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Wednesday 21st February 2018
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Photonic fibres
Feature: 3/4/2002



Specially developed glass-coated fibres could have far-reaching benefits for the high visibility PPE clothing market (director-e News, Thursday 25 April).

Developed by Shandon Hart of Massachusetts Institute of Technology in the US and colleagues, the fibres are said to reflect more light at certain wavelengths than the best metallic reflectors. The fibres can also be drawn out to different diameters to customise their optical properties.

Man-made fibres are robust and versatile, and cheap to produce in large quantities. In contrast, optical devices known as dielectric mirrors are fragile and expensive.

But Hart and colleagues have combined the resilience of synthetic fibres with the properties of an optical device known as a dielectric mirror to create a fibre that could be woven into fabrics on conventional looms to make radiation shields, high visibility reflective fabrics, and filters.

Dielectric mirrors are made of alternating layers of two materials with different refractive indices. These layers give the mirror a 'photonic band gap', so that it reflects light in a certain range of wavelengths, but absorbs light outside this range.

Dielectric mirrors are widely used in optical communications systems to reflect light of selected wavelengths.

Hart's team made the fibres from arsenic triselenide, a glass with a refractive index of 2.8, and polyether sulphone, a polymer with a refractive index of 1.6. The researchers deposited a layer of glass on each side of a sheet of polymer, and rolled the glass-coated polymer sheet around a polymer core several times.

Fibre preform

This produced a fibre 'preform' with 21 alternating layers of glass and polymer, which was then drawn out into fibres with diameters ranging from 175 to 500 micrometres.

The diameter of a fibre determines the thickness of its dielectric layers, and therefore its reflectivity characteristics. This allowed the researchers to make fibres with various photonic band gaps.

Hart's team found that these band gaps arose in the infrared portion of the spectrum at the positions predicted by their calculations. This shows that the fibres contained few impurities, were uniform, and kept their optical properties at the high temperatures used in the drawing process.

The fibres are also sensitive to light approaching from any angle, in contrast with some dielectric mirrors that only respond to light that hits their surfaces at right angles.

Hart and colleagues hope that their fibres will be suitable for a wide range of applications. They even suggest that customised fibres could also be woven into textiles, acting as 'bar codes' that could be read to show that fabrics are authentic.

And the concept of high visibility safety clothing that has inherent reflectivity would certainly seem to be one area worth pursuing
Author: John Gibbon
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