Fiber Optic Hydrogen Sensor (FOHS)

The ability to detect hydrogen gas leaks economically and with inherent safety is an important technology that could facilitate commercial acceptance of hydrogen fuel in various applications. In particular, hydrogen fueled passenger vehicles will require leak detectors to signal the action of various safety devices. Such detectors will be required in various locations within a vehicle, wherever a leak could pose a safety hazard. It is therefore important that the detectors be very economical. For purposes of early detection a fast response time (<–1 second) is also desired. An optical fiber coated with a thin film of a chemochromic (color change induced by a chemical reaction) material offers the possibility of meeting these objectives.

Chemochromic materials such as tungsten oxide and certain lanthanide hydrides can react reversibly with hydrogen in air while showing significant changes in their optical properties. Thin films of these materials applied to the end of an optical fiber have been used as sensors to detect low concentrations of hydrogen in air. The coatings include a thin layer of gold in which a surface plasmon is generated, a thin film of the chemochromic material and a catalytic layer of palladiumthat facilitates the reaction with hydrogen. The gold thickness is chosen to produce a guided surface plasmon wave between the gold and the chemochromic material.

A dichroic beam splitter separates the reflected spectrum into a portion near the resonance and a portion away from the resonance and directs the portions to two separate photodiodes. The electronic ratio of these two signals cancels most of the fiber transmission noise and provides a stable hydrogen signal.

A fiber optic sensor based on the palladium catalyzed reaction of amorphous tungsten oxide and hydrogen was first proposed by Ito (1984). This simple sensor design was found to be adequate in terms of sensitivity but too slow in response time for the intended use. A different design using a surface plasmon resonance (SPR) configuration was therefore investigated. The SPR shifts in response to subtle changes in the refractive index of the coating. This shift can be monitored to give a faster response.