Sapphire Fiber Bragg Gratings Demodulated With Cross Correlation Algorithm for Long-Term High-Temperature Measurement

The sapphire fiber Bragg grating (SFBG) is a promising high-temperature sensor for application in aviation and power industries; however, the multimode characteristics of SFBG result in a broadband reflection envelope, including multiple peaks, which have a strong impact on the sensing accuracy. In this article, we propose and experimentally demonstrate a cross correlation algorithm (CCA) for the demodulation of SFBG, with the benefit of enhancing the stability of wavelength detection, and hence, the sensing accuracy can be improved. The SFBG high-temperature sensor was created by using the femtosecond laser line-by-line method and sealed in an argon gas-infiltrated sapphire tube. Such a device was demodulated by using the CCA, and the findings show that the Bragg wavelength dispersion of ±12 pm could be obtained; furthermore, before the calibration process, the SFBG sensor was annealed at 1500 °C for 50 h to enhance the stability of reflection spectrum. The temperature calibration experiment has been carried out. In that case, a dry-block calibrator with high stability was employed to calibrate the temperature uncertainty of SFBG is ±0.7 °C. Compared to the calibrated thermometer, the maximum difference is less than 2 °C. In that case, a tube furnace was used to test the SFBG; the temperature uncertainty increased to 2.2 °C, and the maximum difference increased to 7 °C. This is due to the larger temperature fluctuation of this tube furnace. A 1000-h (i.e., 42 days), 1500 °C stability test was carried out. The SFBG exhibited excellent long-term high thermal stability (temperature deviation less than 2.0 °C). A cycling temperatures test was performed, which exhibited good repeatability in temperature measurements. Hence, such an SFBG sensor and the demodulation algorithm are prospectives for applications in harsh environments.