Fibre-optic helical beam temperature sensor for coal spontaneous combustion monitoring

Abstract This paper proposes a fibre-optic Fabry–Perot (F–P) temperature sensor based on the thermo-mechanical deflection effect of a bimaterial helical beam. The sensor exploits the difference in thermal expansion coefficients between polyimide and aluminium layers to induce deflection at the free end of the cantilever, which is converted into cavity length variation. According to the F–P interference principle and demodulated using fast Fourier transform (FFT)-based spectral algorithm for temperature demodulation. Thermo-mechanical coupled finite element simulations under a uniform temperature range of 0 °C–150 °C shows a temperature sensitivity of 0.394 μm °C −1 , with the helical beam structure exhibiting superior stability compared with the maze-shaped beam and the maze-shaped perforated beam structure. An F–P displacement measurement platform was constructed, and calibration as well as stability tests were carried out within 26 °C–146 °C. The measured sensitivity reached 0.411 μm °C −1 , in close agreement with the simulated value (0.395 μm °C −1 ). Temperature cycling experiments further demonstrated excellent repeatability of the sensor, with temperature fluctuations confined within ±0.35 °C (≈±0.29% F.S.). Moreover, the FFT demodulation method achieved a temperature resolution better than ±0.005 °C. In summary, the helical beam temperature sensor method features inherent safety, passive operation, immunity to electromagnetic interference, high sensitivity, and rapid response. It can be easily networked through optical fibres and is well suited for high-precision monitoring of weak thermal signals in complex environments.