We demonstrate frequency down-conversions of femtosecond pulses through dispersive wave generation and degenerate four-wave mixing in a gas-filled anti-resonant hollow-core fiber. These are achieved by exploiting the rapid variation of the dispersion in the fiber’s transmission band edge. In this approach, the wavelength of the down-shifted radiation is governed solely by the thickness of the dielectric wall at the core–cladding interface, while other system parameters are accountable only for inducing sufficient nonlinear phase shifts. With the right choice of cladding wall thickness, the concept can be applied directly for generating high-power mid-infrared femtosecond pulses.

We demonstrate the generation of multi-octave-spanning supercontinuum in a gas-filled hollow-core fiber that extends into the mid-infrared. This is achieved by pumping the system with high-energy ultrashort pulses centered at 2 µm wavelength.

We demonstrate a fibre-based approach that generates mid-infrared femtosecond pulses in the 3-4 µm spectral region with microjoule-level single pulse energy. This is realised in a piece of gas-filled antiresonant hollow-core fibre that is pumped by a two-micron light source. A rapid variation of the dispersion near a structural resonance of the fibre creates a phase-matching point in the mid-infrared, which mediates the frequency-down conversion. We generate femtosecond pulses centred at 3.16 µm wavelength with the pulse energy of more than 1 µJ, achieving the conversion efficiency as high as 9.4 %. The wavelength of the radiation is determined solely by the dielectric wall thickness of the cladding elements, while the yield is subject to other experimental parameters. This, combined with high power-handling capability of hollow-core fibres, 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 allfibre-based mid-infrared supercontinuum source, which promises to be a powerful new tool for ultrahigh sensitivity molecular spectroscopy.

We report the generation of five phase-locked harmonics, f₁:2403  nm, f₂:1201  nm, f₃:801  nm, f₄:600  nm, and f₅:480  nm with an exact frequency ratio of 1:2:3:4:5 by implementing a divide-by-three optical frequency divider in the high harmonic generation process. All five harmonics are generated coaxially with high phase coherence in time and space, which are applicable for various practical uses.

We present an antiresonant hollow-core fiber-based bandpass optical filter. The device is realized by tapering down a section of tubular hollow-core fiber to a ratio of less than 0.5. Sweeping of the tube wall thickness-induced resonant bands in the down- and up-transition sections of the taper suppresses the blue side of the spectrum, while the red side filtering exploits the increased confinement loss at the taper waist that depends sharply on the wavelength-to-core-diameter ratio. These working principles of the filter make it possible to customize the location and width of the passband by tailoring the fiber design and taper profile. We achieve a 350-nm-wide bandpass filter with the minimum insertion loss of 1.3 dB in the passband and up to 40 dB suppression in the lossbands. We anticipate the filter to become one of the essential components in all-hollow-core fiberized optical systems.