Optical properties of thin VO2-films at the phase transition: Design aspects for a fiber-optic thermometer

The pronounced variation of the optical reflectance R of nanometer thin VO₂ films with temperature T at the phase transition recently has triggered interest in the development of compact thermo-optical sensors that utilize optical fiber technology. Its immunity against electromagnetic interference effects makes this approach suited for an optical detection of local areas with increased temperature, i.e. so called hot spots, inside of high power transformers. Optical reflectance of thin VO₂-films as function of T throughout the hysteretic phase transition is reported in this work. A modified version of the Preisach hysteresis model is utilized, and combined with experimental optical data. Suited VO₂ film thickness values and wavelengths range for optimum sensor operation can be extracted. The achievable temperature range, as confirmed by experimental observations, is consistent with requirements that apply for power transformers of the first group, according to NBR 5416 (ABNT).