The generation and amplification at wavelengths longer than 1100 nm is not straightforward when using Yb-doped optical fibers, since light emission of ytterbium occurs preferentially in the region of 1020 nm - 1100 nm with a maximum at 1030 nm. One well known approach is to heat the Yb-doped fiber up to temperatures above 100 °C. This increases the re-absorption in the lower emission band and also enhances at the same time the emission at longer wavelengths. Consequently, heating allows to extend the spectral gain-region of Yb-doped fibers by at least 60 nm up to 1160 nm. However, the drawback of this method is that it results in a shorter durability of the fiber, since heating damages the polymer-coating. Moreover, such a laser has a reduced overall efficiency, due to heating, isolation and heat removal issues. It has been reported, that at the presence of an aluminosilca host (silica doped with Al) efficient laser activity at around 1150 nm can be achieved by heating the Yb-doped fiber to only 60 °C. In this work we investigate the spectroscopy of a heated Yb-doped fiber with a high aluminum concentration. The fiber is drawn in our in-house fiber drawing tower. The preforms are produced by the sol-gel-based granulated silica method which allows us to vary the aluminum as well as the ytterbium concentrations within a large range. The fiber is investigated with respect to their spectroscopic data as well as their lasing performance.
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 surrounding 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.
We introduce a new fiber-optical approach for reflection based refractive index mapping. Our approach leads to improved stability and reliability over existing free-space confocal instruments and significantly cuts alignment efforts and reduces the number of components needed. Other than properly cleaved fiber end-faces, this setup requires no additional sample preparation. The instrument is calibrated by means of a set of samples with known refractive indices. The index steps of commercially available fibers are measured accurately down to < 10-3. The precision limit of the instrument is currently of the order of 10-4.