In recent years, the extracted output power of fiber lasers has been scaled from ten watts up to the kW regime. Those devices harness the extreme optical stability of silica glass, the properties of double-clad fibers, and the availability of appropriate high-power diode laser systems that can be used as pump sources. The cited publications prove the scalability in the field of fiber lasers. The progress is due to the outstanding thermal and optical properties of doped fibers. Their large surface-to-core ratio allows for excellent heat removal from the active volume. In general, the beam quality of a fiber laser does not depend on the power level but only on the fiber design, i. e. diameter of the active core, numerical aperture, and the emission wavelength. Furthermore, using ytterbium as dopant reduces the thermal loading during laser action as the quantum defect of these rare earth ions is known to be small. Nonlinear loss effects that are due to the high intensities and large fiber lengths, e. g. stimulated Raman scattering (SRS), prevent from scaling the output power of a fiber laser with a single-mode core with small core diameter. In order to avoid such undesired processes, so-called large-mode-area (LMA) fibers are advantageous. Those fibers spread the laser intensity over a fiber core with a large diameter and a small numerical aperture (NA).
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