Extended‐Range Thermometry via Dual Er Fluorescence of Oxyfluoride Glass‐Ceramic Optical Fibers

Abstract Oxyfluoride glass‐ceramics (GCs) containing LaF ₃ or α‐NaLuF ₄ nanocrystals, co‐doped with 2 mol% Yb ³⁺ and 0.5 mol% Er ³⁺ , are considered as core materials for the implementation of optical fiber photoluminescence (PL) thermometers. A dual‐channel ratiometric thermometry approach combining green (UC, ² H _11/2 → ⁴ I _15/2 vs ⁴ S _3/2 → ⁴ I _15/2 ) and infrared (IR, ⁴ I _13/2 (m´) → ⁴ I _15/2 vs ⁴ I _13/2 (m) → ⁴ I _15/2 ) emissions allows the extension of the sensing temperature range above the UC luminescence quenching (at ≈ 650 K) by over 100 K (LaF ₃ ) or 150 K (α‐NaLuF ₄ ). The IR channel operates at the wavelength of minimum propagation losses of optical fibers, facilitating long‐distance propagation of luminescence signals. The UC channel in LaF ₃ ‐GC shows a maximum absolute sensitivity S _A = 102 × 10 ⁻⁴ K ⁻¹ at 602 K in glass and S _A = 71 × 10 ⁻⁴ K ⁻¹ at 591 K in GC with thermal resolution δ = 1.5–3 K. The α‐NaLuF ₄ ‐glass and ‐GC reach UC S _A = 90 × 10 ⁻⁴ K ⁻¹ at 698 K. The IR channel in both GCs, based on PL intensity ratios at λ = 1498 nm and λ = 1610 nm, exhibits S _A ≈ 30‐10 × 10 ⁻⁴ K ⁻¹ for the 300–800 K range with thermal resolution δ = 4.8‐6.4 K.