Abstract
A new all-optical laser-intensity-amplitude-modulated hydrogen sensor has been developed as a next-generation device to the earlier photopyroelectric hydrogen sensor. When modulated light is incident on a palladium thin film coated on a polymeric membrane, optical reflectance and transmittance signals are generated in photodiode detectors. When hydrogen gas comes into contact with the thin palladium film the gas is absorbed, altering the optical properties of the palladium and producing a signal output dependent on hydrogen concentration in the ambient. The detection range of this sensor is between 0.2 and 100% [H 2] by volume and is intended for sensitive monitoring of the explosive range, 4% by volume H 2 in air. This sensor is very durable and robust; no palladium delamination or blistering occurs even after repeated exposures to pure hydrogen. The signal dependencies on increasing hydrogen concentration were found to be consistent with increased occupation of empty electronic states by electrons associated with absorbed hydrogen atoms, leading to upward shifts of the Pd Fermi level, E F, and to decreasing optical transition probabilities.
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