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 () and S-FBG () 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.
Highlights
Decades of research has established fiber optics-based temperature sensors as viable tools in harsh environments and over long distances [1,2,3]
Fiber Bragg gratings inscribed in single crystalline multimode sapphire fibers (S-fiber Bragg gratings (FBG)) are suitable for monitoring applications in harsh environments up to 1900 ◦C
We have successfully demonstrated the metrological characterization of a high-temperature hybrid sensor using thermal radiation and calibrated sapphire FBG (S-FBG) for process monitoring in harsh environments
Summary
Decades of research has established fiber optics-based temperature sensors as viable tools in harsh environments and over long distances [1,2,3]. One way to improve the peak shape is to change the optical properties of the fiber by applying a cladding, which is able to reduce the number of modes. This can be technically very challenging and, depending on the selected material, can limit the upper operating temperature [20]. The distinct peak, known from single-mode FBGs, becomes a rather wide and asymmetric shape [17] This can be understood by considering the Bragg condition for the wavelength λB for a periodic perturbation in the fiber [24]: λB = 2neffΛ. Due to the strain decoupling in the sensor design, we assume in the following that strain effects are negligible for the temperature determination
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