Faser-Bragg-Gitter für die Hochtemperaturanwendung

Im Rahmen der Dissertation wurden die Eigenschaften von Faser-Bragg-Gittern (FBG) in zwei speziellen Fasern unter dem Gesichtspunkt der Hochtemperaturstabilität (>800°C) untersucht. Die Gitter wurden dabei unter Verwendung eines frequenzverdoppelten Ti:Sa-basierten Femtosekundenlasers in einem Zweistrahl-Phasenmasken-Interferometer zum einen in einer einkristallinen Saphirfaser und zum anderen in einer Alumosilikatglaskernfaser erzeugt. Für die Gitter in der Saphirfaser, konnte experimental Temperaturstabilität bis ca. 1900°C nachgewiesen werden. Die zweite Faser wurde zunächst erst einmal nach einem neuartigen Ansatz aus einem mit Quarzglas umfangenen Saphirstab verzogen. Die so entstandene Alumosilikat-glaskernfaser (AluSi-Faser mit ca. 50 mol-% Al2O3-Anteil im Faserzentrum) zeigt eine Temperaturstabilität der eingeschrieben FBG von bis zu 900°C. Beide Fasern wurden hinsichtlich ihrer Führungseigenschaften Untersucht und auf diesen Ergebnissen basierend die Reflexionseigenschaften der in ihnen erzeugten Faser-Bragg-Gitter modelliert. Auch wenn beide Fasern extrem mehrmodig sind, führen die verscheiden Brechzahlprofile (Stufenindexprofil der Saphirfaser, parabolisches Gradientenindexprofil der AluSi-Faser) zu grundlegend verschiedenen FBG-Charakteristika. Für beide Fasern wurden entsprechend angepasste Auswertealgorithmen genutzt um die temperaturabhängige Braggwellenlängen-verschiebung der FBG aus dem breitbandigen Reflexionssignal (Saphirfaser) und dem äquidistanten Mehrpeakspektrum (AluSi-Faser) zu erhalten. Derart konnten mit beiden Fasertypen Einzelmess- und Wiederholgenauigkeiten von wenigen Kelvin für die FBG-basierten Temperaturmessungen erreicht werden

Two-Step-Model of Photosensitivity in Cerium-doped Fibers

The photosensitivity of various cerium-doped fibers has been experimentally investigated for both excimer- and femtosecond-laser illumination. The results of single-pulse, few-pulse and multi-pulse inscription of fiber-Bragg-gratings with both laser systems and the thermal aging of those gratings demonstrated the restrictions of the conventional color center model for cerium-doped fibers. To explain the short-term stability of single-pulse gratings against long-term stability of multi-pulse gratings, an extension into a two-step-model was deduced

Germania and alumina dopant diffusion and viscous flow effects at preparation of doped optical fibers

We report on germania and alumina dopant profile shift effects at preparation of compact optical fibers using packaging methods (Stack-and-Draw method, Rod-in-Tube (RiT) technique). The sintering of package hollow volume by viscous flow results in a shift of the core-pitch ratio in all-solid microstructured fibers. The ratio is increased by about 5% in the case of a hexagonal package. The shift by diffusion effects of both dopants is simulated for typical slow speed drawing parameters. Thermodynamic approximations of surface dissociation of germania doped silica suggest the need of an adequate undoped silica barrier layer to prevent an undesired bubble formation at fiber drawing. In contrast, alumina doping does not estimate critical dissociation effects with vaporous aluminium oxide components. We report guide values of diffusion length of germania and alumina for the drawing process by kinetic approximation. The germania diffusion involves a small core enlargement, typically in the sub-micrometer scale. Though, the alumina diffusion enlarges it by a few micrometers. A drawn pure alumina preform core rod transforms to an amorphous aluminosilicate core with a molar alumina concentration of only about 50% and a non-gaussian concentration profile

3D laser engineering of molten core optical fibers: toward a new generation of harsh environment sensing devices

International audienceAluminosilicate glasses offer wide-ranging potential as enabling materials for a new generation of optical devices operating in harsh environments. In this work, a nonconventional manufacturing process, the molten core method, is employed to fabricate and study sapphire (Al$_2$O$_3$) and YAG (yttrium aluminum garnet) derived all-glass silicate optical fibers in which a femtosecond (fs) laser is used to imprint oriented nanostructures inside the fiber cores. Both writing kinetics and thermal stability of the laser-modified regions are investigated over a wide temperature range (20–1200 °C). The laser-imprinted modifications in these high alumina-content fibers exhibit improved thermal stability with respect to commercial pure silica and GeO$_2$-doped silica analogs. Furthermore, optical devices in the form of Rayleigh backscattering and fiber Bragg grating sensors are fabricated to demonstrate the high-temperature sensitivity and stability of these nonconventional fibers. This functionalization of aluminosilicate fibers by fs-laser direct writing opens the door to a new generation of optical devices suitable for high-temperature operation

Germania and alumina dopant diffusion and viscous flow effects at preparation of doped optical fibers

We report on germania and alumina dopant profile shift effects at preparation of compact optical fibers using packaging methods (Stack-and-Draw method, Rod-in-Tube (RiT) technique). The sintering of package hollow volume by viscous flow results in a shift of the core-pitch ratio in all-solid microstructured fibers. The ratio is increased by about 5% in the case of a hexagonal package. The shift by diffusion effects of both dopants is simulated for typical slow speed drawing parameters. Thermodynamic approximations of surface dissociation of germania doped silica suggest the need of an adequate undoped silica barrier layer to prevent an undesired bubble formation at fiber drawing. In contrast, alumina doping does not estimate critical dissociation effects with vaporous aluminium oxide components. We report guide values of diffusion length of germania and alumina for the drawing process by kinetic approximation. The germania diffusion involves a small core enlargement, typically in the sub-micrometer scale. Though, the alumina diffusion enlarges it by a few micrometers. A drawn pure alumina preform core rod transforms to an amorphous aluminosilicate core with a molar alumina concentration of only about 50% and a non-gaussian concentration profile

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