We will report on recent advances in fabrication of large volume silica based, doped fiber preform materials synthesized via powder-based processes.
Recently, there has been increased interest for power scaling in fiber based laser applications that requires large core volumes with excellent homogeneity in refractive indices, but also chemical variety (in terms of high dopant concentrations, different dopants). A structural fiber variety requires dedicated large volume core material of reproducible and tailorable chemical composition.
Established technologies such as modified chemical vapor deposition (MCVD) or crucible melting rely on complex thermal processing, and are limited in accessible chemistries, dopant concentration, achievable functionalities, and in case of MCVD in achievable core sizes. The current process development thus targets to overcome such draw-backs by including novel approaches to enable extreme material combinations, enhanced reactivity, or novel functions.
Experimental results on optical properties of Tm-doped oxide glass fibres based on different technologies and host materials are presented. Silica fibres and crystal-derived fibres (CDF) are compared. Crystal-derived fibres offer potentially higher doping levels than those made by Modified Chemical Vapor Deposition technology providing the ability to tune optical properties towards new applications.
A detailed analysis of absorption and fluorescence properties of the 3F4 and 3H4 levels within a concentration range from 0.08 up to 1.52 mol% Tm2O3 is provided resulting in an extensive and specific energy level scheme for 789nm core-pumping.
We investigate Tm-doped double-clad fibers drawn from fused silica preforms with inserted Tm:YAG laser crystal rods. Based on the Molten-Core-Method the drawn crystal-derived fibers show typical amorphous properties covering a Tm concentration range from 0.2 to 0.84 mol% Tm2O3. They are studied in terms of their suitability of for multi-Watt level fiber lasers and compared to a Tm-laser fiber conventionally fabricated by Modified Chemical Vapor Deposition and solution doping. For the crystal-derived fibers, we demonstrate up to 4 W output power around 2 μm emission wavelength and a slope efficiency of 47 %, which are to date the highest achieved values for 790 nm pumping.
Incorporating new optical materials as nanocrystals into glass fibres for new functionalities has recently become a hot research topic. Our team (funded by the European FET Open project NCLAS) investigates the introduction of nanoscale laser crystallites into the core of optical fibres using the glass powder doping method. Active Y2O3:Pr3+ nanocrystals (NCs) were prepared via different synthesis methods, and structurally and spectroscopically characterized. After modification of technological parameters, the optimised NCs have been proposed as a luminescence centres to embed into germanate and silicate glass hosts. Glasses were analysed in terms of optical (transmission, refractive index matching to NCs) and thermal (thermal stability, viscosity, thermal expansion coefficient) parameters. Crystallisation issues during fibre drawing were particularly investigated. In a first step, glass powder-NCs mixing techniques and fibre preform preparation were developed. It was shown that temperature cycle profiles including dwell time and heating/cooling ramp rates influenced the glass-NCs properties and can lead to glass crystallisation or NCs dissolution. The sintering investigations pointed out the melting temperature limits to preserve active NCs in the glasses. In germanate glasses, Y2O3:Pr3+ dissolution was noticed at 800°C. In the case of the silicate glass compositions these regions vary from 700°C to 1050°C. The results allowed to select optical fibre drawing conditions performed by the powder-in-tube method. Their distribution uniformity is not yet sufficient, requiring further optimisation of the drawing kinetics.
Magnetic nanoparticles (MNPs) have a major role as contrast agent in diagnostic imaging and therapeutic monitoring. In order to research on MNP exposition, degradation and elimination of those nano composites as well as the consequences of the MNP exposition in relation with social economic relevant diseases (cancer, infectious diseases), the comprehensive characterization of magnetic and structural properties is of high importance. Within this contribution, the magnetic characterization of theranostic relevant MNPs is introduced. Applying a vibrating sample magnetometer (VSM), it is found, that the nanocomposites show superparamagnetic behavior and the recorded data confirm iron oxide cores (magnetite/maghemite). Employing Raman spectroscopy, the typical fingerprint information of magnetite is detected. By increasing the laser power, the transition to maghemite and hematite due to the oxidation of the magnetic core is illustrated. Moreover, IR spectroscopy is applied to characterize the coating material e.g. starch or other biocompatible polymers. To determine the stability of MNPs as well as the MNP’s elimination under physiological conditions, different buffer systems were tested i.e. simulated body fluid (SBF) and artificial lysosomal fluid (ALF). The investigated MNPs are stable in SBF; thus, the stability in blood after injection of the contrast agent is guaranteed. Finally, the storage in ALF leads to a complete decomposition of the MNPs, which reflects the conditions in lysosomes and guarantee for a fast MNP elimination.
Acknowledgement: We thank the Federal Ministry of Education and Research (BMBF), Germany as well as the Project Management Jülich (PTJ), Germany for funding the research project NanoBEL (03XP0003F).
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