Novel approach for high-performance optical fibers: multiple-doped silica powders with plasma-enhanced processes

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.