Fiber Bragg Grating Sensors for Measuring Temperature and Strain Simultaneously at Cryogenic Temperature

Feasibility of employing Fiber Bragg Gratings (FBG) for measuring thermodynamic parameters of superconducting (SC) coils has been studied. The distribution of mechanical stress and temperature inside the coil are important for an optimized magnet 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. The superimposed dual wavelength bi—metallic recoated Bragg gratings, fabricated in one fiber at same location are better suited for this purpose. Coil temperature and stress will vary the gratings periods which can be read out with a tuneable laser. The spectral position of the reflections may be correlated with the spatial position of the gratings. The shift of the gratings' maximum reflection indicates the change of the gratings' period. This, in turn, measures temperature and stress. The simultaneous temperature and strain measurement response of an aluminium—indium bi—metal coated FBG sensor is reported in this paper.