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

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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.