Experimental Investigation on Evolution of the Beam Quality in a 2-kW High Power Fiber Amplifier

The M
²
value is a general metric to characterize the beam quality of the high power fiber lasers. In this letter, we demonstrated a near single-mode fiber amplifier with 2.04 kW output. To the best of the authors' knowledge, we present the first evolution of the beam quality as a function of the signal output power in a multikilowatt class fiber laser utilizing step-index large-mode-area fiber. The M
²
parameter of was preserved at any power level without mode instability. What is more, the evolution of the beam quality and the mode field experimentally verifies the superior thermal-optical property of the high power fiber lasers compared with other bulk solid-state lasers because of their geometry of the gain medium.