Supercontinuum generation in a methane-filled hollow-core antiresonant fiber.

We report the generation of a multi-octave supercontinuum spanning from 350 nm to 1700 nm with exceptional spectral flatness and high conversion efficiency to both visible and near-infrared regions, by pumping a methane-filled hollow-core antiresonant fiber with 1030 nm laser pulses. The dynamics exhibited signs of both modulational instability (MI) and stimulated Raman scattering (SRS). Fiber lengths ranging from 15 cm to 200 cm were investigated along with gas pressures up to 50 bar and pump pulse durations from 220 fs up to 10 ps. The best supercontinuum, in terms of spectral width and flatness, was achieved with 220 fs pulses, 25 bar filling pressure, and 60 cm propagation length. Comparison with argon-filled fiber with matched nonlinearity and dispersion showed that the Raman contribution enhances the supercontinuum generation process compared to a pure modulational instability-based process. The average power was scaled up by increasing the pulse repetition rate to 50 kHz, but further scaling was hindered by linear and nonlinear absorption, leading to fiber damage.