Our present work develops an all fiber coherent beam combining system to achieve a high energy pulse fiber laser beyond pulse energy limits due to nonlinear effects in rare-earth-doped fibers. Coherent beam combining is a power addition technology that obtains high energy density and high beam quality because it integrates multiple laser beams in a mono-light wave. However, the coherent beam combining using optical fibers is technically difficult because the optical phases and polarizations in optical fibers fluctuate due to disturbances. Therefore, we developed a novel all fiber coherent beam combining system that can precisely control optical path lengths and automatically compensate polarization changes. When the system combined four laser pulses, it achieved a beam-combining efficiency of 96.6% that was about four times as high as a pulse energy before combining. In addition, the system obtained 97.1% beam-combining efficiency of coherent beam combining using femtosecond pulses that have very low coherences by minimizing the optical path-length differences between combined two pulses. Moreover, the system successfully regulated the beam-combining-efficiency changes to less than 2.0 percentage points in full width by suppressing optical path-length fluctuations and tracking-control of the optical phases.
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