Silent callers

Crackly interference often associated with long-distance telephone calls could be eliminated thanks to a special filter developed by researchers at the Optoelectronics Research Centre at Southampton University.

The scientists have demonstrated a filter using superstructured fibre Bragg grating (FBG) technology to remove noise found in signals transmitted through optical fibres at a rate of 160Gbit/s. The fastest installed networks today, according to Dr Periklis Petropoulos, a member of the Southampton team, operate at 10Gbit/s.

‘We are concerned with optical transmission signals that are transmitted over hundreds and thousands of kilometres. These transmission links usually have optical amplifiers, and wherever there is an amplifier, noise is introduced,’ said Petropoulos. ‘In digital communication, this noise can be either in the intensity of the signals, or on the definition of the pulse flows, where each pulse starts and ends. Our technique solves the latter problem.’

The University of Denmark in Copenhagen provided the 160Gbit/s test bed and diagnostic tools for the demonstration of the new technology.

‘We sent a particular data sequence down our system, say 11001, and then at the receiver we tried to detect this data sequence to see if all the bits came to the receiver correctly or if one of them was an incorrect bit, say 10001,’ said Petropoulos. ‘We achieved error-free performance for the system, which means you allow one out of one billion bits to come in error. Without the filter, the rate of errors would be worse than one error to every 10 million bits, which in the lab makes a very big difference.’

In the new technique, a filter shapes the usually Gaussian (bell) shaped digital pulses into rectangular pulses, and a locally generated pulse is transmitted to sit in the centre of the rectangular pulse.

‘If we try to picture the pulse that is transmitted over the long distance as a rectangular pulse, imagine that the vertical lines of the rectangle have noise on them, while the horizontal lines depict time. However, although you know there is noise on the time domain, you’re not sure where the vertical lines fall,’ said Petropoulos.

‘So, we have shaped the noisy pulses and what we do next is switch or gate the shaped pulses with a very clean signal, clean because it hasn’t undergone any transmission. This sits right in the middle of the shaped pulses and is represented by a vertical line in the middle of the rectangle, splitting it in half.

‘These pulses are injected into an optical switch, which carves out the middle part of the rectangular pulse so that all the noise components will be outside it, thus filtering out the noise.’

Existing methods of reducing noise involve the conversion of optical signals to electronic signals so that the noise can be removed using electronic processing techniques. This is an extra step that is avoided by integrating FBG technology directly into the optical fibre wire, and also means that the passive filter does not need a separate power supply.

‘Electronic processing can be done in today’s networks but in future networks that will operate at much faster speeds it is questionable that electronics will manage to keep up,’ said Petropoulos.

‘Optics are faster than electronics and when you transmit more than one wavelength in the same optical fibre, which is a common method of transmitting optical signals, you need an electronic processor for each wavelength.

‘By doing it optically, you immediately solve the problem of speed, and this could be the first step into processing more than one wavelength signals at one time.’

According to Petropoulos, although FBG is a well-known optical filter within fibreoptics, the work carried out at Southampton stands out by using superstructured fibre technology, which allows for precise manipulation of the frequency components of the filters.

Other benefits of FBG include its insensitivity to polarisation, which means that the filter will operate no matter what the state of polarisation in an optical system. This means scientists do not have to constantly monitor and align it to the polarisation. It is also cheaper to fabricate.

‘The filters are fabricated by illuminating the fibre with a UV beam. In the most conventional technique, a phase mask is placed between the fibre and the beam and alternates the phase of the UV beam so that the optical fibres see the right light component,’ he said.

‘We are able to use a standard uniform phase mask by controlling the position of the fibre itself.’

Anh Nguyen