Femtosecond laser inscription of fiber Bragg gratings in lutetium oxide (Lu 2 O 3 ) single-crystal optical fiber for ultra-high-temperature sensing

Lutetium oxide (Lu 2 O 3 ) single-crystal fibers represent a promising platform for extreme-environment optics, offering an exceptionally high melting point (~2450 °C) and a cubic crystal structure that avoids polarization-dependent effects associated with birefringent crystalline fibers such as sapphire. In this work, we demonstrate the first-ever, to the best of our knowledge, inscription of a fiber Bragg grating (FBG) in Lu₂O₃ single-crystal fiber using a femtosecond (fs) laser line-by-line technique. The fabricated Lu 2 O 3 FBG, when spliced to a standard silica interrogation network with a 0.46 dB coupling loss, exhibits a stable spectral response and reversible operation up to 1663 °C. The measured thermal sensitivity of 14.4 pm/°C at 1663 °C is approximately 50% lower than that of a comparable sapphire-based FBG. This reduced sensitivity effectively decreases the thermally induced spectral excursion per sensor, thereby alleviating spectral overlap constraints and potentially increasing the achievable multiplexing density within a fixed interrogation bandwidth. These results establish Lu₂O₃ single-crystal fibers as a viable ultrahigh-temperature sensing platform for high-density distributed sensing applications in aerospace, energy, and other high-temperature industrial environments.