Fiber Bragg Gratings for Sensing Temperature and Stress in Superconducting Coils

We study the feasibility of employing fiber Bragg gratings (FBG) for measuring thermodynamics parameters of super conducting coils. The distributions of mechanical stress and temperature inside the coil are important for an optimized design. Standard sensors with electrical connections like resistance thermometer and strain gauges cannot be placed inside the coil. So it is impossible to access local stress and temperature data. Bragg gratings fabricated at different location in one fiber is better suited for this purpose. Five-mm long gratings with different spatial period are arranged at various positions along a single mode fiber inside the coil. Coil temperature and stress will vary the gratings periods, which can be read out with a tunable laser in a wavelength division-multiplexing (WDM) scheme. The spectral position of the reflections may be correlated with the spatial position of the gratings, and the shift of the gratings' maximum reflection indicates the change of the gratings' period, which in turn measures temperature or stress. We specify the design and technology requirements to adapt the FBG sensor concept for low temperature near 4 K. Initial experiments, which demonstrate the properties of glass FBG at low temperatures, are reported.