Development, manufacturing and lasing behavior of Yb-doped ultra large mode area fibers based on Yb-doped fused bulk silica

At the Photonics West 2008 we presented our rare earth doped fused bulk silica for fiber laser applications [1]. This approach overcame the typical geometrical limitations of other well known production methods for rare earth doped silica materials. Our unique production technique is based on the sintering of Yb-doped granulates of high-purity SiO2 particles. We have processed our Yb-doped bulk silica rods into ultra large mode area (XLMA) multi-mode double cladding laser fibers with an active core diameter in the range of 40 mu m to 100 mu m (depending on the core doping level). In the XLMA fiber the active core is surrounded by a so-called 2D- or 4D-shaped pure silica pump cladding (with diameter between 850 mu m and 900 mu m) and an F-doped outer silica cladding with an outer diameter of 1000 mu m. We have investigated the refractive index and the intrinsic stress profiles of different XLMA laser fibers and their preforms to visualize interface effects. The fiber cross section designs, the quality of all interfaces and the material composition are important factors for the laser fiber performance. The laser properties of these fibers have been investigated in detail. In addition, the preparation of the fiber end-face is important to reduce heat effects and we have developed concepts to mitigate such thermal load at the fiber end face.