Southampton University’s Optoelectronics Research Unit and manufacturer Pirelli have developed a technique for boosting the available bandwidth of fibre optic cable.
The approach, based on a technology called Chirped Fibre Gratings, will provide a cost-effective route to capacity of 50Gbit/s. This is considered necessary to prevent gridlock on telecoms networks as demand grows for Internet services and cable television channels.
CFGs overcome the problem of interference between light pulses down a fibre optic line. At 2.5Gbit/s signals can be transmitted successfully over 1000km of fibre, but the viable distance falls at higher data rates. Thus 10Gbit/s can travel only around 60km without excessive distortion, requiring the addition of amplifiers and dispersion compensation fibre.
Interference occurs when pulses broaden, or `chirp’, because the refractive index of glass differs according to wavelength. Over long distances, this causes different pulses to overlap.
CFGs are short lengths of fibre inserted in the line to correct distortion by exploiting the chirping effect in a controlled way.
They contain a series of graded points of change – or gratings – in the refractive index which reflect the pulse back on itself, at the same time introducing a time delay that progressively increases as wavelengths reduce. The pulse is thus recompressed.
The refractive index changes are achieved by exposing the CFG’s photosensitive core to ultra-violet radiation. The ORC and Pirelli have patented a technique for using two interfering UV beams to achieve a highly accurate gradation.
In field trials, US long distance carrier MCI has used the Pirelli/ORC CFGs to send data at 40Gbit/s down a 450km link between Chicago and St Louis.
Pirelli is expected to bring the product fully to market within two years.