fiber Bragg grating is a periodic modulation of refractive index written into the core of an optical fiber, which reflects one narrow band of wavelengths and passes the rest — and which becomes a sensor because that reflected wavelength shifts as the fiber is stretched or warmed.
The physics is a one-dimensional Bragg condition. Light meets a few thousand index planes spaced by a period Λ; reflections from successive planes add in phase only for the wavelength satisfying λ = 2nΛ, where n is the core's effective index. Stretch the fiber and Λ grows; change its temperature and both Λ and n change; either way the reflected peak moves, and reading the peak reads the local strain and temperature. The grating is written by exposing a photosensitive, usually germanium-doped, core to an ultraviolet interference pattern, so the sensing element is the glass itself. That is the property that matters inside a magnet: the measurement carries no current, is immune to electromagnetic pickup, needs no electrical feedthrough into the 📝cryogenics, and is unbothered by a 20-📝tesla field. The corresponding weakness is that strain and temperature both move the same peak and must be separated by design, and that silica's thermo-optic response weakens sharply at cryogenic temperature, making a bare grating a duller thermometer at 20 kelvin than at room temperature.
The near-neighbor distinction is against the voltage tap, the classical instrument of 📝quench protection. A voltage tap integrates resistance along a whole section of winding and competes with inductive noise; a grating senses one centimeter of conductor directly and says where. Chaining many gratings along a single fiber extends that locality into 📝distributed fiber-optic sensing. Locality is what makes the approach attractive for 📝High-Temperature Superconducting (HTS) Magnets, where a normal zone spreads slowly enough that a 📝quench can smolder while producing almost no measurable voltage. Bartholomew M. Ludbrook and colleagues at the Paihau-Robinson Research Institute of Victoria University of Wellington, with Owen Duke and Erica E. Salazar of 📝Commonwealth Fusion Systems, reported how attenuation and a grating's position within an array affect that measurement in IEEE Transactions on Applied Superconductivity 33(5), 2023 (doi:10.1109/TASC.2023.3244762). A 📝fusion magnet adds a hazard a laboratory magnet does not: neutron and gamma exposure darkens glass, and the resulting attenuation degrades the signal over a machine's life. Duke, Aliya Greenberg, Joseph Desroches and Salazar of Commonwealth Fusion Systems, with Joe Schuyt and Dominic Moseley of the Robinson Research Institute, addressed it in "Reducing Radiation Effects on Fiber Optic Quench Detection Sensors With Optical Annealing," IEEE Transactions on Applied Superconductivity 34(5), 2024 (doi:10.1109/TASC.2023.3347369), recovering transmission by injecting light into the irradiated fiber.