A novel fiber optic sensor has been developed to be used in superconducting magnets for fusion reactors and other large
cable-in-conduit superconductor (CICC) magnet applications. These large superconducting magnets need a diagnostic
that can measure the temperature and strain throughout the magnet in real-time, which was not possible until now.
Simultaneous temperature and strain measurements at cryogenic temperatures have been demonstrated, using
spontaneous Brillouin scattering in an optical fiber. Using an extremely narrow (100 Hz) linewidth Brillouin laser with
very low noise as a frequency shifted local oscillator, the frequency shift of spontaneous Brillouin scattered light was
measured using heterodyne detection. A pulsed laser was used to probe the fiber using Optical Time Domain
Reflectometry (OTDR) to determine spatial resolution. The spontaneous Brillouin frequency shift and linewidth as a
function of temperature agree with previous literature on stimulated Brillouin scattering data from room temperature
down to 4 K. For the first time, the spontaneous Brillouin frequency shift, linewidth, and intensity as a function of strain
have been measured down to 4 K. Analyzing the frequency spectrum of the scattered light after an FFT gives the
Brillouin frequency shift, linewidth, and intensity of the scattered light. 65,000 pulses, with 53 ns pulse widths, were
averaged in under one second, providing a 5 meter spatial resolution along a fiber that was about 100 m long. Measuring
these three parameters allow the simultaneous determination of temperature and strain in real-time throughout a fiber
with a spatial resolution on the order of several meters.