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Objective: Ultrafast fiber laser sources with mJ-level pulse energy and kilo-watt average power are of particular importance for various science fields such as attosecond lasers. Nowadays, several attosecond laser large scale facilities are under construction, including ELI-ALPS in Europe, SECUF in China, NeXUS in America and ALFA in Japan, to name a few. High performance femtosecond driven lasers are crucial for attosecond lasers and various ultrafast laser facilities. Fiber lasers have large surface-to-volume ratio for efficient cooling, and are suitable for high average power amplification. However, due to small mode area of fibers, detrimental nonlinear optical effects such as self-phase modulation, four wave mixing, and stimulated Raman scattering limit the peak power of pulse to hundreds of MW, corresponding to pulse energy of hundreds of μJ for femtosecond pulses in large mode area rod-type fibers. In addition, the average power of fiber lasers is limited by transverse mode instability, which decreases the stability and quality of beams above certain threshold. In rod type fibers, the threshold is around 250 W. Neither average power or pulse energy emitted by single fiber meets the demand of attosecond laser generation.
Methods: Further scaling of average power and pulse energy can be realized by coherent beam combining, which involves splitting pulses offered by an frontend laser and recombining them after amplification. It is essential for coherent beam combining to maintain the coherence of pulse replicas, which usually involves high speed photodiode detectors, piezo-driven mirrors, and other electronics forming a feedback system to actively control the phase of all replicas. We present a high-energy, high-power ultrafast fiber laser system employing filled-aperture coherent combination of four ytterbium-doped rod-type fiber amplifiers. The phase control is achieved using stochastic parallel gradient descent method. The frontend includes a passively mode-locked Yb-fiber oscillator, a stretcher, a pulse picker, and three fiber pre-amplifiers, which delivers 1-MHz stretched pulses centered at 1032-nm with 700-ps duration and 20-W average power. The pulse is split to four replicas by polarization beam-splitter and half-wave plate pairs, and the replicas pass through delay lines formed by piezo-driven mirrors before amplification. The pulse replicas are equally split and amplified to ensure the same accumulated nonlinear phase, and are combined by thin film polarizer and half-wave plate pairs. Small part of combined pulse is split and collected by a photodiode detector after filtered spectrally and spatially as signal for phase control. The combined pulse is compressed by a double-pass diffraction grating pair compressor including two 1739 l/mm gratings. Results and Discussions: At 1-MHz repetition rate, our 4-channel Yb-fiber coherent beam combining system generated a combined average power of 753-W with 87% combining efficiency. Utilizing tunable pulse stretcher together with compressor produces 0.67-mJ, 242-fs near transform-limited pulses with 89% compressing efficiency. The compressed pulse is centered at 1032-nm, and the spectrum width is 8.8-nm. The root-mean-square of average power is below 1% over 30-minute measurement, while the residual phase error is less than λ/23, indicating excellent stability at different time scales. The beam quality factor of the 0.67-mJ compressed pulses is 1.17×1.11. At 500-kHz, we obtained 1.07-mJ, 247-fs pulses with average power of 534-W, with similar efficiency, long-term stability and beam quality. The residual phase error decreases to less than λ/29, illustrating a better short-term stability. Further power and energy scaling can be achieved by increasing the number of channels. By adding delay and pointing stabilization system which are under development, it is possible to generate 1-kW, 2-mJ pulses using 8-channel CBC system.
Conclusions: In this paper, we implemented a 4-channel coherent beam combining system based on SPGD method, and obtained compressed 673-W, 673-µJ, 242-fs pulses at 1-MHz and 534-W, 1.07-mJ, 247-fs pulses at 500-kHz. Both power and energy can be further improved by increasing the channel number, and the delay and pointing stabilization system is also under construction. By adding coherent pulse stacking amplification technique, coherent beam combining system is supposed to generate up to 100-mJ pulse energy, which constitutes an enabling source for applications such as laser wake-field acceleration. -
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