At the beginning of February, 2015, a new technique to process fiber optic signals was published by UCL(London’s Global University) researchers. This new technique can double the distance at which data travels error-free through transatlantic sub-marine cables. There is no doubt that it is a new milestone of fiber-optic communications. During the past two months, this new technique was reproduced or discussed by people in many communities or news sites of fiber-optic communication industry. However, we may not know much about it and still have doubts with it. How can it achieve the performance of doubling the distance of fiber-optic communications? And will this new technique greatly change the fiber-optic communication? Today, we are going to talk something about it.
A Brief Introduction
With the exponential growth in communications, caused mainly by the wide acceptance of the Internet, people’s demands of bandwidth and large data capacity have increasingly grown. However, from a technical perspective, fiber attenuation, dispersion and nonlinearity can significantly limit the bit rate and the spanning distance of the optical communication. With the improvement of fiber manufacturing and the invention of EDFA(erbium-doped fiber amplifier), the war against attenuation has won, while dispersion and nonlinearity are still taken into main consideration in today’s high speed optical communication systems. The achievable transmission capacity of conventional fiber-optic communication systems is limited by nonlinear distortions due to the Kerr effect and the difficulty in modulating the optical field to effectively use the available fiber bandwidth. According to the research, due to the impact of fiber nonlinearities, the capacity increases in wavelength division multiplexing (WDM) research have slowed to approximately 20% per year over the last decade. However, the fundamental limit to the nonlinear channel is still unknown to this date that means it is still a subject of investigation. Therefore, recent research on maximising the single fiber core capacity has focused on increasing the information spectral density of each WDM channel, while simultaneously employing advanced coding and fiber nonlinearity mitigation techniques to maximise the achievable transmission distance.
Nowadays, the widely used techniques to increase information spectral density (ISD) of an optical network is to use advanced modulation formats with high cardinality or to reduce the frequency spacing between WDM channels. However both techniques are accompanied by significant limitations. Increasing the cardinality of the modulation format comes at the expense of requiring a higher signal to noise ratio (SNR), which places stringent demands on the transmitter and receiver subsystems. Alternatively, as the frequency spacing between WDM channels is reduced, inter-channel interference begins to cause significant performance penalties due to linear crosstalk. Although tight filtering can be employed to constrain the bandwidth (BW) of each WDM channel, the filtering process itself results in significant inter-symbol interference (ISI) within each channel. However, if an appropriate filter shape is used, for example a sinc shaped pulse with a corresponding rectangular spectrum, then the Nyquist criterion for ISI can be met.
In order to achieve a high ISD, while simultaneously maintaining transmission reach, multi-channel fiber nonlinearity compensation and spectrally efficient data encoding must be utilized. In this work, the researchers used a “16QAM super-channel” made of a set of frequencies which could be coded using amplitude, phase and frequency to create a high-capacity optical signal. Effective nonlinearity mitigation is achieved using multi-channel digital back-propagation (MC-DBP) and this technique is combined with an optimized forward error correction implementation to demonstrate a record gain in transmission reach of 85%; increasing the maximum transmission distance from 3190 km to 5890 km, with an ISD of 6.60 b/s/Hz. From 3190 km to 5890 km, it is a breakout to double the transmission distance of fiber-optic communication. We can expect that this new method has the potential to reduce the costs of long-distance fiber-optic communications as signals wouldn’t need to be electronically boosted on their journey, which is important when the cables are buried underground or at the bottom of the ocean. This is why this new technique has been highly concerned. For wider range of applications of this research result, the researchers will test their new method on denser super-channels commonly used in digital cable TV (64QAM), cable modems (256QAM) and Ethernet connections (1024QAM).
Applications & Prospects
People always strive to solving the limitation of fiber-optic long-haul transmission. It is no doubt that, the WDM system is the main trend. However, due to the impact of fiber nonlinearities, the development of WDM system has to slowed down. Additionally, the cost of WDM system is still quite expensive. These factors limit the optical long-haul transmission and make nowadays fiber-optic communication not satisfy the increasing demands of people. As the technique can correct the transmitted data if they are corrupted or distorted on the journey, it could also help to increase the useful capacity of fibers. This is done right at the end of the link, at the receiver, without having to introduce new components within the link itself. Increasing capacity in this way is important as optical fibers carry 99% of all data and demand is rising with increased use of the internet, which can’t be matched by the fibers’ current capacity, and changing the receivers is far cheaper and easier than re-laying cables, especially useful for the cables buried underground or at the bottom of the ocean. This new finding will greatly improve the efficiency of fiber-optic communications by doubling the transmission distances. However, it’s just the beginning. We may face many challenges when using this new technique in further applications. And among them, overcoming the capacity limits of optical fibers cables is a large part of solving that problem. Despite there are many difficulties, doubling the distance of fiber-optic communications is no longer a dream and it is becoming closer to us. We believe that one day, there are more advanced techniques demonstrated and applied.