New Optical Sensor Suite for Ultrahigh Temperature Fossil Fuel Applications

Development of practical, high-temperature optical claddings for improved waveguiding in sapphire fibers continued during the reporting period. A set of designed experiments using the Taguchi method was undertaken to efficiently determine the optimal set of processing variables to yield clad fibers with good optical and mechanical properties. Eighteen samples of sapphire fibers were prepared with spinel claddings, each with a unique set of variables. Statistical analyses of the results were then used to predict the set of factors that would result in a spinel cladding with the optimal geometrical, mechanical, and optical properties. To confirm the predictions of the Taguchi analysis, sapphire fibers were clad with the magnesium aluminate spinel coating using the predicted optimal set of factors. In general, the clad fibers demonstrated high quality, exceeding the best results obtained during the Phase I effort. Tests of the high-temperature stability of the clad fibers were also conducted. The results indicated that the clad fibers were stable at temperatures up to 1300 C for the duration of the three day test. At the higher temperatures, some changes in the geometry of the fibers were observed. The design, fabrication, and testing of a sapphire sensor for measurement of temperature was undertaken. The specific sensor configuration uses a polished sapphire wafer as the temperature-sensitive element. The wafer is attached to a sapphire fiber (clad or unclad), and interrogated as a Fabry-Perot sensor. Methods for assembling the sensor were investigated. A prototype sensor was fabricated and tested at room temperature and elevated temperatures. Results were difficult to interpret, due to the presence of modal noise which was found to result from the use of a spectrometer that was not designed for use with multimode fibers. A spectrometer optimized for use of multimode fiber has been obtained, and further evaluation of the sapphire temperature sensor is continuing