of a LPG are shown to be sensitive to the hydrogen absorption in palladium.
We demonstrate a measurable wavelength shift in the position of the LPG lossband when exposed to low (150-10000ppm) concentrations of hydrogen. The dependency of the absorption shift with the optical properties of thin film palladium is explored. Leading to the characterisation of the optical properties of thin film palladium on exposition to hydrogen, through a combination of ellipsometry and surface plasmon resonance. The theoretical response of palladium coated LPGs are explored demonstrating maxim response to hydrogen for certain LPG-Pd combinations .
Initial data for the effect of long term drift and temperature-age interdependence is presented demonstrating match to expected results.