Beam quality and noise properties of coherently combined ytterbium doped single frequency fiber amplifiers

Collinear coherent combination of multiple single frequency fiber amplifiers is a promising approach to realize the high power laser sources required for 3rd generation gravitational wave detectors (GWD), as long as the stringent requirements on the beam quality and noise properties can be met. Here, we report the beam quality and noise properties of two coherently combined 10 W single frequency amplifiers with respect to the requirements of GWD. The combining efficiency was larger than 95% with 97% of the combined beam in the fundamental spatial mode. There was no significant noise increase compared to the fluctuations of the single amplifier.