ACTIVE HYDROGEN SENSING USING A PALLADIUM COATED OPTICAL FIBER BRAGG GRATING SENSOR

The use of liquid hydrogen as a fuel in both ground and in space requires low-cost multi-point sensing of hydrogen gas for leak detection well below the 4% explosion limit of hydrogen.In this thesis, we demonstrate an agile multi-functional active fiber optical hydrogen sensor for all temperature operations. Fiber-optical hydrogen sensors offer a number of advantages over other hydrogen sensor including explosive proof, low-cost, multiplexing, and capability of working in hostile environments. They are very suitable for mission-critical applications such as for hydrogen sensing and leak detection in aerospace vehicles. In this thesis, we demonstrate a multi-point, one-feed through, in-fiber hydrogen sensor capable of hydrogen detection below 0.5% concentration with a response time of less than 10 seconds. Our solution entails use of a fiber Bragg grating (FBG) coated with a layer of hydrogen-absorbing palladium which, in turn, induces strain in the FBG in the presence of hydrogen. The hydrogen-induced stress was detected by the shift of FBG wavelength. The responsivity of fiber optic hydrogen sensor was calibrated for hydrogen concentration from 0.1% to 10% and temperature range from -120oC to 120oC. The optimal sensitivity and response time of the sensor was found to be highly sensitivity to the temperature. The response due to the presence of hydrogen is imperceptible until the temperature reaches about -20oC. To develop a fiber-based hydrogen sensor for operation at all temperature, infrared power laser light carried by the same fiber containing FBG was used to induce localized heating in the palladium coating which dramatically decreases sensor response time and increases the sensor's sensitivity at low temperatures. At -50oC localized heating yields 57% of sensitivity of that at room temperature. This technology promises an inexpensive fiber solution for a multi-point hydrogen detection array with only one fiber feed-through operation for all-temperature hydrogen sensing