Metrological Characterization of a High-Temperature Hybrid Sensor Using Thermal Radiation and Calibrated Sapphire Fiber Bragg Grating for Process Monitoring in Harsh Environments

Abstract

Fiber Bragg gratings inscribed in single crystalline multimode sapphire fibers (S-FBG) are suitable for monitoring applications in harsh environments up to 1900 ∘C. Despite many approaches to optimize the S-FBG sensor, a metrological investigation of the achievable temperature uncertainties is still missing. In this paper, we developed a hybrid optical temperature sensor using S-FBG and thermal radiation signals. In addition, the sensor also includes a thermocouple for reference and process control during a field test. We analyzed the influence of the thermal gradient and hotspot position along the sensor for all three detection methods using an industrial draw tower and fixed point cells. Moreover, the signal processing of the reflected S-FBG spectrum was investigated and enhanced to determine the reachable measurement repeatability and uncertainty. For that purpose, we developed an analytical expression for the long-wavelength edge of the peak. Our findings show a higher stability against mechanical-caused mode variations for this method to measure the wavelength shift compared to established methods. Additionally, our approach offers a high robustness against aging effects caused by high-temperature processes (above 1700 ∘C) or harsh environments. Using temperature-fixed points, directly traceable to the International System of Units, we calibrated the S-FBG and thermocouple of the hybrid sensor, including the corresponding uncertainty budgets. Within the scope of an over 3-weeks-long field trial, 25 production cycles of an industrial silicon manufacturing process with temperatures up to 1600 ∘C were monitored with over 100,000 single measurements. The absolute calibrated thermocouple (Uk=2≈1K…4K) and S-FBG (Uk=2≈10K…14K) measurements agreed within their combined uncertainty. We also discuss possible strategies to significantly reduce the uncertainty of the S-FBG calibration. A follow-up measurement of the sensor after the long-term operation at high temperatures and the transport of the measuring system together with the sensor resulted in a change of less than 0.5 K. Thus, both the presented hybrid sensor and the measuring principle are very robust for applications in harsh environments.

Hysteresis Compensation in Temperature Response of Fiber Bragg Grating Thermometers Using Dynamic Regression

“…Until recently, systematic examination of temperature response of these devices including detailed characterization of measurement uncertainties has been lacking. Several authors have examined the behavior of type-I and type-II fiber Bragg gratings (FBG) 1 sensors at high temperatures and found that the sensors undergo significant hysteresis that is dependent upon both temperature and duration of excursion [13,14,15]. These results are broadly in agreement with earlier research on FBG fabrication processes that suggests the fabrication process creates shallow trap states in the bandgap that are "erased" at temperatures higher than 450 K [16,17,18,19].…”

“…Long term hysteresis or ageing effects in photonic thermometers [13,14,15,12] represent a significant measurement science challenge to the adoption of photonic thermometry in-lieu of resistance thermometers. In this study we demonstrate that guided by device physics we can deploy proven statistical techniques to model the ageing effects and successfully reduce the measurement uncertainty by up to 70%.…”

Optical Fiber Sensors for High-Temperature Monitoring: A Review

“…After packaging and calibrating, the above SFBG arrays can be used for stable temperature measurements. In 2022, Eisermann et al developed a hybrid high-temperature sensor combining a thermocouple, blackbody radiation, and SFBG to improve the stability of measuring the wavelength shift of SFBG [ 157 ]. The sensor was placed in a fiber draw tower for 3 weeks and over 100,000 measurements were conducted with temperatures up to 1600 °C.…”

Draw tower furnace diagnostics applying a sapphire fiber Bragg grating probe

“…In sapphire fibers fiber Bragg gratings can be inscribed using fs-Laser radiation 2,3,4 . It was shown that air-clad sapphire fiber Bragg gratings (SFBG) are stable at temperatures above 1500°C 5 and can be applied in industrial processes 6 . The sapphire fibers are extremely multi-mode.…”

“…It was demonstrated that single mode interrogation of these fibers is possible 4,7 . In 6 it was shown, that a fiber Bragg wavelength can be calculated from the unconventional multi-mode SFBG spectra with high precision that allows temperature measurements with an uncertainty below 5K according to metrological standards.…”