Response of Distributed Fiber Optic Temperature Sensors to High-Temperature Step Transients

Experiments were performed to test fiber optic temperature sensors under thermal conditions similar to those that would be experienced in the transient reactor test facility (TREAT) at the Idaho National Laboratory. Specifically, four single-mode optical fiber sensors were dropped into a pre-heated furnace to simulate a step temperature transient, for steps from room temperature to indicated furnace temperatures, ranging from 700 °C to 1100 °C. The four fibers were a standard SMF-28 fiber, a fiber with inscribed type-I fiber Bragg gratings (FBGs), and two fibers with inscribed type-II FBGs. The two type-II FBG fibers were different in the initial backscatter signal of the gratings; one fiber had gratings with greater backscatter reflection than the other. The type-I FBG sensor and standard SMF-28 sensor measured temperature for temperature transients with a final temperature of 700 °C. The low reflection type-II FBG sensor measured temperature transients with final temperatures up to 950 °C. The high-reflection type-II FBG sensor successfully measured temperature for short times with final temperatures up to 1050 °C. For long times at 1050 °C, there was a drift in the temperature sensing. We conclude that the high-reflection type-II FBG sensor would perform acceptably for application in the thermal environment of TREAT, or for other step temperature transients from room temperature to final temperatures up to 1000 °C.