The development of energy-efficient, ultra-high capacity communication networks capable of connecting people and businesses seamlessly everywhere is one of the most important challenges facing modern society. The traffic on the global communications infrastructure keeps rapidly increasing, typically at a rate of 40% per annum, driven by the communication services applications that are drastically increasing in number ( e.g. Twitter, YouTube, Facebook, etc.) and demand on bandwidth (e.g. HDTV, 3D,...). This continuously increase of available bandwidth/ capacity in a single optical fibre has been ensured by the enormous progress in optical communication systems over the years (e. g. employing many wavelengths or new types of complex modulated signals). When transmitting these new types of signals, current networks need to change their corresponding hardware (e. g. implementing new transmitters and receivers). However, the ideal network should handle them at no extra hardware cost. The project SPEED proposes to investigate all-optical solutions that are compatible with the existing fibre technology and functionalities to guarantee that the network can handle signals that may be developed and used in coming years. This will guarantee that the consumer can continue enjoying new bandwidth-hungry services that have being offered at no extra cost. So far, the proposed all-optical solutions are mainly based on coherent mixing in single mode nonlinear media. SPEED aims to develop a new nonlinear platform technology, the multi-mode one, with the vision to propose novel low-cost and energy-efficient solutions for the future-proof upgradable transmission systems discussed above. A detailed study will be conducted to demonstrate the advantages and the enhanced functionality offered by multi-mode nonlinear platform as compared to the widely developed single-mode one in a variety of disruptive applications in many key areas. The added degree of freedom given by the spatial dimension of few-mode waveguides will improve the system performance, mainly in terms of broadband operation and noise by a factor that is proportional to the number of modes.

The aim of this fellowship is to develop disruptive approaches through theory and experiment to unlock the capacity of future information systems. To go beyond current channel limits is arguably the greatest challenge faced by digital optical communications. To target it, the proposed research will combine techniques from information theory, coding, higher-dimensional modulation formats, digital signal processing, advanced photonic design, and machine learning to make possible breakthrough developments to ensure a robust communications infrastructure beyond tomorrow. Optical communications have to-date been able to fulfil the ever-growing data demand whilst simultaneously reducing cost and energy-per bit. However, it is now recognised that systems are rapidly approaching the fundamental information capacity of current transmission technologies, a trend with potential negative impact on the economy and social progress. To meet future demands with prospective cost and energy savings and avoid the impending exhaust of fibre capacity, the only solution is the emergent technology of spatial division multiplexing (SDM). It provides much wider conduits of information by offering additional means for transporting channels over one single fibre, using multi-mode and multi-core fibres. However, SDM has not yet found a viable path to access this much higher information capacity. State-of-the-art SDM transceivers are only compatible with few-mode/few-core fibres (~10 paths) given the requirement to multiplex/demultiplex over all the fibre pathways to successfully estimate and unravel pathways crosstalk and walk-off. This completely defeats SDM's purpose, the installation of new fibres must allow for several decades of capacity growth to offset the high deployment costs of new cables. This fellowship envisages how to transform SDM technology to drive future optical networks by addressing the key issue overlooked by the research community since the introduction of SDM concepts: optical transceivers must undergo >100-fold integration to enable the benefits of multi-mode/core. Focus on new transceivers capable of digital space modulation will enable scalability of all data pathways to reduce the cost and energy-consumption per bit. Digital spatial modulation in novel coherent transmission schemes, i.e. the pathway index itself is used to carry information, will open fundamentally new theoretical and experimental possibilities up to now unexplored. These new transceivers will be capable of exploiting the multidimensional channel properties in the linear and nonlinear regimes through new spatial modulation formats and coding guided by new information theory and nonlinear science methods. Two main challenges are to construct a high-speed digital spatial modulator capable of dynamically addressing different groups of paths (potentially with tens of paths) in massive multi-path fibres and to develop new learning algorithms (guided by new theory methods) suitable of being embedded in spatial-adaptable transceivers to reach the ultimate capacity of nonlinear multi-dimensional channels.

