High-power pulses that are shorter than 100 fs in the mid-IR region (wavelengths ~2-5 microns) will be extremely valuable for time-resolved spectroscopy and generation of soft x-rays, among other things. However, high-power lasers in that spectral region do not generate such short pulses. An alternative approach to production of mid-IR radiation is stimulated Raman scattering in hollow-core microstructure fibers. In this paper, Loranger and co-authors report surprising and remarkable results for the generation of ultrashort pulses that also have implications for practical sources. The authors launched 600-fs pulses from a fiber laser into a hollow-core fiber filled with hydrogen to pressures around 30 atmospheres. The 1030-nm input pulses were converted to 1800 nm with 50% quantum efficiency, which is a nice but not remarkable result. However, measurement of the output produced a surprise: the output pulse actually consists of a 40-fs spike that contains 40% of the energy at 1800 nm along with a low pedestal of ~600-fs duration. Numerical simulations reveal that the vibrational wave builds up in the hydrogen molecules during the course of the input pulse. Late in the input pulse, the combination of a strong vibrational wave and strong field produces abrupt conversion to the Stokes wave, which yields the sharp 40-fs spike in the output. The generation of sub-100-fs mid-IR pulses with ~50-megawatt peak power starting with readily-available femtosecond fiber lasers is a simple and robust approach that will be attractive for applications.

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