A new design of a fiber optic sensor using Palladium as a sensitive layer is presented. In this new configuration, a transducer layer is deposited on a multimode fiber (without the optical cladding). The transducer layer is a multilayer stack based on a silver, a silica and a Pd layer. The spectral modulation of the light transmitted by the fiber allows to detect hydrogen. The sensor is only sensitive to the Transverse Magnetic polarized light and the Transverse Electric polarized light can be used as a reference signal. The multilayer thickness defines the sensor performance. The Silica thickness tunes the resonant wavelength, whereas the silver and Pd thickness determines the sensor sensitivity. We present some results obtained for different multilayer Pd configurations.

For many hydrogen related applications it is preferred to use optical hydrogen sensors above electrical systems. Optical sensors reduce the risk of ignition by spark formation and are less sensitive to electrical interference. Currently palladium and palladium alloys are used for most hydrogen sensors since they are well known for their hydrogen dissociation and absorption properties at relatively low temperatures. The disadvantages of palladium in sensors are the low optical response upon hydrogen loading, the cross sensitivity for oxygen and carbon, the limited detection range and the formation of micro-cracks after some hydrogen absorption/desorption cycles. In contrast to Pd, we find that the use of magnesium or rear earth bases metal-hydrides in optical hydrogen sensors allow tuning of the detection levels over a broad pressure range, while maintaining a high optical response. We demonstrate a stable detection layer for detecting hydrogen below 10% of the lower explosion limit in an oxygen rich environment. This detection layer is deposited at the bare end of a glass fiber as a micro-mirror and is covered with a thin layer of palladium. The palladium layer promotes the hydrogen uptake at room temperature and acts as a hydrogen selective membrane. To protect the sensor for a long time in air a final layer of a hydrophobic fluorine based coating is applied. Such a sensor can be used for example as safety detector in automotive applications. We find that this type of fiber optic hydrogen sensor is also suitable for hydrogen detection in liquids. As example we demonstrate a sensor for detecting a broad range of concentrations in transformer oil. Such a sensor can signal a warning when sparks inside a high voltage power transformer decompose the transformer oil over a long period.