Fundamental mode operation of a 19-core phase-locked Yb-doped fiber amplifier.

We demonstrate, theoretically and experimentally, a high power/energy 19-core Yb-doped fiber amplifier that operates in its fundamental in-phase mode. The calculated result using an improved coupled mode theory with gain shows that, with a Gaussian beam as seed, the in-phase supermode dominates. Experimentally, we use a Q-switched single-core fiber laser with single transverse mode as seed, and amplify it with a 5.8 m 19-core fiber. The measured near and far fields are close to the in-phase supermode. The measured M2 factor of the amplified beam is 1.5, which is close to the theoretical value. A pulse energy gain of 20 dB is obtained with the amplified pulse energy up to 0.65 mJ at a repetition frequency of 5 kHz. No appreciable stimulated Brillouin scattering is observed at this power level.