Coherent Optical SIgnals for extremely high-capacity NEtworks (COSINE)

Lead Research Organisation: University College London
Department Name: Electronic and Electrical Engineering

Abstract

High-speed fibre-optic cables link cities, countries and continents across the globe, underpinning the Internet and the fixed and mobile phone networks that enable and enrich our lives today. Historically, increasing the overall data rate transmitted on a single optical fibre has dramatically reduced the cost of data transmission, and this is one factor that has enabled high data rate connections to be available at reasonable cost to end users. As services like social networking, music downloads, and video-on-demand capture the public's attention, they in turn create demand for greatly increased capacity on these networks. We can expect this cycle to continue as the installed fibre capacity is pushed to its limit.To achieve high transmission capacity on a single fibre, data is transmitted on several wavelength channels (wavelength division multiplexing, WDM). For compatibility with the components used in existing systems, and to avoid having to manage a huge number of wavelengths, it is preferable to increase the amount of data transmitted by increasing the data rate per channel, rather than by packing the wavelength channels closer and closer together. WDM networks with 40 Gb/s line rate are now being deployed, and there is currently considerable activity directed towards research and standardisation of 100 Gb/s Ethernet (100 GE) on a single wavelength.By extending the approach proposed for 100 GE by using advanced modulation schemes like those used in wireless communications, it may be possible to squeeze data transmission rates of several hundred Gb/s onto each wavelength, but the technological challenges posed will be significant. To move beyond this - towards 1 Tb/s (1,000 Gb/s) per wavelength - will require new techniques.In this work, we will investigate one approach to achieving this, which also eases some of the stringent demands on the optical transmitters and receivers imposed by current methods. Each wavelength channel will be divided into a number of sub-channels, and advanced modulation formats used to transmit data at a high rate in the narrow spectral band of each sub-channel. It will be necessary to generate the optical signals that define the sub-channels at the transmitter efficiently and cost-effectively, and to produce synchronised optical signals at the receiver to recover the data. To do so, we will generate all the sub-channels at the transmitter from a single laser that defines the frequency of the overall channel, and we will use one sub-channel to transmit information to allow an identical set of optical signals for channel demodulation to be created at the receiver. In this way, the sub-channels are synchronised (phase locked) to each other within the overall channel, as are the transmitter and receiver. This means that the sub-channels can be packed as closely together as possible and behave as a single unit, while recovering the data at the receiver is simplified.By this means we expect to increase the overall fibre transmission capacity to 135 Tb/s, more than an order of magnitude greater than the current state of the art for commercial long haul transmission systems. The work will mainly be carried out experimentally, investigating the key technical elements of the proposal in stages before combining them to show that the full scheme could deliver the anticipated increase in transmission capacity if fully implemented. Areas that will be examined include new ways of generating phase-locked sub-channels at the transmitter; methods for generating and synchronising the corresponding optical signals at the receiver; and modulation and de-modulation techniques giving high data rate transmission in a narrow spectral band. The experimental demonstration will be supported by computer simulations of the system, which will also allow new applications enabled by the approach - too advanced to be demonstrated experimentally at this stage - to be investigated.

