A fundamentally new type of optical fibre has been developed by physicists at Bath University in the UK. The fibre takes advantage of a hollow honeycomb structure to create a "photonic band-gap" effect that prevents light of certain wavelengths escaping (Science 282 1476). The new fibre displays "extraordinary properties" according to Jonathan Knight, a member of the team that developed it.
Fibre-optic cables usually consist of an inner core of highly refractive glass sheathed inside glass with a lower refractive index. Light travels along the inner core fibre by total internal reflection. However, despite their popularity, standard optical fibers have a number of limitations. They are difficult to manufacture, light can leak from the inner core, and it is only possible to change the optical properties of the fibre at one wavelength.
In the approach developed at Bath, solid silica rods and hollow silica capillary tubes are stacked in hexagonal arrangement and heated to 2000 Celsius. The silica is then stretched into a fibre that has a pattern of submicron air holes over its cross sectional area. Instead of travelling along a region with a high refractive index, light travels through a region of the fibre containing an extra air hole. Moreover, these holes create a photonic band-gap effect which means that only certain wavelengths can propagate in the fibre, as predicted by researchers from the Technical University of Denmark.
The photonic waveguide has a number of advantages for optoelectronics applications. For example, whereas normal optical fibres rely on doping with impurities such as krypton to change their refractive index, the wavelength sensitivity of the new fibre can be changed by adjusting the spacing, temperature or pattern of the rods in the manufacturing process. Doping is not necessary.
The photonic band-gap fibre also handles the polarization of light beams in a completely different way to normal fibers. According to Knight, it allows light to travel much more slowly along its length, which can lead to increased nonlinear effects. This could lead to a reduction in the size of devices that rely on optical fibres, such as gyroscopes.
“In the near term I suspect it will be used in niche markets” says Knight. Some companies have already approached the Bath team about using the fibres made in a similar way for laser welding equipment. The challenge for commercial manufacturers will be to make long lengths of the fibre. The Bath team have only just succeeded in making fibres many metres long with only small fluctuations in the fibre width.
