Laser-Heated Pedestal Growth and Optical Attenuation Mechanisms of Magnesium Aluminate Spinel Single-Crystal Fibers for Radiation-Hardened Fiber Bragg Grating Sensing up to 1600 °C

In situ sensing in extreme environments requires waveguide media that can withstand ultrahigh temperatures, intense radiation, and large mechanical strains. Here, [100]-, [110]-, and [111]-oriented magnesium aluminate spinel (MgAl2O4) single-crystal fibers (SCFs) were fabricated by the laser-heated pedestal growth (LHPG) method for the first time, with diameters down to 50 μm and lengths exceeding 1 m. High-temperature exposure produces extensive surface deposition together with aligned linear features and triangular faceted surface features. Sapphire-tube protection markedly suppresses surface deposition and reduces both high-temperature and irreversible attenuation, identifying external deposits as a major source of optical loss. The triangular features remain compositionally close to the MgAl2O4 matrix, whereas Ar treatment produces isolated Mg-depleted, Al-rich faceted crystallites. Furthermore, MgAl2O4 SCF Bragg gratings (MFBGs) were fabricated by femtosecond-laser inscription and evaluated for high-temperature sensing. The [111]-oriented MFBG exhibits a temperature sensitivity of 39.54 pm/°C at 1600 °C and no observable Bragg wavelength drift after γ-ray irradiation to 21.6 kGy. The MFBGs also exhibit a stable strain response, with a sensitivity of 1.63 pm/με at 1600 °C under strains up to 1440 με. These results demonstrate the potential of MgAl2O4 SCFs and the corresponding MFBGs for multi-parameter sensing in extreme environments.