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 paper, we propose and experimentally demonstrate a cross-correlation algorithm (CCA) for demodulation of SFBG, benefiting to enhance the stability of wavelength detection. And hence, the sensing accuracy can be improved. The SFBG high temperature sensor was created by 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 case that a dry block calibrator with a 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 case that a tube furnace was used to test the SFBG, the temperature uncertainty increases to 2.2 °C, and the maximum difference increases 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 exhibits an excellent long-term high thermal stability (temperature deviation less than 2.0 °C). Cycling temperatures test was performed, which exhibits a good repeatability in temperature measurements. Hence, such a SFBG sensor and the demodulation algorithm are prospective for applications in harsh environments.