Microjoule-Level Mid-Infrared Femtosecond Pulse Generation in Hollow-Core Fibers

A fiber-based approach that generates mid-infrared femtosecond pulses in the 3-4 mu$\umu$m spectral region with microjoule-level single pulse energy is demonstrated. This is realized in a piece of gas-filled antiresonant hollow-core fiber that is pumped by a two-micron light source. A rapid variation of the dispersion near a structural resonance of the fiber creates a phase-matching point in mid-infrared, which mediates the frequency-down conversion. Femtosecond pulses centered at 3.16 mu$\umu$m wavelength with the pulse energy of more than 1 mu$\umu$J are generated, achieving a conversion efficiency as high as 8.2%. The emission wavelength is determined solely by the dielectric wall thickness of cladding elements, while the yield is subject to other experimental parameters. This, combined with high power-handling capability of hollow-core fibers, makes it possible to power scale the mid-infrared output by either increasing the pulse energy or repetition rate of the pump. The technique presents a new pathway to build an all-fiber-based mid-infrared supercontinuum source, which promises to be a powerful new tool for ultrahigh sensitivity molecular spectroscopy.