Fiber Optic Sensing for Very High Temperatures

Fiber optic sensing continues to grow as a valuable temperature measurement tool, thanks to the fiber’s small diameter, small mass and robust operation in harsh environments.  The nuclear industry, for example, has shown interest in fiber optic sensors for very high temperature applications, such as monitoring fuel performance during irradiation or even embedding fiber in commercial reactor components.

One option to further extend the temperature range of fiber optic sensors is single crystal sapphire optical fiber (α-Al₂O₃), due to its very high melting temperature (in excess of 2000 °C). However, there are certain problems that need to be overcome for most sensing techniques to work. For example, with Optical Frequency Domain Reflectometry (OFDR), utilized by Luna’s OBR 4600, the multimode nature of sapphire optical fiber distorts the measurement by introducing additional modes in the fiber. Another issue is that the sapphire single crystal fiber has almost perfect crystal structure, therefore, the Rayleigh backscatter signal level is too low to perform the OFDR-based sensing.

However, researchers from Ohio State University, working with Luna’s OBR 4600, are developing a solution that could lead to sapphire fiber sensors for high temperature sensing.  The researchers discovered that sapphire fiber can be cladded through the reaction of ⁶₃Li(n,α)₁³H in a reactor.  High energy alpha particles and tritons slightly modify the index of refraction of sapphire in its periphery, making it single-mode in nature. Moreover, the fast neutrons interaction with the fiber core induce displacement damage throughout the fiber. The defects formed from these interactions create a Rayleigh backscatter signal of a high enough level to be detected by the OBR 4600, which has the noise floor at -130 dB.  The scatter sites anneal out at the temperatures exceeding 300 °C, however, there is a potential of measurements up to 1500 °C with type-II Bragg gratings written in a sapphire fiber.

More details on OSU’s research can be found published in the IEEE Sensors Journal.