In this Letter, to overcome these limitations, the authors cleverly assemble a few existing technologies: chirped pulse amplification (CPA), coherent combination of spatially separated amplifiers, and large-pitch fibers (LPFs) that significantly reduce the peak power of pulses in the amplifier both in the temporal and spatial domains and prevent nonlinear effects. A brief description of the method is as follows. The authors first use a mode-locked solid-state oscillator to generate femtosecond seed pulses for amplification, and then stretch the seed pulses to about 2 ns using a grating stretcher. This significantly reduces the peak power of pulses prior to amplification. Once the pulses are stretched in the time domain, three pre-amplifiers and four main amplifiers are employed to amplify the stretched seed pulses. Particularly, the authors use six LPFs for two pre-amplifiers and four main amplifiers following one fiber-based pre-amplifier. Since LPFs can have a large effective mode area with single mode operation based on preferential gain only for the fundamental mode, a higher pulse energy can be held in them without compromising the quality of the pulses. Additionally, coherent combination of four spatially separated main amplifiers is used to further increase the pulse energy. This combination method consists of first dividing the pre-amplified pulse into four pulses, then amplifying each in a spatially separated amplifier, and finally recombining coherently these four pulses into one pulse. The separation into four pulses effectively increases the total beam size four times and also reduces thermal load, consequently leading to a higher pulse energy and average power in the amplifier. Following all the amplification processes, the amplified pulses are compressed in the time domain using a grating compressor, eventually leading to a gigawatt peak power with an average power of 530W.
In summary, the authors demonstrate the generation of gigawatt femtosecond pulses with half a kilowatt average power using a femtosecond fiber CPA system based on LPFs with coherent combination of spatially separated amplifiers. As the authors note, it is clearly expected that the coherent combination with the help of the development of fiber laser technology can further improve the performance of femtosecond laser amplifier in the future.
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