For high power fiber lasers, codoping with Al, P or both is necessary to prevent rare earth (RE) clustering in the silica network of the laser active core material. Here, we present a complementary infrared (IR) based multispectral method combined with elemental analysis data on core/cladding to describe the structure of the doped core material as fabricated by chemical vapor deposition with gas phase doping of Al and P. The resulting 2D image and its corresponding 3D visualization of the data enable an alternative and convenient way to characterize the main species of the dopants aside from NMR measurements.

The long-haul subsea applications have driven recent development to focus on incorporation of chlorine replacing germania as index adapting component in ultralow loss fibers. We present a method to dope silica glass with a chlorine level above the one expected from thermodynamical equilibrium consideration. Gaseous SiCl4 was used as dopant for the treatment of porous green body prepared by the powder sinter technique (REPUSIL). To be able to optimize the chlorination process steps, the exhaust species of the individual steps had been analyzed using an online process monitoring of exhaust gases via IR spectroscopy. Finally, we could realize a chlorine level of 1.2 wt% Cl with an initial pressure of SiCl4 of 0.1 bar while the calculation of thermodynamic chemical equilibrium predicted only a maximum level of less than 1.0 wt% Cl in SiO2 under those optimized conditions.

The incorporation of phosphorus into silica soot material strongly changes during the multistep preparation process of the MCVD technology in combination with solution doping for Al and rare earths. We report on the influence of various process parameters on the phosphorus concentration, the bond types of phosphorus atoms and the relative density of the soot material. By optimization of the process the phosphorus concentration of the presintered soot could be increased by around 10% in comparison to the conventional treatment. The understanding of the interdependencies allows an improvement of the preparation process of phosphorus co-doped RE doped silica laser fibers with MCVD technology.

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.

In this paper we will discuss the influence of atmosphere pressure microwave plasma on the background loss and the radial distribution of several dopants in specialty, especially rare-earth (RE) doped, preforms and fibers for high power application. In conclusion we were able to demonstrate a significant improvement in the homogeneity of the distribution of the codopants within the silica matrix. Furthermore, we used the plasma process for the functionalization of pure and doped particles as basic raw material in the powder-based reactive powder sintering of silica (REPUSIL) process. In further experiments we will use plasma technology for the all-in-one doping of both active and passive dopants for a brightness adjustment of the refractive index of specialty fibers.

It is well known that the efficiency of silica-based erbium/ytterbium-doped fibre lasers and amplifiers can be greatly enhanced by a high level of phosphorus codoping due to an improved Yb to Er transfer efficiency. The manufacture of these types of rare earth (RE)-doped silica fibres with a high P concentration is principally carried out via modified chemical vapour deposition (MCVD) technology in combination with solution doping. The supply of RE ions for the P-doped active core of the fibre preform occurs via the liquid phase during the multi-stage preparation process. The incorporation of phosphorus into the silica matrix is determined and strongly influenced by the chemical equilibrium, evaporation by formation of gaseous phosphorus oxides, and diffusion during the process steps. The knowledge and understanding of these interaction processes are very important for the optimisation of the fabrication process for high power laser and amplifier fibres. Here, we report on a systematic investigation of phosphorus incorporation into the silica matrix during the MCVD process in combination with a solution doping technique. The P2O5-doped silica soot material of the individual steps was prepared with a wide range of different process parameters (gas concentration of the starting compounds POCl3 and SiCl4, pre-sintering temperatures, porosity). The samples were investigated concerning their porosity, morphological characterisation, and composition via scanning electron microscopy, Fourier transform infrared spectroscopy and energy dispersive x-ray spectroscopy.

