Hollow-Core Fibres for Stable Trains of Wavelength-Tuneable Ultraviolet Pulses at 10 MHz Repetition Rate

Nonlinear dynamics in gas-filled hollow-core fibres (HCFs) provide a practical route for versatile and efficient sources of ultrashort pulses in the ultraviolet (UV). In recent years, resonant dispersive wave (RDW) emission and four-wave-mixing in such a system have enabled the generation of few-femtosecond radiation covering the spectral range from 90 nm to the visible, with energies from 10s of nJ to 100s of μJ, average powers up to the W-level and conversion efficiencies up to 38% [1]–[4]. At low repetition rates (in the kHz range), such sources can be as stable as the pump laser but become increasingly unstable at higher repetition rates due to ionisation-induced long-lived effects [1]. Here, we show that by designing the system so as to operate at lower average powers and minimise ionisation of the fibre-filling gas, we can obtain stable trains of ultrashort pulses tuneable across the near ultraviolet at a repetition rate of 10 MHz and with average powers exceeding 100 mW.