240 W high-average-power square-shaped nanosecond all-fiber-integrated laser with near diffraction-limited beam quality

We report an all-fiber-integrated high-average-power square-shaped nanosecond pulse laser operating at 1068 nm based on the master oscillator power amplifier configuration. The seed source is a passively mode-locked Yb-doped fiber laser with fundamental cavity repetition rate of 1.86 MHz. Output pulses with a square shape can be tuned in pulse width from 271 ps to the nanosecond level. The average output power reaches to 9.21 W after three preamplifiers. Finally, a main amplifier is developed to boost the average output power to 240 W, and the corresponding pulse energy and peak power are $\sim 129.3\mathrm{\mu J}$ and 36 kW, respectively. The efficiency of the main amplifier is $\sim 61.3\%$, and the beam quality represented by $M^2$ factors is below 1.3 and 1.2 in the X and Y directions.