Contributors: Simon Pevec, Stephan Krenek, René Eisermann 

During the three-month period from January to March 2025, Dr. Simon Pevec, Assistant Professor at the University of Maribor, worked as a visiting scientist in PTB's Photonic Thermometry Working Group. The synergy of his expertise and PTB's world-renowned photonic thermometry capabilities is driving the research on fibre-optic temperature sensors based on Fabry-Pérot interferometers (FPIs) and advancing the INFOTherm project. These sensors are a promising alternative to the more commonly used Fibre Bragg Grating (FBG)-based temperature sensors. 

Together we continued our research on fibre optic temperature sensors based on FPIs. Our research focused on sensor performance at temperatures up to 700 °C, with the aim of achieving long-term stability through new sensor designs and various ageing and annealing techniques. We conducted high-temperature tests using innovative pure SiO2 FPI sensors made from coreless, dopant-free fibres that prevent drift due to diffusion-related effects at elevated temperatures. 

In addition, we investigated the pressure-temperature effects on the performance of both FPI and FBG temperature sensors. With the support of PTB, we were able to carry out tests under high metrological standards in terms of pressure and temperature conditions, as both were monitored with high accuracy. The tests were carried out in nitrogen gas, at temperatures ranging from 0 to 600 °C and pressures from vacuum to 80 bar. Both at room temperature and at elevated temperatures, the tests demonstrated the effect of pressure on the temperature reading.      
 
The experimental results were in close agreement with the theoretical model for pressure tests performed at room temperature and 0 °C, but significant deviations were observed at elevated temperatures. To better understand this, the data for the temperature dependence of Young's modulus, Poisson's ratio and photoelasticity of amorphous SiO2 need to be further investigated. 

The partial results of this work and the wider efforts of the INFOTherm project have already been applied in the MQB-Pascal project, where Dr Pevec and the University of Maribor continue to collaborate with PTB. The understanding of the pressure/temperature dependency of Fabry-Perot-based fibre temperature sensors has led to design modifications that prevent pressure/temperature crosstalk, thereby improving sensor performance for practical applications. 

Dr. Pevec's work as a visiting scientist exemplifies the value of international collaboration, as the combined expertise of Dr. Pevec and PTB has led to significant advancements in the field of photonic thermometry.