New twist on fibre optics

By twisting fibre optic strands into helical shapes, researchers have created unique structures that can precisely filter, polarise or scatter light.

Compatible with standard fibre optic lines, these hair-like structures may replace bulky components in sensors, gyroscopes and other devices. While researchers are still probing the unusual properties of the new fibres, tests show the strands impart a chiral, or 'handed', character to light by polarising photons according to certain physical properties.

In conventional optical fibres, light is transmitted from one end to the other through a round core housed within a concentric outer cladding. But, because a circular core does not develop handedness when twisted, the research team wound rectangular-core fibres to create a double helix.

When the team tested the twisted fibre, they discovered that some photons left the core and entered the cladding. Photons with the same handedness as the fibre entered the cladding whereas photons with handedness opposite that of the fibre remained in the core.

With only a relatively loose twist - roughly 100 µm to form a complete turn - photons with a handedness that coincides with the fibre's twist scatter out of the core at a shallow angle and are trapped in the cladding. With a tighter twist, photons with the same handedness as the fibre scatter at a wider angle, allowing the photons to escape from the cladding into the surrounding space.

Only light of a single polarisation remains in the fibre. At the tightest twists, roughly one-millionth of a metre to complete a turn, photons with the same handedness as the structure are reflected backwards in the core.

Because the environment surrounding the fibre affects the wavelength of the light embedded in the cladding, 'loosely' twisted fibres can serve as sensors for pressure, temperature, torque and chemical composition.

With moderately twisted fibres, researchers can manipulate the resulting polarised light in useful ways, leading to a range of applications such as gyroscopes for navigation systems, current meters for electric power stations and chemical and materials analysis equipment.

For tightly wound fibres, the amount of twist determines the precise wavelength of the light remaining in the fibre, producing light that is suitable for filter and laser applications.