FEATURE PHOTONIC CRYSTAL FIBRES


Guided by holes


Rachel Berkowitz on the latest developments made in photonic crystal fibres, optical fibres with unique light guiding properties


I


n the early 19th century, Daniel Colladon’s fountain of light established the core principles of modern optical fibre: light trapped by total internal


reflection followed the curved path of a water jet, thus illuminating the water for an audience. Similarly, modern optical fibres comprise a solid core surrounded by a transparent cladding material with a refractive index lower than that of the core. The light stays in the core by total internal reflection, which causes the fibre to act as a waveguide.


But solid optical fibre requires a large


core to transmit higher power, which gives undesirable physical properties. Thin fibre bends because the glass at the core is very thin; but as it gets thicker it behaves less like a fibre and more like a glass rod. In the 1990s, Phillip St J Russell pioneered a microstructured photonic crystal fibre (PCF) as an alternative. PCF guides light by confining it within


an array of microscopic air holes that run along the entire length of a fibre. The core provides a waveguide for the trapped light, with the photonic crystals forming a


cladding around the core. There are also versions in which the light is confined in a core with a lower index than the cladding – these fibres can even have a hollow core. The tuneable refractive index and


refractive index contrast means the new technology allows for a wider range of optical properties than are possible in standard optical fibres. Nonlinear effects can be adjusted to select specific properties: for amplification, light can be more strongly confined in polarisation- maintaining fibres; or a hollow core can be used to propagate wavelengths that would be absorbed by standard fibre materials. One of the fastest growing applications of PCF is use in high-power fibre lasers and amplifiers. Scientists and engineers are working to incorporate these advanced materials into fibre lasers for industrial use and more specialised applications.


Stacks of spaghetti The Fraunhofer Institute for Applied Optics and Precision Engineering IOF in Jena, Germany, holds fibre laser performance records, using fibres developed jointly with PCF pioneer NKT Photonics. This summer, Fraunhofer IOF opened a new PCF facility with the mission of conducting application- oriented research while producing its own fibres. An ordinary optical fibre starts with


PCF Fibre output, showing bimodal behaviour 22 Electro Optics November 2017


a ‘preform’, or a thick segment of silica comprising an inner core and outer cladding designed to have a specific refractive index. The preform is loaded at the top of a fibre drawing tower. Here, its tip gets melted until a molten segment falls down due to gravity, thus elongating and solidifying into a thin fibre as it cools. ‘You take the large chunk of glass, heat it, and it becomes thinned out like a piece


“The difficult question is how to get a preform that has holes”


of cheese would,’ said Falk Eilenberger, research coordinator at Fraunhofer IOF. The composition of the preform is preserved in the final fibre. That’s a straightforward process for a solid piece of glass – but it’s more difficult for PCF, where high-pressure gasses are needed to prevent the holes from collapsing. ‘That’s totally feasible. But the difficult question is how to get a preform that has holes,’ added Eilenberger. He explained how stacking patterns of


smaller silica rods ‘that look like spaghetti’ leads to a preform with a microarray of holes. Heating and elongating the resulting structure, as with solid core fibres, makes the ‘adjacent small spaghettis fuse together, with holes where you left out the rods. In this way, you can very much define the geometry and light-guiding properties of the fibre’. Fraunhofer develops continuous wave


lasers that emit on the order of a kilowatt from a single fibre, for use in any material


@electrooptics | www.electrooptics.com


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