Max Planck scientists couple high-power lasers into hollow optical fibres

Researchers have demonstrated an 'optical trapping system' that offers a new way to launch high-power laser light into hollow-core optical fibres. The simple, self-aligning method could have potential applications in laser cutting and basic physics research.

The work, carried out by researchers from the Max Planck Institute for the Science of Light (MPL) in Germany, showed, for the first time, that laser light can be used to manipulate a glass optical fibre tapered to a sharp point smaller than a speck of dust, in the middle of an optical fibre with a hollow core.

The scientists demonstrated that optical forces cause the sharp point, or 'nanospike', to self-align at the centre of the hollow core, trapping it more and more strongly at the core centre as the laser power increases.

'Launching very high power laser light into an optical fibre, especially a hollow-core fibre, can be very difficult and usually requires extensive electronics and optics to maintain alignment,' explained Philip Russell, director at the MPL and leader of the research team. 'This can be accomplished with our new system by simply pushing the nanospike into the hollow core and then turning up the laser power slowly. Once the nanospike self-stabilises, you can turn up the laser power and nothing will move or get damaged.'

As reported in the journal Optica, almost 90 per cent of the laser light was transferred from the nanospike to the hollow-core fibre.

To create the nanospike, the researchers started with an ordinary single-mode glass optical fibre about 100 microns in diameter. They heated this fibre so that they could stretch it to form a tapered portion and then etched the fibre’s tip with hydrochloric acid to create a nanospike around 100nm in diameter and less than 1mm long.

The team then created the optical trap by inserting the nanospike into the hollow core fibre and launching a high-power 1,064nm laser beam into the single-mode fibre. When the laser light enters the tapered portion of the fibre it begins to spread out beyond the nanospike into the empty space inside the hollow core fibre. As the taper gets smaller and smaller, the light begins to sense the boundary of the larger fibre core, which causes the light to reflect inwards towards the tapered fibre. This reflected light exerts a mechanical force on the nanospike, forming an optical trap.

'The nanospike is held in place by the light at exactly the right place to perfectly launch the light into the hollow core without any electronics or other systems to keep it in place,' Russell said. 'If any of the components move a little, there’s no effect on the laser light because the nanospike self-aligns and self-stabilises.'

The new work could increase applications for hollow-core fibres, a new class of fibre that features a hollow core rather than one made of glass like traditional optical fibres. Hollow-core fibres are especially good at handling high-power lasers, making them useful, potentially, for laser machining and cutting of metals, plastics, wood and other materials.

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