The road to ultra-high-capacity optical-based Internet communications

In recent decades, research on data transmission using optical fibres has enabled exponential growth of Internet traffic. Today, optical fibres are used to connect cities, countries, and continents. As more than 99.9% of all Internet traffic is at some point transported over optical fibres, making them the backbone of our communication and information-driven society, and enabling video streaming, cloud computing, and 5G mobile access.

For his PhD thesis, Menno van den Hout explored how future ultra-high-capacity optical communication systems could be implemented. To this end, two research directions were explored: ultra-wideband transmission and space-division multiplexing.

Ultra-wideband transmission and space-division multiplexing

Current optical transmission systems exploit several physical dimensions available to an optical fibre: the amplitude, phase, wavelength, and polarisation of light are used to increase the capacity.

Ultra-wideband transmission aims to increase the number of different wavelengths that can be multiplexed onto a single fibre by employing new optical amplifiers. This benefits network operators, as only terminal equipment needs to be replaced, and the costly deployment of new fibres is unnecessary. On the contrary, the gains in capacity are limited to 5 to 10 times.

Space-division multiplexing (SDM) uses an optical fibre's last unexploited physical dimension: space. By altering the glass profile of the fibre, novel fibres can guide multiple modes, have multiple cores, or have a combination thereof. As a result, SDM fibres can increase capacity over standard single-mode fibres by orders of magnitude. However, this requires network operators to install new fibres and the required digital signal processing (DSP) complexity at the receiver increases.

Key highlights

Transmission over long distances using multi-mode fibres is challenging due to propagation effects occurring in the fibre. Therefore, long-distance transmission using multi-mode fibres used to be limited to six modes.

In his research, van den Hout demonstrated for the first-time transmission using 15 modes over a distance of 1001 km yielded a data rate of 273.6 terabit/s. Specially designed mode mixing techniques allowed the receiver DSP complexity to be reduced.

Moreover, a novel fibre with nineteen cores (compared to one core for standard fibre) was used to demonstrate a data rate of 1.7 petabit/s over 64 km, the highest data rate ever measured in a standard-sized fibre. The fibre properties of the 19-core fibre were also beneficial, requiring less DSP complexity compared to the 15-mode experiment.

Finally, van den Hout combined both ultra-wideband and SDM transmission in a single experiment. By using a fibre with a slightly increased diameter, an ultra-wideband signal could be transmitted over 114 spatial channels (3 modes times 38 cores). This resulted in a data rate of 22.9 petabit per second, which is, to date, the highest data rate measured in any optical fibre and twice as much as the previous record.

This data rate is about 20 times the global internet traffic per second. To put this in context, it is the same as roughly 1 billion people watching 4K quality Netflix streams simultaneously.

Notable output

The novel transmission techniques presented in van den Hout’s thesis are expected to enable future ultra-high-capacity optical transmission links that form the backbone of the Internet.

The research was conducted at the High-Capacity Optical Transmission Laboratory at TU/e and resulted in more than 45 publications, including several post-deadline papers and nominations for best (student) paper awards.

Part of the research was conducted during a visit to the National Institute of Information and Communications Technology (NICT) in Tokyo, Japan. The KPN-TU/e Smart Two program partly funded this research.

Title of PhD thesis: Ultra-wideband and Space-division Multiplexed Optical Transmission Systems. Supervisor: Chigo Okonkwo.