2.4 kW, narrow-linewidth, near-diffraction-limited all-fiber laser based on a one-stage master oscillator power amplifier

In this paper, we demonstrate a high-power, narrow-linewidth, near-diffraction-limited all-fiber laser based on a one-stage high power amplifier configuration. The seed source is a fiber oscillator with a short cavity length in order to suppress the spectral broadening effect, the linewidth is 0.027 nm at the laser power of 7.3 W. Then only one-stage high-power amplifier is used to scale up the seed power. It is found this simple laser structure can obtain an output power of 2.4 kW with a linewidth of 0.24 nm. Meanwhile, the center wavelength only shifts 0.025 nm during the amplification process, which is about 0.1 times of the linewidth. The output laser at maximum power has a near-diffraction-limited beam quality, the M2 is 1.27 in the x-direction and 1.35 in the y -direction, respectively. Besides, for the first time, we discover the beam fluctuations (different from the mode instability) with a frequency of a few Hz, this new phenomenon limits the further improvement of output power.