'Inverse laser drilling' to tailor dispersive properties in hollow-core fibres

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Inverse laser drilling is being used to manufacture preforms for hollow-core fibres with advantageous geometrical shapes. (Image: Fraunhofer ILT) 

Researchers are using a laser process known as ‘inverse laser drilling’ to enable the production of complex structures within hollow-core fibres in order to tailor their dispersive properties.

The process will be used to create structured fibre preforms – the glass rods from which the fibres are then drawn – over 200mm long.

Structured hollow-core fibres can be used to transmit very intense laser beams that would otherwise destroy solid fibres, for example in ultrashort-pulsed laser materials processing.

Currently, such fibres are produced by stacking preforms and drawing them to length – also known as the ‘stack-and-draw' process.

However, this process limits the structure within the fibre to a hexagonal shape. It is also very difficult to automate.

The partners of the recently launched ‘LAR3S’ project, including the Max Planck Institute for the Science of Light (MPL), Fraunhofer ILT and the Fraunhofer Institute for Silicate Research (ISC), are therefore improving an existing process known as ‘inverse laser beam drilling’ to produce more complex and thus potentially more advantageous structures within hollow-core fibres.

In this process, a scanner-manipulated laser beam is focused through the glass material of a fibre blank onto its rear side, where it effectively drills a hole backwards into the glass. Using the new technique, a wide range of structures with large aspect ratios could be produced, delivering tailored dispersive properties to fibre preforms over 200mm long. Such structures could one day be calculated on a computer using artificial intelligence and then directly manufactured using a laser, meaning the process could be completely automated, according to the researchers. 

3D microstructures by selective laser-induced etching

The project partners are also developing another process, known as selective laser-induced etching, with which focused ultrashort-pulsed laser radiation is used to structure the volume and surfaces of transparent materials.

The process will enable such surfaces to be structured ‘crack-free’, thereby changing their properties in a way that they can later be selectively processed using wet chemical etching. This two-part process will provide users with a high degree of geometrical freedom.

The project partners are looking to optimise the process primarily for new geometrical shapes in the manufacture of laser microresonators. Such sub-millimetre structures can be used in telecommunications and quantum technology, for example. As couplers, converters or sensors, they enable the further miniaturisation and integration of optical components.

Selective laser-induced etching can be used to manufacture microresonators, e.g for frequency comb generators. (Image: MPL)

The LAR3S project (Laser-generated three-dimensional photonic components – Resonant and antiresonant devices for shaping and guiding light) will run for three years and is funded by the Fraunhofer Max Planck Cooperation Program.

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