Fiber Bragg gratings are formed by periodic variations of the refractive index in the core of an optical fiber. These periodic variations allow a fiber Bragg grating to pass the majority of light propagating through a fiber while reflecting back a narrow band of the incident light with a particular peak wavelength called the Bragg wavelength. In response to induced disturbances, the Bragg wavelength changes, making the grating a versatile sensing device. However, because they are “written” in germano-silicate fibers, the gratings are expected to fail at high temperatures because of the devitrification (crystallization) of the host material and the dissipation of the gratings themselves.
To assess the stability of Bragg gratings at high temperatures, researchers at the NASA Glenn Research Center evaluated the performance of the devices at temperatures up to 1000 °C using commercially available polyimide-coated high-temperature gratings annealed by the manufacturer to 300 °C. The gratings were placed in a furnace, and the signals generated by them were sent to photodetectors and, from there, to a spectrum analyzer. After that, the signals were fed into a computer equipped with LabVIEW software. This software was used to control and monitor the equipment as well as to process the data.
The tests included thermal cycling from room temperature to 750 and 1000 °C as well as prolonged exposure of the gratings to 1000 °C. The thermal limits of the tests were restricted by the test configuration and could be extended to even higher temperatures.
Left: Test results from continuous operation of Bragg gratings at 1000 °C after 400 hr. Right: Formation of a stable response during the first 25 hr.
The left graph shows test results from continuous operation of Bragg gratings at 1000 °C for over 400 hr. The right graph describes events that happened during the first 25 hr of testing. It also shows the transition from degradation in the grating reflectivity to a formation of a stable response after 15 to 16 hr. The following graph demonstrates test results from continuous thermal cycling of Bragg gratings from room temperature to 1000 °C. It clearly shows the first run (cycle) during which the transient phenomenon and the formation of the stable response occur. After that, the consequent runs show very high repeatability. During the tests, the gratings were able to track the temperature measured by reference s-type thermocouples within 3-percent accuracy.
Test results from thermal cycling of Bragg gratings from room temperature to 1000 °C.
The tests confirmed the formation, at high temperatures, of secondary thermally stable gratings in the germania-doped glasses. These secondary gratings were formed in place of the primary ones originally written by the ultraviolet light. Our results were corroborated by similar data obtained independently and simultaneously by other research groups, including Sabeus Photonics, United States; Royal Institute of Technology, Sweden; and Communication Research Center, Canada.
Juergens, J., et al.: Thermal Evaluation of Fiber Bragg Gratings at Extreme Temperatures. AIAA-2005-1214, 2005.
Juergens, Jeffrey, et al.: Thermal Evaluation of Fiber Bragg Gratings at Extreme Temperatures. NASA/TM--2005-213560 (AIAA-2005-1214), 2005. http://gltrs.grc.nasa.gov/cgi-bin/GLTRS/browse.pl?2005/TM-2005-213560.html
Dr. Grigory Adamovsky, 216-433-3736, Grigory.Adamovskyemail@example.com
Author: Dr. Grigory Adamovsky
Headquarters program office: Aeronautics Research, VSP
Programs/Projects: LEAP, AEFT
Last updated: October 16, 2006
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