Simulation of a temperature-compensated palladium-based fiber optic hydrogen sensor and comparison with measurements

A temperature-compensated sensor architecture for a fiber optic hydrogen sensor consisting of a partly palladium-coated pi-shifted fiber Bragg grating was modeled and compared with measurements. The transfer matrix formalism was used to calculate the spectral line shape of the pi-shifted FBG with a hydrogen-induced, non-homogeneous strain distribution along the grating axis. The temperature response of the grating itself can be compensated by referencing the notch to the flank wavelength. In addition, the hydrogen solubility in Pd shows a non-linear temperature dependence that was also included in the sensor performance calculations. For the investigated H2 concentration range of 200 ppm to 20000 ppm and between 15 °C and 40 °C, measurement data fit well to the simulation above 3000 ppm but become diffuse below, indicating deviations from the expected dependence according to Sieverts’ square root law.