Applications addressed by femtosecond (fs) laser sources are requiring increasing pulse energies and increasing average powers. Ytterbium-doped fiber amplifiers are excellent candidates to generate high average powers at high repetition rates, but present strong disadvantages in terms of peak power. Indeed, the tight confinement of the beam over long interaction length induces nonlinear effects at high peak-powers that affect the overall performances of fiber systems. This work describes coherent combining methods that can be used to scale the performances of femtosecond laser sources.Coherent beam combining has been widely used in CW regime and more recently in the nanosecond range. It consists in splitting a single seed into N beam replicas, amplified each by independent amplifiers in parallel. Their respective outputs are combined in free space into one single beam that carries the power of the N amplifiers without cumulating nonlinearities. This architecture allows scaling both peak and average powers of the amplification systems. We have studied and demonstrated the efficiency of active coherent beam combining in the fs regime with two fiber amplifiers, which are peak-power limited. The experiments show the preservation of the temporal/spectral/spatial properties of the combined pulses, with high combination efficiencies.Coherent beam combining methods require phase-matching between all the beams to combine. This is usually achieved by an active feedback loop on each amplifier along with a phase detection scheme. We demonstrate that a Sagnac interferometer can be used to ensure perfect and stable phase-matching over time, which considerably simplifies the setup. Finally, another passive combining method known as divided-pulse amplification, acting in the temporal domain, is studied and demonstrated in the fs regime. It is coupled with the passive spatial combining method described above to scale the number of pulse divisions. All these experiments show the compatibility of coherent combining concepts in the fs regime and provide new opportunities for fiber amplifier systems.
