High Power Tandem Pumped Ytterbium-Doped Fiber Amplifier Employing Low-NA Fiber

The power scaling of tandem pumped ytterbium-doped fiber lasers (YDFLs) is mainly limited by the nonlinear effect such as the stimulated Raman scattering (SRS). Applying a kind of YDF featuring a large mode field area (LMA) and few guided modes is one of the effective methods to improve the SRS threshold while achieving mode control. With the benefits of small V values, few guided modes and LMA, low-numerical aperture (NA) fibers have potential for effective mode control and high SRS threshold. In this contribution, a low-NA YDF with a core/ inner cladding diameter of 40/250 mu m and core NA similar to 0.051 is fabricated for high power tandem pumped fiber laser. Compared with the conventional YDF, the low-NA fiber amplifier reaches a maximum output power of 7.1 kW with beam quality M-2 similar to 2.3 using a forward tandem pumping scheme, providing significant advantages of improving beam quality and transverse mode instability (TMI) threshold. This work indicates the great potential of low-NA fibers applied in tandem-pumped fiber lasers for high-power laser systems requiring further power scaling and excellent beam quality, such as precision industrial machining.