Planned Impact

The proposed work is focused on the goal of increasing the capacity of fibre-optic networks, particularly the high-speed backbone that underpins the Internet. As such, it is highly applied engineering research with the aim of having potential for significant impact on society in the longer term. Historically, improvements in the capacity of the physical layer of telecommunication networks have driven down cost by maximising the utilisation of expensive infrastructure - the installed optical fibre network. This has enabled the development and low-cost delivery of the feature-rich internet and telecommunications services that we have come to take for granted. Such benefits would also be expected to follow from this work if it is successful, and subsequently exploited commercially, to the direct advantage of network providers and end users alike, by reducing the cost to them of network provision and access respectively, and by enabling delivery of services that are not currently cost-effective - or those that have yet to be imagined. Lower cost availability of higher capacity networks can be expected to have a significant impact on life, both in the UK and globally, by encouraging further development of the digital society. Economic performance and competitiveness will be advanced through e-commerce and improved global communications, education and training will be improved through enhanced e-learning opportunities, and quality of life will be improved by the provision of new services, such as higher quality online entertainment. By facilitating the provision of immersive virtual presence, improvements in network capacity can even be expected to contribute to overcoming the challenge facing humanity from global warming, by reducing travel and the associated carbon dioxide emissions. The most immediate beneficiaries of the proposed work are expected to be members of the research community working in the field of optical fibre communications. Industry visionaries have recently challenged this community to consider how the capacity of a single optical fibre might be increased to ten times that currently being standardised (from 100 Gb/s per wavelength to 1 Tb/s per wavelength). The proposed work represents one view of how this goal might be achieved, and will contribute to what can be expected to be a vibrant debate over the next few years, as alternative approaches are presented and demonstrated, until eventually one solution becomes accepted as a standard. Exploitation of the proposed research, if adopted and developed by telecommunications equipment manufacturers, would directly benefit these companies by contributing to sales of optical equipment, the global market for which was around US$15 billion in 2009. To facilitate this knowledge transfer the proposal partners with Oclaro, one of the world's leading suppliers of optical technology for communications and u2t Photonics UK Ltd., a supplier of advanced optical modulators. The timescale anticipated for impact to be observable is within a decade. In addition to the beneficiaries identified above, knowledge gained through the project is likely to enable us to engage with businesses operating in related areas through collaboration or consultancy. An example of this from our previous work on coherent optical receivers using optical phase locking is the assistance we have been able to give to u2t Photonics UK Ltd. on testing prototype devices aimed at complex modulation formats. Following intellectual property rights protection where necessary, engagement with the potential beneficiaries will be done in the first instance by publication of the results of the research in leading peer-reviewed journals (such as Optics Express, IEEE Journal of Lightwave Technology) and by presentations at important international technical conferences (e.g. OFC/NFOEC, ECOC). It is expected that this will lead to opportunities for more direct discussion and collaboration.
 
Description Central to the research hypothesis and goals of the COSINE project was the use of optical comb sources to generate the ultra-high-capacity and highly spectrally efficient optical communication systems required to keep the development of this vital underpinning technology for today's Internet on track. The idea was that, by using comb sources, several lower rate channels could be combined to form a superchannel with overall capacity approaching 1 Tb/s. This concept, while not completely novel when the project was conceived, was also far from being mainstream at that time. However, over the project's duration the comb-based superchannel transmitter was widely taken up for systems with per-channel bit rates above 200 Gb/s.

The COSINE project contributed to this rapid and worldwide acceptance of the superchannel approach through numerous publications in the academic literature and at scientific conferences. This published work covers many of the anticipated activities of the project, from developing comb generation techniques and analysing their performance to investigating algorithms required for the sophisticated digital signal processing (DSP) techniques used to recover the data at the receiver. As part of the project, we also extended our investigations of 'coolerless' semiconductor laser sources, which may lead to systems with lower power consumption, a major and increasing challenge.

We also made progress with our goals of making use of the well-defined phase relationships between lines in an optical comb, demonstrating that a receiver (local oscillator) comb could be generated that is phase locked to the transmitter comb. However, for most applications currently investigated, this does not appear to offer significant benefits over using multiple receiver lasers or unlocked combs. Full phase locking introduces significant complexity, while comb sources and stable lasers allow frequency errors to be reduced to levels that can be easily corrected in the receiver DSP. In addition, receiver technology and bandwidths have moved on, making it possible to consider capturing the whole superchannel in a single receiver.
Exploitation Route As noted, results from COSINE and similar projects around the world have led to the rapid adoption of comb-based techniques, enabling academic groups to take the concepts further, with the UCL Optical Networks Group (one of the COSINE co-investigators) reporting the highest throughput (1.125 Tb/s) ever recorded using a single coherent receiver, just a year after the close of the COSINE project. In the future, we can expect these approaches to be picked up and developed into commercial products by communications equipment manufacturers.
Sectors Digital/Communication/Information Technologies (including Software)

 
Description This grant pioneered the use of optical frequency combs including the super-channel concept in optical communications. Several industrial research groups, worldwide, have applied these concepts to the design of the next generation coherent optical communications that will underpin the future internet. Further developments of the concepts researched has led to the development of "Flexgrids" an approach to have channels of variable bandwidth tailored to capacity required for a particular service, rather than fixed spacing wavelength division multiplex. The new approach increases fibre utilisation enabling increased capacity within the existing deployed fibre network.
First Year Of Impact 2015
Sector Aerospace, Defence and Marine,Digital/Communication/Information Technologies (including Software),Electronics
Impact Types Societal,Economic