Thin-film beam combining technology is an effective approach to improve output power while maintaining beam quality. However, the lack of comprehensive research into the key factors affecting the beam quality in systems makes it challenging to achieve a practical combined beam source with high brightness. This paper clearly established that the temperature rise of dichroic mirrors (DMs) and sub-beam overlapping precision are the main factors affecting the beam quality of the system, with quantified effects. Based on this understanding, a combined light source of four channels of 3 kW fiber lasers was achieved, and an output power of 11.4 kW with a beam quality of M2x = 1.601 and M2y = 1.558, using three high-steepness low-absorption DMs and the active control technique. To the best of our knowledge, this is the best beam quality for a 10 kW light source. This study offers a solution for practical high-power laser sources in the tens of kilowatts range.
