A Dy:YAG coated fiber optic thermal sensor - proof of concept
The goal of the research was to develop a non-intrusive optical thermal sensor that could withstand the high temperature, reacting flow environment of operational turbines. A fiber optic thermal sensing design was chosen because of two main attributes, one was the remote/non-intrusive sensing and the other was the distributed sensing potential (the ability to measure temperature as a function along the fiber).
In the prototype sensor design a Dy: YAG layer was coated onto the core of a fused silica optical fiber. The Dy: YAG is a ceramic material which fluoresces in the visible when excited with ultra-violet radiation. The visible emission has a direct correlation with temperature up to the melting point of the ceramic (~2100 K). The basic mechanism used to excite the Dy: YAG is the evanescent wave coupling of the guided pump radiation out of the core and into the coating. This mechanism was utilized to produce bound rays within the fiber from the Dy: YAG emission signal which in turn are guided to a detector. The pump radiation was launched into the fiber core to remove optical path difficulties in turbines allowing a spatially distributed sensor capable of measuring the temperature as a function of distance along a single fiber.
The ceramic material (dysprosium doped yttrium aluminum garnet) was synthesized utilizing a precipitation reaction and x-ray diffraction patterns confirmed the existence of a YAG crystalline structure. An excimer pulsed laser deposition coating technique offered the best thin-film uniformity and adhesion properties (as in the Eu: Y₂O₃ thin film), but the existing facilities could not coat the Dy: YAG thin film due to temperature limitations of the substrate heater. Therefore, the fibers were coated with a thick film using a slurry bonding technique.
The main objective of the experiments conducted as part of this study was to evaluate the effectiveness of the evanescent pump/emission wave coupling and external pumping of the coated fibers. The external pumping of the coated fibers showed that the emission signal can be separated from the background noise with a rather fast, 20 picosecond per point sampling rate, oscilloscope. This was not the case for the majority of the measurements (the noise was greater than the signal) taken with an oscilloscope which could not resolve the pulse (5-6 data points resolution vs. ~5,000). Therefore, further investigation is needed to determine the feasibility of the Dy: YAG coated fiber optic thermal sensor.