March 2015
Spotlight Summary by Paul Steinvurzel
Passive coherent combining of CEP-stable few-cycle pulses from a temporally divided hollow fiber compressor
Lasers have been an integral part of the researcher’s toolkit for more than half a century now, and no matter how much optical power they deliver, there is always a drive for more. Laser systems are themselves susceptible to overheating, optical nonlinearities, and optical damage, and clever strategies must be employed to generate MW-level CW power or multi-mJ pulses with well-controlled spatial and temporal profiles. Many of these strategies rely on some form of coherent combination, where copies of the beam are spatially or temporally separated, amplified, and then recombined. The coherent aspect of coherent combination is what makes this a non-trivial process – the recombined beams must all be in-phase, requiring ps-scale timing accuracy (mm-scale path length matching over several meters) for CW systems down to attosecond (as) scale accuracy (sub-µm scale path length matching) for short pulsed systems. Since shorter pulses contain more bandwidth, timing accuracy alone is insufficient, dispersion must also be well matched. For ultrashort pulses, the carrier envelope phase (CEP) stability, which is crucial for metrology applications, must be preserved as well.
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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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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Article Information
Passive coherent combining of CEP-stable few-cycle pulses from a temporally divided hollow fiber compressor
Hermance Jacqmin, Aurélie Jullien, Brigitte Mercier, Marc Hanna, Frédéric Druon, Dimitrios Papadopoulos, and Rodrigo Lopez-Martens
Opt. Lett. 40(5) 709-712 (2015) View: Abstract | HTML | PDF