Optical Thermometry Using Absorptions in Thulium-doped YAG

A general approach for modeling optical absorptions in rare-earth-doped crystals is applied to transitions in Tm³⁺:YAG and demonstrates their use as an optical thermometer at cryogenic temperatures. The modeling of the optical absorption, using two temperature-dependent processes – Boltzmann population shifts and homogeneous line broadening – is sufficient to generate predictions for absorption spectra that are in close agreement with observations. The model allows the temperature dependence of Tm³⁺ absorptions to be calibrated and used as an optical thermometer. Absorbed light is analyzed for three spectral ranges: 650-725 nm which includes absorption into the ³F₃ manifold, 725-850 nm which includes absorption into the ³H₄ manifold, and 1100-1300 nm which includes absorption into the ³H₅ manifold at temperatures between 10 K and 300 K. The modeling of the temperature dependence of multiple absorption bands allows for a single crystal to be used as a multi-range temperature sensor.