Novel special optical fibers nowadays can take advantage of several new preform production techniques. During the last years we have devoted our attention to the granulated silica method. It is one of the variants of the powder-in-tube technique and potentially offers a high degree of freedom regarding the usable dopants, the maximum possible dopant concentration, the homogeneity of the dopants, the geometry and minimal refractive index contrast. We developed and refined an approach for the production of doped granulated silica material based on the sol-gel process.Here, we present material analysis results of an ytterbium (Yb) doped, aluminum (Al) and phosphorous (P) co-doped glass on the basis of our sol-gel glass based granulated silica method as well as first measurements of two LMA fibers obtained from this material. For the material analysis we used advanced analysis techniques, such as HAADF-STEM and STEM-EDX spectroscopy to determine the composition of the material and the distribution of the dopants and the codopants. The chemical mapping of the STEM-EDX shows an extremely homogeneous distribution of the dopants and co-dopants in nano-scale. Based on self-made LMA fibers, we measured the refractive index contrast of the sol-gelbased granulated silica derived core compared to the pure silica cladding. In addition we quantified optical characteristics such as the emission and absorption spectrum. The measured upper state lifetime of the optical active dopant ytterbium was 0.99ms, which in turn confirms the homogeneous distribution of the Yb atoms. The propagation losses were determined to be 0.2dB/m at 633nm and 0.02414dB/m at1550nm.

The applications of fibre lasers demand for increasing power. Limits are set by various nonlinear effects. Leakage channel fibres (LCF) are one approach to this problem. With this type of fibre, most nonlinear effects can, in principle, be mitigated simultaneously by increasing the mode field area and by maintaining the single mode regime. For its implementation, we propose to use the powder-in-tube preform technique. While the microstructure consists of commercial pure silica rods, the surrouding is filled with index-raised aluminum-doped silica oxide granulate. For the fabrication of the latter, we tested two different methods. For the first one, the oxide precursors were mixed in pure powder form. In the other method, the material was produced with the helps of the sol-gel process, where the mixing takes place in liquid phase, thus resulting in an expected improved homogeneity. Prior to the fabrication of a prototype, their feasibility has been tested with the help of a finite-difference method simulation tool (Lumerical MODE Solutions). Two such fibres have been fabricated according to this results. The influence of the granulate mixing method and of the grain size on the homogeneity in refractive index has been tested. Although the produced fibres do not yet show the desired performance, the produced prototypes prove that LCFs can indeed be realised with this approach.

No abstract

The sol-gel based granulated silica method has several advantages for the production of active optical fibers. It offers a high degree of freedom regarding the usable dopants and co-dopants, the maximum possible dopant concentration and the homogeneity of the dopants. The freedom in controlling the co-dopant concentration enables the full control to tailor the refractive index of the core. Several ytterbium (Yb) doped, aluminum (Al) and phosphorus (P) co-doped double cladding silica fibers with varying Al and P co-dopants concentrations have been produced by the sol-gel based granulated silica method, in order to study the influence of the different co-dopants concentrations on the fibers performance. To do so, we fixed the Yb concentration to 0.3 at% in all the fibers, as well as the cores and claddings diameters to 10 and 125 micrometer respectively. The variation of the core-cladding refractive index steps due to the different Al and P co-dopants concentration have been confirmed by measuring the one dimensional and two dimensional refractive index profile of every fiber by two different measurement apparatus, resulting in core-cladding refractive index steps that correspond well with the fibers compositions. In addition, the effect of the different co-dopants concentration on the fibers performance have been investigated by measuring the upper state lifetimes and the lasing performance (slope efficiency) of the fibers. We observed different fluorescence lifetimes among the differently co-doped fibers, and different slope efficiencies that are well correlated with the corresponding lifetime of each fiber. One of the fibers featured 60% slope efficiency at 1030nm by pumping the fiber at 976nm by a fiber-pigtailed laser diode (LD).

The sol-gel based granulated silica preform fabrication method is presented as a versatile "rapid prototyping" platform for specialty optical fiber production, enabling arbitrary geometries, large flexibility of doping composition and concentration, and homogeneous dopant distributions.