Coherent combination is especially important in the cutting edge field of attosecond science, where few-cycle or sub-cycle pulses are used to investigate and control electronic processes at the shortest time-scales ever measured, with significant applications in physical chemistry, metrology, and strong-field physics, to name just a few. A common way to generate ultrashort pulses is to spectrally broaden a femtosecond pulse via self-phase modulation in a gas-filled hollow capillary and temporally compress it to its transform limit. It was recently proposed that this scheme is amenable to coherent combination, where the pulses are temporally divided, individually (and identically) broadened, recombined, and compressed. The authors’ calculations showed that few-cycle, 10 mJ pulses should be possible. A French collaboration between researchers at the Ecole Polytechnique, Thales Optronique, and Université Paris-Sud have now experimentally demonstrated that this idea may be feasible. They measure 95% combination efficiency and produce 6 fs, 062 mJ pulses after compression. More crucially, they show the CEP stability of their input pulses is preserved by the combination process. The pulse division and combination is based on thin birefringent plates, where angle tuning alone provides sufficient control for path matching, and the thinness suffices to mitigate against uncompensated dispersive effects. Their system, which includes no active stabilization, shows remarkable stability in both phase and pulse energy. More advanced implementations with multiple pulse divisions are in the works and promise CEP-stable few cycle pulses at the highest energies to date.
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