The past decade has seen a great number of advances in the field of gas-based nonlinear optics. Earlier experiments had to be performed in free space, where the laser beam diverges, so high intensity cannot be maintained over long distance; or in hollow tubes where the beam does not diverge, but instead experiences high propagation loss. This changed with the advent of fibers with a hollow core, surrounded by a carefully engineered microstructure that prohibits divergence while still allowing low loss propagation.

Compared to nonlinear optics in fibers with a solid glass core, hollow-core fibers rarely suffer from ultraviolet induced damage of the glass. The light propagates mainly in the gas filled core, with only a negligible fraction propagating in the glass. This is actually also a problem, because gases have much lower nonlinearity than glass. To compensate for the lower nonlinearity, it is advantageous to make the core very small to raise the light intensity. It is very challenging to make small-core hollow-core fibers directly during drawing, so the authors of this paper instead post-processed a fiber by tapering (done by heating until softening, while pulling) it, so that the core decreased from 25.6 µm to 10.6 µm. This meant that ultraviolet light could be generated with a pump pulse energy of only 125 nJ, showing an interesting potential for making compact wavelength-tunable UV sources.

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