Ultra-wideband and Space-division Multiplexed Optical Transmission Systems

Over the past thirty years, research on data transmission using optical fibres has enabled the vast rise of the Internet, which would drastically change our society. By exploiting the wide bandwidth of optical fibres, employing both polarisations of the light and using advanced modulation techniques supported by coherent detection, the per-fibre throughput has grown exponentially. In fact, over the last thirty years, the per-fibre throughput has seen tremendous growth, reaching the order of 100 Tb/s, almost a factor 10,000 higher compared to thirty years ago. These data rates are close to the theoretically predicted maximum capacity of a standard single-mode optical fibre. On the other hand, the demand for transmission capacity is still exponentially increasing, requiring the development of novel optical transmission technologies.

Two potential candidates for next-generation optical networks are ultra-wideband (UWB) transmission and space-division multiplexing (SDM). For UWB transmission, the entire available bandwidth inside an optical fibre is used, supported by the recent development of novel optical amplification techniques. The capacity increase of UWB transmission is limited to an order of magnitude of 10 times. However, it allows upgrading currently deployed fibre links, as the used optical fibre stays the same. For SDM transmission, deployment of new fibre types is required, but increases in capacity of 100 times have already been demonstrated. For SDM transmission, optical fibres are altered to have multiple spatial paths inside a single shared cladding. These spatial paths can be either multiple cores, spatial modes, or a combination thereof.

This thesis presents a broad overview of high-speed optical transmission systems and presents and demonstrates novel transmission techniques to enable next-generation state-of-the-art ultra-high-capacity transmission links. As it is essential to increase the per-fibre capacity and reduce the cost-per-bit simultaneously, novel approaches to reduce the hardware complexity of a transmission link are also discussed. An optical transmitter employing a digital resolution enhancer is demonstrated to reduce the hardware requirements of an optical transmitter, and an optical receiver based on the Kramers-Kronig (KK) coherent detection scheme is presented and demonstrated to work with SDM transmission, reducing the receiver hardware complexity. As both UWB and SDM transmission require new components and subsystems, novel switches, wavelength-selective switches and amplifiers have been characterised and evaluated. Furthermore, an extensive set of state-of-the-art experiments demonstrates the potential of UWB and SDM transmission. For short-distance transmission, terabit-per-wavelength transmission is shown over a link of 130 km consisting of different types of multi-mode fibre. A record capacity of 1.7 Pb/s has been demonstrated by transmitting signals in the C- and L-band over the first randomly-coupled multi-core fibre with 19 cores. Long-distance transmission experiments using recirculating loops illustrate the application of UWB and SDM for long-haul systems. Transmission of 273.6 Tb/s over 1001 km of 15-mode fibre is established, resulting in the record highest data rate for long-haul multi-mode transmission and a record-high capacity-distance product. The potential of UWB and SDM has been combined in a 4-core recirculating loop experiment, where every core contained signals spanning the S-, C-, and L-band, resulting in the transmission of 138.9 Tb/s over a distance of 12,345 km, resulting in the highest-ever measured capacity-distance product in standard cladding diameter fibres. Finally, the extremes of multiplexing in both space and wavelength have been investigated. Transmission of S/C/L-band signals over a few-mode multi-core fibre resulted in a data rate of 22.9 Pb/s, the highest data rate ever measured in a single fibre when writing this thesis.

In summary, this thesis provides novel state-of-the-art transmission techniques that have the potential to enable the continuation of the exponential growth in per-fibre data throughput, enabling next-generation ultra-high-capacity links in the coming decades.