Development of ultrashort pulse amplification system by all fiber passive-phase-matching coherent addition technology

Ultrashort pulse fiber lasers (FLs) are required to have higher power to achieve high quality laser processing of difficult-to-cut materials such as carbon fiber reinforced plastics (CFRPs). Our present work develops an all-fiber coherent addition system to achieve a high power ultrashort pulse FL beyond pulse-power limits due to nonlinear effects in rare-earth-doped fiber [1]. Coherent addition is a technology that divide a single pulse into multiple pulses, and amplify the power by fiber amplifiers, and combine the amplified pulses again into a single pulse [2]. However, the coherent addition of ultrashort pulses is technically difficult because ultrashort pulse lasers have very low coherence due to their broad spectrums. Therefore, we have developed a novel all-fiber coherent addition system that can passive-matching of phase and polarization, and attempted high-efficiency and high-power by femtosecond pulse.