The active core diameter in silica preforms can be significantly increased by the deposition of ytterbium (Yb) and the most important codopant aluminum (Al) in the gas phase through the high-temperature evaporation of an Yb chelate compound and Al chloride in the Modified Chemical Vapor Deposition (MCVD) process. Here, we report on systematic investigations of the incorporation of Yb and Al into silica by gas phase doping technique. Preforms and fibers were prepared in a wide range of Yb and Al concentrations. The samples were characterized concerning the radial distribution of the refractive index and dopant concentrations, the efficiency of the deposition, and the absorption and emission properties in the NIR region. First laser experiments have demonstrated a slope efficiency of 80%, which is comparable to fibers made by MCVD/solution doping and powder sinter technology.

The photodarkening resistivity of ytterbium doped alumosilicate fibers can be remarkably improved by cerium codoping. Here we report on systematical investigations of the influence of cerium on an optimized fiber design. Fibers with different ytterbium, aluminium and cerium contents have been prepared both under reducing and oxydizing conditions and characterized concerning refractive index, absorption and emission from UV to NIR. Typical spectral features in the UV and visible range have been analysed with respect to the ratio of Ce3+ / Ce4+. Photodarkening tests have been accomplished in order to correlate the power stability with the Ce content and valency state.

Up to now, the role of divalent ytterbium ion has been controversially discussed in the literature concerning its influence on the photodarkening of ytterbium doped high power laser fibers. In general, however, the experimental findings are relatively sparse and some discussions are based more on speculations than on examined facts. Here we report on systematical investigations concerning the formation of Yb2+ during the fabrication process of preforms and fibers. By Modified Chemical Vapor Deposition, fibers with different codopants (additional to the active ytterbium doping) have been prepared in a well-defined manner, regarding process parameters and glass composition. The comprehensive characterization of the samples involves the ytterbium absorption in the NIR, the UV absorption and UV excited emission. The typical spectral features in the UV and visible range have been analysed and correlated with the presence of Yb2+. The amount of formed divalent ytterbium ions shows a strong dependence on the process route and varies remarkably with the kind and concentration of the codopants. Photodarkening tests have been accomplished in order to correlate the power stability with the Yb2+ content. Moreover, the formation of Yb2+ during the process of UV radiation darkening was investigated.

Ytterbium-doped high-power fiber lasers with high beam quality are promising devices for a variety of applications. Extreme power load and complicated fiber structures make great demands on material properties and preparation technology. Recently, with the further increase of output power, the problem of photodarkening has been identified as a critical issue for fiber laser devices. Detailed knowledge of optical properties of materials and fibers are needed for the successful development of laser fibers, aimed at increasing efficiency and power.It is well known that the properties of silica based rare earth doped fibers can be influenced and remarkably improved by the incorporation of further dopants. Here, the influence of combined aluminium-phosphorus codoping on the optical properties of Yb doped laser fibers was investigated. Preforms and fibers were prepared by MCVD and solution doping both with phosphorus and aluminium excess. The samples were characterized concerning the radial distribution of refractive index and dopant concentrations and the absorption and emission properties in the UV/VIS/NIR region. The observed spectral effects and active properties (laser efficiency, photodarkening) were correlated with changes in the fiber composition. It could be shown, that the combined doping leads to effects which deviate from a simple additivity and which can be beneficially utilized for the improvement of the laser fiber performance.

In the last years, the performance of ytterbium doped high power silica fiber lasers has been remarkably improved. Increasing attention has been paid to the role of material composition, impurities and atomic defects, because the extreme power load and the complicated fiber structure make great demands on material properties and preparation technology. Recently, with the further growth of output power, the problem of photodarkening has been identified as a critical issue for fiber laser devices. Here we report on developments aimed at increasing efficiency and power stability of ytterbium doped laser fibers. The optical properties of preforms and fibers made by MCVD and codoped with high phosphorus contents were investigated for a wide range of the ytterbium concentration. Moreover, the influence of the collapsing atmosphere on the properties was studied. The observed spectral effects, photodarkening and laser efficiency were correlated with changes in fiber composition and process conditions. It could be shown that phosphorus codoping leads to remarkably different and advantageous fiber properties compared with the usual aluminium codoping.