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

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.

First-order fiber Bragg grating inscription in indium fluoride fiber using a UV/Vis femtosecond laser and two-beam interferometry.

Fiber gratings are among key components in fiber-based photonics systems and, particularly, laser cavities. In the latter, they can play multiple roles, such as those of mirrors, polarizers, filters, or dispersion compensators. In this Letter, we present the inscription of highly reflective first-order fiber Bragg gratings (FBGs) in soft indium fluoride-based (InF3) fibers using a two-beam phase-mask interferometer and a femtosecond laser. We demonstrate an enhanced response of InF3-based fiber to a visible (400 nm) inscription wavelength compared to ultraviolet irradiation at 266 nm. In this way, FBGs with a reflectivity >99.7% were inscribed at around 1.9 µm with the bandwidth of 2.68 nm. After thermal annealing at 393K, the Bragg wavelength demonstrates stable thermal shift of 20 pm/K in the temperature range 293-373K. These observations suggest a potential extension of InF3 fiber-based laser components to an operational range of up to 5 µm.

100 W average power femtosecond laser at 343 nm.

We present a femtosecond laser system delivering up to 100 W of average power at 343 nm. The laser system employs a Yb-based femtosecond fiber laser and subsequent second- and third-harmonic generation in beta barium borate (BBO) crystals. Thermal gradients within these BBO crystals are mitigated by sapphire heat spreaders directly bonded to the front and back surface of the crystals. Thus, a nearly diffraction-limited beam quality (M2 < 1.4) is achieved, despite the high thermal load to the nonlinear crystals. This laser source is expected to push many industrial and scientific applications in the future.

High temperature sensing with fiber Bragg gratings in sapphire-derived all-glass optical fibers.

A structured sapphire-derived all-glass optical fiber with an aluminum content in the core of up to 50 mol% was used for fiber Bragg grating inscription. The fiber provided a parabolic refractive index profile. Fiber Bragg gratings were inscribed by means of femtosecond-laser pulses with a wavelength of 400 nm in combination with a two-beam phase mask interferometer. Heating experiments demonstrated the stability of the gratings for temperatures up to 950°C for more than 24 h without degradation in reflectivity.

Inscription of first-order sapphire Bragg gratings using 400 nm femtosecond laser radiation.

The paper describes the implementation of fiber Bragg gratings inscribed by femtosecond laser pulses with a wavelength of 400 nm. The use of a Talbot interferometer for the inscription process makes multiplexing practicable. We demonstrate the functionality of a three-grating multiplexing sensor and the temperature stability up to 1200 °C for a single first-order Bragg grating.