High-sensitivity mid-infrared temperature sensors based on femtosecond laser-inscribed indium fluoride FBGs at 4.6 μm

We demonstrate high-sensitivity mid-infrared (MIR) temperature sensing using femtosecond-laser-inscribed fiber Bragg gratings (FBGs) in single-mode indium fluoride (InF₃) optical fibers. Linear and repeatable thermal responses are obtained at long MIR wavelengths, with a sensitivity of 51.96 pm/°C at 4.61 µm, enabled by operation near the long-wavelength transmission edge of the material. High-reflectivity Bragg resonances exceeding 97% at 3.45 µm and 98% at 4.61 µm are achieved using plane-by-plane inscription through the polymer coating, preserving fiber integrity. First-order FBG operation beyond 4.5 µm is demonstrated for the first time, to our knowledge, in InF₃ fiber, while higher-order harmonics in the near-infrared enable indirect interrogation using standard telecom-band instrumentation. These results establish InF₃ FBGs as an effective platform for integrated MIR temperature sensing and fiber-based photonic systems.