Atoms are the building blocks of all materials. Therefore, the common sense suggests that microscopic devices cannot be fabricated with the precision better than an angstrom, the size of an atom. However, the performance of optical microdevices is usually determined by the average position of a very large number of atoms. The progress in measurement technologies allows this average to be determined with the ground-breaking picometre (one hundredth of the atomic size) precision. It has recently been recognized that similar picometre precision may become a must for the fabrication of a range of emerging photonic microdevices promising to revolutionize computer, communication, and sensing technologies. However, the problem of robust and scalable fabrication of microdevices with such astonishing precision remains open since major modern manufacturing technologies have achieved a precision plateau of several nanometres (tens of angstroms). SNAP (Surface Nanoscale Axial Photonics), a unique technology invented by the Principal Investigator of this project, allows the fabrication of miniature photonic devices at the surface of an optical fibre with unprecedented subangstrom precision. In contrast to the propagation of light in regular optical fibres, in SNAP devices, light is spiralling along the perimeter of the fibre and slowly propagating along its length. Recently, we demonstrated new SNAP fabrication methods and proposed unique microdevices for applications in communications, optical signal processing, and ultraprecise sensing. However, the SNAP devices demonstrated to date have been the products of breakthrough experiments. To bring these devices to realistic applications and further increase their precision, it is necessary to develop a robust manufacturing process attaining both ultra-accurate reproducibility and scalability. The goal of this project is the development of this process, which requires the insight into the depth of associated physical phenomena, as well as the design and fabrication of new microdevices critical for the future communication, optical signal processing, microwave, and sensing technologies. We will (i) develop a technology for scalable manufacturing of microphotonic devices with unprecedented picometre-scale precision and (ii) demonstrate SNAP microdevices including miniature optical delay lines, dispersion compensators, frequency comb generators, microwave photonics filters, as well as optical microfluidic sensors and manipulators with outstanding performance for applications ranging from food industry to fundamental science. If successful, this project will not only bring in a new revolutionary technology but also deliver miniature optical devices with performance not previously possible to achieve and ready for practical applications. We envision a high need for the miniature optical devices we plan to design and fabricate in this project in future applications in the information and communication technologies, precise manufacturing, microwave, and sensing technologies. Lack of reliable, scalable manufacturing processes with the required picometre precision remains a major obstacle for their mass manufacturing. SNAP devices, which we plan to fabricate in this project, have real potential to address this need due to their highest to date precision and exceptional performance. We anticipate that the developed robust, unprecedently precise, and scalable manufacturing process with the UK-owned IP, as well as miniature optical devices we plan to deliver, will have broad industrial, scientific, and social impact centred in the UK.

Over the last decade, much interest of scientists and engineers working in optics and photonics has been attracted to the research and development of miniature devices based on the phenomenon of slow light. The idea of slow light consists in reducing its average speed of propagation by forcing light to oscillate and circulate in specially engineered microscopic photonic structures (e.g., photonic crystals and coupled ring resonators). Researchers anticipated that slow light devices will have revolutionary applications in communications, optical and radio signal processing, quantum computing, sensing, and fundamental science. For this reason, the research on slow light has been conducted in many academic laboratories and industrial research centres including telecommunications giants IBM, Intel, and NTT. However, in spite of significant progress, it had been determined that current photonic fabrication technologies are unable to produce practical slow light devices due to the major barriers: the insufficient fabrication precision and substantial attenuation of light. To overcome these barriers, this project will develop a new photonic technology, Surface Nanoscale Axial Photonics (SNAP) which will allow us to demonstrate miniature photonics devices with unprecedentedly high precision and low loss. SNAP is a new microphotonics fabrication platform invented by the PI of this project. In contrast to previously considered slow light structures based on circulation of light in coupled ring resonators and oscillations photonic crystals, the SNAP platform employs whispering gallery modes of light in an optical fibre, which circulate near the fibre surface and slowly propagate along its axis. The speed of axial propagation of these modes is so slow that it can be fully controlled by dramatically small nanoscale variations of the fibre radius. This project will develop the advanced SNAP technology for fabrication of ultraprecise, ultralow loss, tuneable, switchable and fully reconfigurable miniature slow light devices establishing the groundwork for their revolutionary applications in future Information and Communication Technologies. The success of the project will place the UK in the centre of this revolutionary development.

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.

