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.

We report the efficient generation of broadband and ultra-flat supercontinuum using deuterium-filled anti-resonant hollow-core fibers dual-pumped with pulses centered at 515 nm and 1030 nm wavelength.

We report the flexible on-target delivery of 800 nm wavelength, 5 GW peak power, 40 fs duration laser pulses through an evacuated and tightly coiled 10 m long hollow-core nested anti-resonant fiber by positively chirping the input pulses to compensate for the anomalous dispersion of the fiber. Near-transform-limited output pulses with high beam quality and a guided peak intensity of 3 PW/cm 2 were achieved by suppressing plasma effects in the residual gas by pre-pumping the fiber with laser pulses after evacuation. This appears to cause a long-term removal of molecules from the fiber core. Identifying the fluence at the fiber core-wall interface as the damage origin, we scaled the coupled energy to 2.1 mJ using a short piece of larger-core fiber to obtain 20 GW at the fiber output. This scheme can pave the way towards the integration of anti-resonant fibers in mJ-level nonlinear optical experiments and laser-source development.

Supercontinuum generation in optical fibres is a well-established route towards broadband white-light sources with high spatial coherence and brightness, as required for a variety of applications in science and industry. The use of gas-filled hollow-core anti-resonant fibres [1], allows for tight confinement of both laser pulses and gas over long interaction lengths, with broadband guidance, enabling supercontinuum generation with extreme bandwidth [2]–[4]. Access to such supercontinua has been mainly achieved by pumping in the anomalous dispersion region and driving the electronic nonlinear response (optical Kerr effect) of gases, leading to modulational instability [2] and soliton effects [3], [4]. However, the former produces spectra with low temporal coherence, while the latter require very short pump pulses (few or tens of femtoseconds). Meanwhile, pumping in the normal dispersion region can result in limited spectral broadening through self-phase modulation.

Gas-filled hollow-core fibres have proven to be a very effective system for nonlinear optics in many areas such as frequency conversion, supercontinuum generation, and optical pulse compression. Hollow-core anti-resonant fibres in particular offer the benefit of low loss and high transmission bandwidth with strong mode confinement [1], enabling efficient broadband supercontinuum generation. This is typically achieved by using noble gases and pumping in the anomalous dispersion region to access modulation instability [2], or soliton self-compression dynamics [3], [4]. When pumping noble gases in the normal dispersion region, spectral broadening is driven mostly by self-phase modulation, offering limited spectral broadening due to optical wave breaking.

We report the on-target delivery of 40 fs, 200 µJ laser pulses at 800 nm through a 10 m long hollow-core nested anti-resonant fibre, with a guided peak intensity of 3 × 10 15 W/cm 2 .

We report recent advances in pulse compression, frequency upconversion to the ultraviolet, polarisation control, and flexible pulse delivery experiments in the ultrafast regime, using different types of hollow-core fibres.

We report on recent advances in versatile and efficient light sources based on gas-filled hollow-core fibers. © 2021 The Author(s)

We experimentally demonstrate the efficient generation of circularly polarized pulses tunable from the vacuum to deep ultraviolet (160-380 nm) through resonant dispersive wave emission from optical solitons in a gas-filled hollow capillary fiber. In the deep ultraviolet, we measure up to 13 µJ of pulse energy, and from numerical simulations, we estimate the shortest output pulse duration to be 8.5 fs. We also experimentally verify that simply scaling the pulse energy by 3/2 between linearly and circularly polarized pumping closely reproduces the soliton and dispersive wave dynamics. Based on previous results with linearly polarized self-compression and resonant dispersive wave emission, we expect our technique to be extended to produce circularly polarized few-fs pulses further into the vacuum ultraviolet, and few to sub-fs circularly polarized pulses in the near infrared.

Gas-filled hollow-core fibres, both with and without microstructure, provide a versatile system for ultrafast nonlinear optics. We report recent advances in deep and vacuum ultraviolet generation, sub-cycle pulse compression, and supercontinuum generation.

Pulse compression and frequency conversion are key ultrafast technologies. Few-cycle near-infrared pulses, combined with conversion across the ultraviolet to infrared, have enabled femtosecond and attosecond pump-probe experiments, advancing the field of ultrafast science. Soliton dynamics underlie a new class of technologies which can bring important new capabilities to this field [1] . They provide access to even shorter driving pulses, with sub-femtosecond and sub-cycle pulse duration produced by self-compression, while maintaining high energy and peak power. They also provide a highly efficient broadband frequency conversion technique, generating few-femtosecond, µJ-scale pulses, tunable across the vacuum ultraviolet to near-infrared spectrum.

Circularly polarized ultra-short laser pulses in the deep ultraviolet region (DUV, 200-400 nm) are commonly obtained via a two-step process requiring frequency up-conversion in nonlinear crystals followed by polarization conversion using quarter-wave plates. Due to strong dispersion in bulk media, this method suffers from limitations in phase-matching bandwidth, phase compensation and achromatic birefringence. Here, we demonstrate a direct process for the generation of ultra-short, circularly polarized DUV pulses via soliton dynamics in gasfilled stretched hollow capillary fibers [1] , driven by circularly polarized pulses centered at 800 nm. Frequency up-conversion occurs via resonant dispersive wave (RDW) emission with inherent spectral tunability (here we demonstrate 223-377 nm) through control of the gas (Ar) pressure. Our technique overcomes the limitations inherent to crystal-based approaches and allows energy up-scaling and extension to the vacuum UV (100-200 nm) and other spectral regions while permitting ultra-short duration, because the polarization conversion is performed at 800 nm, where material dispersion is low and the quality of commercial phase retarders is high.

We demonstrate an efficient scheme for the generation of broadband, high-energy, circularly polarized femtosecond laser pulses in the deep ultraviolet through seeded degenerate four-wave mixing in stretched gas-filled hollow capillary fibers. Pumping and seeding with circularly polarized 35 fs pulses centered at 400 nm and 800 nm, respectively, we generate idler pulses centered at 266 nm with 27 µJ of energy and over 95% spectrally averaged ellipticity. Even higher idler energies and broad spectra (27 nm bandwidth) can be obtained at the cost of reduced ellipticity. Our system can be scaled in average power and used in different spectral regions, including the vacuum ultraviolet.

We demonstrate broadband wavelength up-conversion (240-320 nm) based on a seeded four-wave mixing scheme in gas-filled stretched hollow-capillary fibers with 50% conversion efficiency. Our technique is scalable in energy from the nJ to mJ level.

We report a remarkably efficient experimental scheme for the generation of high energy ultra-short pulses by means of four-wave mixing in long stretched hollow capillary fibers filled with helium. We thoroughly investigate the role of strong and weak seeding fields in a degenerate up-conversion scheme to the deep ultraviolet. In the weak seed regime we demonstrate the tunable emission of up to 30 μJ in ultrashort pulses (~8 fs) in the 250-300 nm range, corresponding to pump energy conversion of up to 30%, from pump pulses with energies readily available from high-average power lasers. In the strong seed regime, we obtain higher pump conversion efficiencies, up to 42%, together with a spectral bandwidth supporting few femtosecond pulses and a record high deep-ultraviolet pulse energy exceeding 70 μJ. The energy can be further scaled by using stretched hollow-core fibers with larger core diameters.