More than 6  kW near single-mode fiber amplifier based on a bidirectional tandem pumping scheme

We demonstrate the first all-fiber monolithic bidirectional tandem pumping amplifier, to the best of our knowledge, based on a 30/250 µm conventional ytterbium-doped double-clad fiber. By optimizing the bidirectional pumping power distribution, an output power of 6.22 kW is obtained with near single-mode beam quality (M²=1.53), and no transverse mode instability is observed. This work could provide an excellent reference for high-power, higher-brightness fiber lasers.