Crystal clear signals

Scientists at Brookhaven National Laboratory, New York have built liquid crystal devices on the tips of optical fibres to correct signal distortions in high-speed optical communications.

Scientists in the US have built tiny liquid crystal devices on the tips of optical fibres to correct signal distortions in high-speed optical communications.

The new device, which is said to use liquid crystals instead of lithium niobate, could make optical communications more affordable in the future.

The researchers are at the US Department of Energy’s Brookhaven National Laboratory and at Bell Laboratories, Lucent Technologies’ Research and Development arm.

‘Most designs for these distortion-correcting devices rely on lithium niobate in spite of the high cost associated with these materials,’ said Ron Pindak, a physicist at the National Synchrotron Light Source at Brookhaven Lab and a co-author of the study. ‘Our device has many advantages: its speed is fast enough for these corrections, it is also reset free, and it has a potential to be low in cost.’

In an optical fibre, the signal is carried by utlrashort pulses of light. Initially, in each pulse, the light’s electric field follows a given direction. Then, because the optical fibre is not perfectly circular, the electric field’s direction, or polarisation, splits into two components that propagate at different speeds, causing the pulse to spread, an effect referred to as polarisation mode dispersion (PMD).

External mechanical vibrations (caused by a passing train or high winds, for example) cause the PMD to vary with time. At very high transmission rates – which can reach beyond 40 billion pulses per second, these time-varying distortions are so severe that they need to be compensated for to achieve reliable operation of the optical transmission.

Current optical transmission systems include, at regular intervals, PMD-compensating devices, which incorporate a device to control the polarisation state of the optical pulses. Most of the existing designs of polarisation controllers rely on lithium niobate because of its high speed, which is needed to keep up with the mechanical vibrations and other effects that cause the distortions.

‘Conventional wisdom suggested that liquid crystals could never achieve the necessary speeds,’ explains John Rogers, director of the Nanotechnology Research Department at Bell Laboratories in Murray Hill, New Jersey, and a co-author of the study. ‘Our work shows not only that liquid crystals can be fast enough, but also that the devices themselves can be built right on the tip of an optical fibre, in a very compact and attractive geometry.’

The researchers reportedly devised a new approach to correct the optical polarisations fast enough to compensate for disturbances in the fibre.

‘Say you want to rotate polarisation by one degree,’ said Bharat Acharya, the study’s lead author and a post-doctoral contract physicist working at Bell Labs as part of the US National Science Foundation’s academic-liaison-with-industry program. ‘In our approach, you initially apply an electrical ‘overdrive pulse’ that is oriented to turn the liquid crystal molecules by 70 degrees, but then you immediately stop the pulse after the molecules have rotated by only one degree.

‘In this way, the molecules rotate by one degree much faster than if you had applied a pulse with the same speed to turn them by only one degree.’

Successful tests of this device, performed in collaboration with Lothar Möller, another physicist at Bell Labs, are said to have generated considerable interest. Bell Lab scientists are working with industrial partners to explore the manufacture and commercialisation of these devices.