To keep optical communications technology, which carries 99% of global data traffic, ahead of the projected growth in demand for bandwidth, researchers have turned their attention to tackling nonlinear distortions of signals travelling in fibre cables which is claimed to be the main cause for the continued expansion of data transmission rates. The researchers used a technique called ‘optical phase conjugation’ (OPC) to reduce the undesirable effect of nonlinearities in a high-capacity transmission link. The result was a 60% increase in the distance which information could travel and still be successfully received.
OPC is a method in optics to undo distortions in optical waves introduced by the medium in which they travel. It has been known for several decades but until now it had not been tested in optical communication systems with data rates on the order of Tbyte/s, which are expected for the next generation of communications technology. In this study the researchers used a single OPC device in a 2000km fibre link to show for the first time, in systems with data rates from 800Gbytes/s to 4Tbytes/s, that OPC can successfully deal with nonlinearities, improving signal quality and allowing information to travel further.
The main drawback of OPC is that it requires a symmetry in the transmission link which is said to be hard to achieve. This symmetry is needed because an OPC device manipulates optical signals in a way which allows them to “erase” distortions and noise accumulated in the first half of the link as they travel through the second. “This is identical to Netwon’s prisms, where the first of two identical prisms spreads white light out into a rainbow, and the second inverted prism collects the colours back together again. However, since we don’t yet know how to build an ‘inverted fibre’ we need to use a special ‘mirror’, the optical phase conjugator, to perform this trick,” explained Professor Andrew Ellis, who led the experiment.
Current commercial optical communications systems use amplifiers at regular intervals to boost the signal as it travels through the fibre. As they amplify the signal they break the symmetry in a transmission link crucial for OPC to operate effectively. However, as optical amplification technology improves and new types of components like Raman amplifiers enter the market, it is expected that the symmetry challenge will be largely resolved.
Prof Ellis added: “Some product development is clearly needed, but we are now really only waiting for the market demand to catch up with the technology, which it will within a couple of product cycles.”