It is recognised that global communication systems are rapidly approaching the fundamental information capacity of current transmission technologies. Saturation of the capacity of the communication systems might have detrimental impact on the economy and social progress and public, business and government activities. The aim of the proposed research is to develop, through theory and experiment, disruptive approaches to unlocking the capacity of future information systems that go beyond the limits of current optical communications systems. The research will combine techniques from information theory, coding, study of advanced modulation formats, digital signal processing and advanced photonic concepts to make possible breakthrough developments to ensure a robust communications infrastructure beyond tomorrow. Increasing the total capacity of communication systems requires a multitude of coordinated efforts: new materials and device bases, new fibres, amplifiers and network paradigms, new ways to generate, transmit, detect and process optical signals and information itself - all must be addressed. In particular, the role of fibre communications, providing the capacity for a lion share of the total information traffic, is vital. One of the important directions to avoid the so-called "capacity crunch", the exhaust in fibre capacity - is to develop completely new transmission fibres and amplifiers. However, there is also a growing need for complimentary actions - innovative and radically novel approaches to coding, transmission and processing of information. Our vision is focused on the need to quantify the fundamental limits to the nonlinear channels carried over optical fibres and to develop techniques to approach those limits so as to maximise the achievable channel capacity. The information capacity of a linear channel with white Gaussian noise is well known and is defined by the Shannon limit. Wireless systems can approach this limit very closely - to within fractions of a dB. However, the optical channel is nonlinear. Fibre nonlinearity mixes noise with signal. Therefore, results of the linear theories on capacity can be applied in fibre channels only in the limit of very small nonlinear effects. Optical communication systems are undergoing another revolution with the development of techniques of coherent detection, the ability to detect both the amplitude and the phase of a transmitted signal and use of digital signal processing techniques to reconstruct the original signal. Use of the optical phase in emerging coherent transmission schemes opens up fundamentally new theoretical and technical possibilities most as yet unexplored. The challenge is to understand to what degree optical nonlinearity can also be compensated or, indeed, used to unlock the fibre capacity, maximise both the information transmission rate and the total bandwidth, to determine the fundamental Shannon limit for nonlinear channels and to develop methods to approach this capacity. We propose to explore fundamentally new nonlinear information technologies and to develop a practical design framework based on integration of DSP techniques, novel modulation formats, and novel source and line coding approaches tailored to the nonlinear optical channels. We believe this to be the key to designing the intelligent information infrastructure of the future.

Optical networks underpin the global digital communications infrastructure, and their development has simultaneously stimulated the growth in demand for data, and responded to this demand by unlocking the capacity of fibre-optic channels. The work within the UNLOC programme grant proved successful in understanding the fundamental limits in point-to-point nonlinear fibre channel capacity. However, the next-generation digital infrastructure needs more than raw capacity - it requires channel and flexible resource and capacity provision in combination with low latency, simplified and modular network architectures with maximum data throughput, and network resilience combined with overall network security. How to build such an intelligent and flexible network is a major problem of global importance. To cope with increasingly dynamic variations of delay-sensitive demands within the network and to enable the Internet of Skills, current optical networks overprovision capacity, resulting in both over- engineering and unutilised capacity. A key challenge is, therefore, to understand how to intelligently utilise the finite optical network resources to dynamically maximise performance, while also increasing robustness to future unknown requirements. The aim of TRANSNET is to address this challenge by creating an adaptive intelligent optical network that is able to dynamically provide capacity where and when it is needed - the backbone of the next-generation digital infrastructure. Our vision and ambition is to introduce intelligence into all levels of optical communication, cloud and data centre infrastructure and to develop optical transceivers that are optimally able to dynamically respond to varying application requirements of capacity, reach and delay. We envisage that machine learning (ML) will become ubiquitous in future optical networks, at all levels of design and operation, from digital coding, equalisation and impairment mitigation, through to monitoring, fault prediction and identification, and signal restoration, traffic pattern prediction and resource planning. TRANSNET will focus on the application of machine techniques to develop a new family of optical transceiver technologies, tailored to the needs of a new generation of self-x (x = configuring, monitoring, planning, learning, repairing and optimising) network architectures, capable of taking account of physical channel properties and high-level applications while optimising the use of resources. We will apply ML techniques to bring together the physical layer and the network; the nonlinearity of the fibres brings about a particularly complex challenge in the network context as it creates an interdependence between the signal quality of all transmitted wavelength channels. When optimising over tens of possible modulation formats, for hundreds of independent channels, over thousands of kilometres, a brute force optimisation becomes unfeasible. Particular challenges are the heterogeneity of large scale networks and the computational complexity of optimising network topology and resource allocation, as well as dynamical and data-driven management, monitoring and control of future networks, which requires a new way of thinking and tailored methodology. We propose to reduce the complexity of network design to allow self-learned network intelligence and adaptation through a combination of machine learning and probabilistic techniques. This will lead to the creation of computationally efficient approaches to deal with the complexity of the emerging nonlinear systems with memory and noise, for networks that operate dynamically on different time- and length-scales. This is a fundamentally new approach to optical network design and optimisation, requiring a cross-disciplinary approach to advance machine learning and heuristic algorithm design based on the understanding of nonlinear physics, signal processing and optical networking.