Imre Seres, Barnabás Gilicze, Szabolcs Hack, Attila Czirják, Péter Jójárt, Tamás Bartyik, Zsolt Kovács, Zsolt Bengery, Rajaram Shrestha, Levente Ábrók, Ádám Börzsönyi, Katalin Varjú, Gábor Szabó, Zoltán Várallyay
We demonstrate enhanced spectral broadening of femtosecond pulses (330 fs, 1030 nm, 5-200 kHz) in a hollow-core fiber (HCF) using CO2/helium gas mixtures. While increasing pure CO2 pressure broadens the spectrum and induces a Raman redshift, it causes substantial optical loss. Introducing a helium buffer enables strong broadening at reduced CO2 pressures, significantly decreasing loss. Propagating pulses through a 1-m-long, 300-μm-core HCF, we show that helium enhances spectral width, extends the Raman shift, and improves transmitted power by facilitating spatial cooling. This binary molecular/noble gas approach provides a robust, low-loss route for spectral engineering in HCFs.