The remarkable success of the internet is unquestioned, touching all aspects of our daily lives and commerce. This success is fundamentally underpinned by the tremendous capacity of unseen underground and undersea optical fibre cables and the technologies associated with them. Indeed, the initial surge in web usage in the mid 1990s coincides with the commissioning of the first optically amplified transatlantic cable network, TAT12/13 that allowed ready access to information otherwise inaccessible. Similarly, the remarkable growth of social media is supported by the introduction of optical fibres into data centres, allowing their tremendous growth. Exponential growth has been a characteristic of data communications since their first introduction in the 1970's and has been fuelled by the gradual introduction of radical technologies, such as optical amplification, wavelength-division multiplexing and coherent modulation. All of these technologies are today routinely deployed and it is widely acknowledged that fibres are becoming full. The limit to fibre capacity has its origin in the fact that the intense signals are significantly distorted by nonlinearly (a similar effect to overdriving loudspeakers). This distortion limits the maximum amount of information which may be transmitted across and optical fibre link, and unless combated, the nonlinear response will result in a capacity crunch, limiting access to the internet to today's levels. Faced with the ongoing exponential growth in demand, unless these restrictions are lifted many parallel systems will be required, resulting in exponentially increasing energy consumption, until the cost of this resource becomes prohibitive and finally curtails growth. Only one technology, optical phase conjugation (acting like a mirror for colours), has been shown to offer the prospect of supporting continued internet growth without the need for widespread use of multiple fibres and the associated growth in energy consumption. Very much like Newton's Prisms, optical phase conjugation allows the distortion of one fibre (analogous to spectral spreading in Newton's prisms) to be compensated by a second identical fibre. In PHOS, we will - Optimise the devices which perform this conjugation, both in terms of the assessment of fundamental nonlinear materials and in terms of optimised sub-system configuration. - Demonstrate orders of magnitude increase in the capabilities of optical fibres for both practical point-to-point links with non-uniform span lengths and for optical networks with a plethora of diverse routes. - Verify that the use of optical phase conjugation is cost effective, both in terms of reducing the cost of a network deployment compared to existing products and in terms of enhancing the service provided to customers through higher capacity with lower latency. Furthermore, as optical phase conjugation will transform the capabilities of the network, PHOS will work to remove bottlenecks within the network transmitters and receivers, increasing their performance by an order of magnitude, resulting in 10 times faster connections. The approach of compensating impairments in the optical domain, combined with simplified digital signal processing and enhanced exploitation of fibre bandwidth will reduce the cost, size and power consumption associated with providing 10's of Tbit/s of capacity per optical fibre. If successful, PHOS will enable massively increased data capacities from the employment of Optical Phase Conjugation, giving the UK the most advanced optical communication network and a strong position to become a leading supplier of the technology worldwide.

Technologies underpin economic and industrial advances and improvements in healthcare, education and societal and public infrastructure. Technologies of the future depend on scientific breakthroughs of the past and present, including new knowledge bases, ideas, and concepts. The proposed international network of interdisciplinary centre-to-centre collaborations aims to drive scientific and technological progress by advancing and developing a new science platform for emerging technology - the optical frequency comb (OFC) with a range of practical applications of high industrial and societal importance in telecommunications, metrology, healthcare, environmental applications, bio-medicine, food industry and agri-tech and many other applications. The optical frequency comb is a breakthrough photonic technology that has already revolutionised a range of scientific and industrial fields. In the family of OFC technologies, dual-comb spectroscopy plays a unique role as the most advanced platform combining the strengths of conventional spectroscopy and laser spectroscopy. Measurement techniques relying on multi-comb, mostly dual-comb and very recently tri-combs, offer the promise of exquisite accuracy and speed. The large majority of initial laboratory results originate from cavity-based approaches either using bulky powerful Ti:Sapphire lasers, or ultra-compact micro-resonators. While these technologies have many advantages, they also feature certain drawbacks for some applications. They require complex electronic active stabilisation schemes to phase-lock the different single-combs together, and the characteristics of the multi-comb source are not tuneable since they are severely dictated by the opto-geometrical parameters of the cavity. Thus, their repetition rates cannot be optimised to the decay rates of targeted samples, nor their relative repetition rates to sample the response of the medium. Such lack of versatility leads to speed and resolution limitations. These major constraints impact the development of these promising systems and make difficult their deployment outside the labs. To drive OFC sources, and in particular, multi-comb source towards a tangible science-to-technology breakthrough, the current state of the art shows that a fundamental paradigm shift is required to achieve the needs of robustness, performance and versatility in repetition rates and/or comb optical characteristics as dictated by the diversity of applications. In this project we propose and explore new approaches to create flexible and tunable comb sources, based on original design concepts. The novelty and transformative nature of our programme is in addressing engineering challenges and designs treating nonlinearity as an inherent part of the engineering systems rather than as a foe. Using the unique opportunity provided by the EPSRC international research collaboration programme, this project will bring together a critical mass of academic and industrial partners with complimentary expertise ranging from nonlinear mathematics to industrial engineering to develop new concepts and ideas underpinning emerging and future OFC technologies. The project will enhance UK capabilities in key strategic areas including optical communications, laser technology, metrology, and sensing, including the mid-IR spectral region, highly important for healthcare and environment applications, food, agri-tech and bio-medical applications. Such a wide-ranging and transformative project requires collaborative efforts of academic and industrial groups with complimentary expertise across these fields. There are currently no other UK projects addressing similar research challenges. Therefore, we believe that this project will make an important contribution to UK standing in this field of high scientific and industrial importance.