1.3 kW Yb-doped fiber laser with excellent beam quality

We report on a 1.3 kW output power from an ytterbium-doped-multiclad-large-modearea fiber laser. Even at this power level no limitations due to nonlinearity or thermal load are observed. The beam quality is close to diffraction-limited. 02004 Optical Society of America OCIS codes: (140.3510) Lasen, fiber(060.2280) Fiber design and fabrication In the recent few years the output power of double-clad fiber lasers has been increased tremendously [ 1-41, This fact is owing to a signifcant progress in fiber manufacturing technology and reliable high-power diode-laser pump sources. In general rare-eanh-doped fibers are a scalable solid-state laser concept. Their main performance advantages are due to the'outstanding thermo-optical properties of a doped fiber. The large ratio of surface to active volume of such a fiber ensures excellent heat dissipation. Furthermore, the small quantum defect of Ybdoped lasers reduces the thermal load. However, the large product of intensity and interaction length inside the fiber core enforces nonlinear effects, such as stimulated Raman scattering (SRS), which restrict power scaling even in the continuous-wave regime. Novel fiber designs, so called low-NA large-mode-area fiber, offer power scaling capabilities due to the significantly reduced intensity inside the fiber CO=. In this contribution we report on power scaling of single fiber laser output to well above the 1 kW level. Power handling is sigmilcantly improved by a multi-clad pump core design. Up to 1.3 kW continuous-wave output power is achieved with excellent beam quality and without any thenno-optical or nonlinear limitations, which will allow further power scaling in the future. The core diameter of the fiber applied in the experiment is 38 pm with a numerical aperture of 0.06. The core is doped with ytterbium and neodymium. However, in the herein presented results we just used pump sources at 940 and 976 MI. Therefore, the neodymium ions are not utilized. The active core is surrounded by a D-shaped 600 pm inner cladding. Additionally an F-doped silica layer was added between the 600 pm pump core and the polymer cladding, yielding to an overall numerical apertun: of 0.35 of the pump core. The absorption length of this stlllcture is50 ma t 940 nm. A fiber laser in its simplest form is built up using 50 m length of the above described fiber with a high reflecting mirror on one side and using the 4% Fresnel reflections on the other. The fiber is pumped from both sides by imaging fiber-coupled diode lasers (Laserline GmbH) into the multi-clad pump core stmcture. At a total launched pump power of 2.2 kW we obtained up to 1.3 kW of continuous-wave output power at approx. 1090 MI emission wavelength. Figure 1 shows the output characteristics of the laser. No stimulated Raman scattering is observed at this power level. The beam quality is close to diffraction-limited. The M2-value is characterized to be less than 3. The temporal behavior of the high-power fiber laser is stable. The laser shows no self-pulsing tendency. To our knowledge this is the highest extract& power from a single fiber ever reported and the presented experiment is just limited by available pump power. Thus, further power scaling should be possible in future experiments. CPDDL 1200 3 slope efficiency= 65 % .