Amplitude and phase characterization of 10-fs pulses generated by hollow-core fiber pulse compression

For many nonlinear optics and high-field experiments, including high-order harmonic generation, X-ray lasers and particle acceleration, energetic pulses on the order of 10 fs and shorter would result in the observation of completely new phenomena. For example, calculations suggest that high-order harmonic generation with a 5 fs pulse would result in soft x-ray pulse with a duration on the order of 100 attoseconds. While 10 fs pulses are readily available at low energy (nJ) directly from Kerr-lens mode-locked laser oscillators, gain narrowing and high-order phase compensation have limited the pulse duration from ultra-short pulse laser amplifiers to about 20 fs. The route we are pursuing toward the goal of energetic sub-10 fs pulses is to compress amplified 20 fs pulses after self-phase modulation in a noble-gas-filled hollow-core dielectric waveguide, first used by Nisoli et al [1] for much longer input pulses. Along with the development of this technique, we have adapted the transient-grating frequency-resolved optical gating (TG-FROG) technique [2] for ultrashort, wide bandwidth pulse characterization. Since the guided pulse spectrum is broadened under the simultaneous action of several linear and nonlinear processes, including self-phase modulation, linear dispersion and self-steepening, the use of the FROG technique provides information necessary for optimal compression. Perhaps more important is that such coherent detection dramatically broadens the scope of experiments and applications using these pulses. For example, full characterization of amplitude and phase is crucial for control of the complex field profile.