Photonic crystal at core of breakthrough

A new technique for manufacturing optical fibres will allow transmission of higher-power and multicolour laser light. Conventional optical fibres consist of a core of silica glass doped with germanium, surrounded by cladding of pure silica. The core has a higher refractive index than the cladding and light travelling along the core is unable to escape […]

A new technique for manufacturing optical fibres will allow transmission of higher-power and multicolour laser light.

Conventional optical fibres consist of a core of silica glass doped with germanium, surrounded by cladding of pure silica. The core has a higher refractive index than the cladding and light travelling along the core is unable to escape because total internal reflection occurs at the boundary with the cladding.

The latest breakthrough, photonic crystal fibre, was first developed by the University of Bath optoelectronics group, with funding from Dera and BT, though many companies are now doing research in this area.

Photonic crystal fibre is made by stacking together an array of glass capillaries and solid rods, which are then heated to 2,000 C and drawn down in a succession of operations to produce fibres tens of kilometres long with holes as small as 50nm, one thousandth the width of a human hair.

In the simplest form of the fibre, a solid rod is used at the centre of the array so that after extrusion a solid area of glass is surrounded by a larger area of silica with holes running through it.

These two regions now behave like the core and cladding of a conventional optical fibre, but with an important difference. In a conventional fibre, different wavelengths of light travel at different speeds. So the fibre has to be ‘tuned’ to a given colour or there will be interference and degradation of the information transmitted.

In the photonic crystal fibre, light of all colours travels at the same velocity. The fibre can thus transmit multi-coloured laser light with unprecedented precision over long distances. Because no doping is required, the fibre is also easier to make.

But it can also carry higher power lasers without damage, making it potentially important in applications such as surgery, laser machining and long distance telecommunications.

The drawing process allows multiple cores to be incorporated and this leads to other potential applications. Researchers at Bath are investigating the use of three-core fibres as sensors.

A multiple fibre could be attached to a bridge or built into an aircraft wing made of composite material, for example, and used to detect flexing and strain.

As the structure flexed, the relative length of the cores would change, so that the interference patterns produced by the light would also change. This would automatically detect overstressing or damage which would otherwise be hidden.