Fibre Bragg gratings (FBGs) transform a conventional multi-mode optical fibres into side-emmitting light sources with controllable emission angles, which find applications in endoscopy, (bio-)photoreactors and spectroscopy.
The FBGs were inscribed in of soft-glass indium fluoride-based optical fibres with a two-beam phase-mask interferometer and a 266-nm femtosecond laser. The scattering pattern was imaged in Fourier space accessed by inserting a Bertrand lens in the beam path. Fibre rotation during the imaging yields a 360-deg all-around view.
The FBGs far-field scattering pattern demonstrated discrete broken bands, or scattering cones with different opening angles, for the different laser colours. Furthermore, multiple cones could be observed for the case of complicated, higher harmonics grating refractive index profiles, which provides additional tool to design tailored fibre light emitters or guided light analyser gratings.
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.
This paper presents an innovative one-step doping approach for the preparation of Al-Yb co-doped silica glasses for fiber preforms. Today, fiber-lasers are of great interest in industry due to highest precision and flexibility in system design combined with high power output and excellent beam quality. Industrially established processes such as modified chemical vapor deposition (MCVD), outside vapor deposition (OVD) and reactive powder sintering technology (REPUSIL) are used to fabricate co-doped silica glasses for laser fibers. However, none of these processes is able to simultaneously incorporate laser active dopants increasing the refractive index (rare earth elements, RE), glass matrix modifiers (e.g. aluminum, Al2O3) and dopants reducing the refractive index (e.g. fluorine, F). Instead, the incorporation of the individual refractive index changing dopants, into a silica glass matrix, has to be carried out in subsequent and separate steps. The novel approach pursues to overcome this limit by application of atmospheric-pressure microwave plasma with oxygen used as reactive gas in combination with a powder sintering process, targeting the preparation of tailored rareearth doped preforms for high power fiber-laser applications. As a proof of principle, silica powders doped with Al3+ and Yb3+ have been synthesized successfully. These have been proven to perfectly suit the subsequent processing via the powder sintering process. The plasma generated Al2O3 doped SiO2 particles have an averaged particle size of 30 nm a specific surface area of about 55 m2/g, at an Al2O3 concentration of up to 3 mol%. In a second set of experiments, microwave atmospheric pressure plasma-based co-doping of SiO2 with Al and Yb species has been successfully demonstrated for the first time.
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