With their advantages like good beam quality, easy thermal management, high robustness and compact size, fiber lasers are one of the most promising solid state laser concepts for high power scaling with excellent beam quality. One issue of further power scaling is the reduction of nonlinear effects, especially Raman scattering, which consequently led to increased mode field areas. However, for large mode area fibers, new challenges, namely transversal mode instabilities (TMI) have to be taken into account. Beside our investigations in the power scaling of ytterbium doped fiber amplifiers up to 4.4kW output power, we present our investigations of the TMI threshold in dependence on bend diameter and absorption length of a well-known, commercial fiber. Within this scope, we used a 13m piece of the fiber and gradually reduced the bend diameter from 60cm slightly below 14cm within a pump wavelength of 976nm. Furthermore, we increased the fiber length to 30 m, presuming the bend diameter of 14 cm and all experimental conditions. However, in a next step, we detuned the pump wavelength up to 980 nm in order to increase the pump absorption length As a result, we achieved 2.9kW of single mode output at a bend diameter of 14cm. The 4.4kW result was obtained with a separately manufactured low-NA fiber, allowing for a slope efficiency of 90% with regards to the absorbed pump light and an extremely temporal stability.
We present the fabrication and properties of active fiber laser materials fabricated by a newly developed solution doping technique. The contribution focusses on Aluminum, Phosphorus, Ytterbium as well as Boron doped SiO2 for the use as fiber laser material. More specifically low doping concentration in the vicinity of the molar ratio of Al2O3:P2O5 = 1:1 will be elucidated. The effect of fabrication parameters on optical properties like refractive index, absorption and emission properties will be covered. Currently it is possible to achieve cw output powers greater than 4 kW using Al, P, Yb doped fibers fabricated with this method. Fibers additionally codoped with Boron are as well suitable for kW class applications as well.
We present our current results on the fabrication of arbitrary shaped fiber tapers on our tapering rig using a CO2-laser as heat source. Single mode excitation of multimode fibers as well as changing the fiber geometry in an LPG-like fashion is presented. It is shown that this setup allows for reproducible fabrication of single-mode excitation tapers to extract the fundamental mode (M2 < 1.1) from a 30 μm core having an NA of 0.09.
Fiber amplifiers are representing one of the most promising solid state laser concepts, due to the compact setup size, a simple thermal management and furthermore excellent beam quality. In this contribution, we report on the latest results from a low-NA, large mode area single mode fiber with a single mode output power beyond 4 kW without any indication of mode instabilities or nonlinear effects and high slope efficiency. Furthermore, we quantify the influence of the bending diameter of our manufactured low NA fiber on the average core loss by an OFDR measurement and determine the optimal bending diameter in comparison to a second fiber with a slightly changed NA. The fibers used in the experiments were fabricated by MCVD technology combined with the solution doping technique. The investigation indicates the limitation of the step index fiber design and its influence on the use in high power fiber amplifiers. We demonstrate, that even a slightly change in the core NA crucially influences the minimum bending diameter of the fiber and has to be taken into account in applications. The measured output power represents to the best of our knowledge the highest single mode output power of an amplifier fiber ever reported on.
We demonstrate a quasi-continuous wave laser amplifier with a peak output power of 6.8 kW pumped by an industrial thin-disk laser. A high slope efficiency of 84 % has been obtained within a duty cycle of 10 % at a signal wavelength of 1071 nm. For cw-pumping we measured a maximum output power of 985W. The amplifier fiber had a step index profile with a core diameter of 45μm and a pump core diameter of 120μm. A signal to ASE peak ratio of 48 dB could be determined.