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Advanced digital signal processing for ultra-high-capacity optical transmission

To meet bandwidth demand, optical systems will have to transmit vastly more data, while also enabling the transition towards energy-efficient green networks. Currently, trans-oceanic fibre-optical cables can transmit 10s of Tb/s per fibre pair, enabled by multiplexing of physical dimensions: amplitude, phase, wavelength, and polarization. However, one physical dimension is not yet exploited: space.

A multiplexing technique known as ‘spatial multiplexing’, or space-division multiplexing, can transmit independent channels separated in space. This makes it possible to support future Petabit per second per fibre optical transmission links. To exploit this requires advanced digital signal processing (DSP).

In his PhD thesis, Sjoerd Van der Heide developed an advanced digital signal processing chain at Eindhoven University of Technology. The digital signal processing chain was used in single-mode optical transmission experiments using a recirculating fibre-loop.

200 Gb/s per wavelength transmission was achieved over 11,700 km of fibre using advanced four-dimensional modulation formats. The digital signal processing chain was also used for multi-mode experiments, transmitting 1 Terabit per second per wavelength over 130 km without in-line optical amplifiers.

Sjoerd Van der Heide designed and fabricated an all-fibre multiplexer to interface single-mode fibres with novel three-core coupled core fibres. These multiplexers were used to transmit 172 Terabits per second over 2040 km - equivalent to approximately 10 million ultra-high-definition video streams.

Finally, he implemented a real-time optical receiver capable of receiving 5 Gb/s with advanced digital signal processing on a commercial off-the-shelf GPU using CUDA. The concept has been tested using a 91 km long field trial link in Tokyo, Japan and in a laboratory link over 10,000 km straight-line transmission fibre link.

“Since my internship at ADVA Optical Networking in Germany during my Electrical Engineering MSc, I have been very interested in digital signal processing,” explains Sjoerd. “I am especially fascinated by the relationships between physical optical effects and digital solutions. Space-division multiplexing as a niche is a lot of fun because you are actually looking far into the future. This makes many short-term constraints - complexity and costs, essentially - less important and gives you more freedom to think up solutions.”

“To an extent, this research is more of an evolution in the field than a revolution, so you have an idea of where it is going, but often you don't know in advance where the limitations are. There are always surprises. You can make an educated guess as to what limiting factors are, but often you only find out where you need to focus your attention as you go along. System characterisation for space-division multiplexing is far less advanced than for single-mode systems.
This simply means that you don't have as good a picture of what will happen in a given transmission scenario.”

Sjoerd Van der Heide

“These findings add to our knowledge of space-division multiplexing. They confirm that such systems are technically possible and could be implemented. However, more research and engineering is required to bring this to commercialisation. The inherent scaling advantages of space-division multiplexing will lead to it being used in the future. This research once again proves this is a realistic prospect.

The research was conducted at the High-Capacity Optical Transmission Laboratory at Eindhoven University of Technology. Part of the research was conducted in collaboration with international partners during two research internships at Nokia Bell Labs in Holmdel, New Jersey, USA, and at the National Institute of Information and Communications Technology (NICT) in Tokyo, Japan. It culminated in more than 50 publications, receiving two student paper awards, a best paper award, and a Nokia Bell Labs Innovation Project Award. The investigated techniques are intended for use in future ultra-high-capacity optical transmission links.

Space-division multiplexing uses modes of for example multi-mode optical fibres to modulate data on to, increasing data rates significantly. Exploiting space-division multiplexing requires advanced digital signal processing (DSP). Light in multi-mode fibres experiences linear and non-linear effects: the receiver sees a scrambled combination of transmitted signals. Multiple-input multiple-output (MIMO) filtering, similar to that used in Wi-Fi and 5G, is required to unravel mode mixing in the optical fibre transmission channel.