This study reports on the temperature dependent behavior of absorption bands generated in optical fibers via hydrogen exposure at 800 °C. Hydrogen exposure at 800 °C resulted in the generation of two large absorption bands in the 1-2.5 μm wavelength range at ~1.4 μm and ~2.2 μm. These bands showed temperature dependent behavior when in the temperature range of 20–800 °C such that at higher temperatures the absorption intensity in these two bands was smaller than at room temperature. The temperature dependent behavior was shown to be reversible and repeatable under an array of testing conditions including thermal cycling and long periods of time without hydrogen exposure. The reversibility suggests that no chemical change is taking place while the repeatability suggests that no permanent structural change in the glass is taking place. Although both absorption bands are associated with hydroxyl groups and exhibited similar temperature dependence, variations were observed with respect to time and exposure environment. Therefore, we surmised that the observed behaviors were not exclusive to the hydroxyl bond and/or structural modifications. In this paper, we discuss the possible mechanisms responsible for the observed phenomena and, conversely, the conditions that would be necessary to induce the structural changes that would induce changes in the absorption intensities.
A temperature sensor link based on wavelength-multiplexed fiber Bragg grating (FBG) was designed and fabricated for distributed temperature measurement in a jet engine nozzle under field conditions. Eight FBGs with different Bragg wavelengths ranging from 1520 nm to 1560 nm were fabricated along one single-mode fiber which was packaged inside a stainless steel tube. The reflected signal from the sensor link was simultaneously collected by an optical sensing interrogator and converted into temperature information. The steel tube was embedded in a steel flange assembly attached to a jet engine. Three engine cycles were performed from 55% (idle) to 80% of the engine’s full power to test the sensor response under high temperature, vibration and strong exhaust flow conditions. Test results show good survivability of the sensor, and the temperature around the nozzle was measured up to 290 °C. The system has a temperature measurement range from 20 °C to 600 ° and the response time is less than 1 second.
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