SPOC wins Horizon 2020 prize

Optical transport networks form the backbone of our information society on which the Internet is built. 

Every day in 2016, we create and transmit more data than was generated from the dawn of time up until the year 2000, and the data traffic is still growing at double-digit percentage levels. The backbone of the Internet is optical fibre, and more than 100 million people now enjoy broad-band fiber optic connections directly to their homes. In addition, optical fibres also link up the majority of wireless cell towers, where the data signals from billions of mobile phone users are directly converted to infrared photons which then travel down a vast network of optical fibers that connect cities, span countries and bridge continents. Today hundreds of billions of optical bits are transmitted around the globe each second.

The biggest challenges of the Internet today are the growing capacity demands and the accompanying energy consumption. Today, more than 2% of the global CO2 emission stems from the Internet, and the Horizon Prize is conceived to help address this challenge.

Current installed fibre channels face severe limitations with respect to scaling up the data throughput. The maximum throughput, or the capacity, of standard single-mode fibre (SSMF) is limited by the amount of optical power it can handle, which corresponds to ~100 Tbit/s per fibre core. Going beyond that invokes severe nonlinear distortions of data signals and eventually a phenomenon called fibre fusing occurs, where the fibre core simply melts. To overcome this looming capacity crunch, a new multiplexing technology using new optical fibres is urgently needed.

With current technology, one can increase the capacity 1000 times by installing 1000 parallel systems, but this would also increase cost, energy consumption, and space occupancy by the same factor, which is wasteful of energy and undesirable. In our proposed approach, we suggest to break the optical transmission barrier by building optical communication systems based on high-count, single-mode, multi-core fibre (HC-SM-MCF) for long-haul transmission. This approach  can deliver orders of magnitude more capacity than the state-of-the-art commercial transmission systems and at the same time is expected to reduce the cost-per-bit, energy-per-bit and space-per-bit by a factor of 10 or even more.

Figure: The essential technologies in the Prize-winning proposal. Top: A transmitter chip broadens the spectrum of a laser seed source, so as to feed multiple wavelength channels. Middle: A high-count multi-core fibre (here 30-cores) is used to transmit Pbit/s-class data. Bottom: A multi-core fibre amplifier is essential in long-reach transmission. All the parts have been experimentally demonstrated, e.g. the two top technologies in a 661 Tbit/s transmission demonstration. In the 661 Tbit/s transmission experiment, a single pulsed laser sends optical pulses with a spectral width of 3 nm into an AlGaAs (aluminium gallium arsenide) waveguide chip. This chip is so nonlinear that it broadens the input spectrum to about 40 nm relying on a nonlinear optical process known as self-phase modulation. From this broadened spectrum, 80 wavelength channels are carved out and used to carry data in the form of 16 QAM (Quadrature Amplitude Modulation) signals where both the phase and the amplitude is modulated. The 80 wavelength channels are additionally multiplexed in polarisation and finally in space sending the signals through 30 individual fibre cores. 

The proposed HC-SM-MCF approach combined with multi-core erbium doped fibre amplifiers (MC-EDFAs) can enable ultra-high capacity (> 1 Pbit/s) optical transmission over one thousand km with significant collective energy and cost savings. Additionally, our proposed single photonic chip-based supercontinuum source also leads to massive energy and cost savings, as many individual lasers at different wavelengths, including cooling, are replaced by a single laser source. In short, our proposed work is innovative in terms of transmission capacity, transmission reach, energy savings and also offers low complexity, low cost and better integration.

When the proposed techniques are eventually ready to be deployed in real transmission systems, it will be to the benefit of billions of Internet users offering cheaper and greener ultra-broadband connections, and will become a foundation and driving force for future omnipresent connectivity through 5G, Internet of Things and cloud services.

Core technologies in the proposal 

• In space-division multiplexing (SDM) the signal power is distributed over several spatial channels, thus lowering the intensity per channel and allowing for increased data throughput. Such SDM fibres can allow for orders of magnitude higher capacity through a single SDM fibre, whilst keeping the fibre diameter on a reasonable scale (less than twice the diameter of SMF) to ensure that the mechanical reliability is kept sufficient for practical deployment.  With the introduction of SDM, the world has witnessed an explosion in reported records of data transmission throughput, such as the 1 Pbit/s landmark reached in 2012. One approach to SDM is to use multiple optical modes in the same multi-moded fibre, and to then use advanced multiple-input/multiple-output (MIMO) processing to separate the channels digitally. Another approach is to use multiple single-mode cores closely packed within the same fibre cladding, where the MIMO processing for spatial mode demultiplexing is usually not required, as the individual cores can be designed and fabricated with very little crosstalk and with very high core count. Combining the multi-mode with multi-core approach has also been explored reaching an extraordinary number of spatial channels (above 100), though this usually requires a larger fibre diameter compromising mechanical reliability as well as advanced MIMO processin

• For long-haul transmission above 1000 km, amplification is inevitable, and there are today clear signs of considerable collective energy savings when designing SDM fibre amplifiers. Additionally, as cost and energy consumption are becoming limiting factors in high-capacity systems, using fewer lasers with accompanying less overall system energy consumption has become highly desirable. 

• Within our proposal, we suggest to break the optical transmission barriers by building optical communication systems based on high-count, single-mode, multi-core fibre (HC-SM-MCF) for long-haul transmission enabled by multi-core erbium doped fibre amplifiers (MC-EDFAs), and lit up by a single aluminum gallium arsenide (AlGaAs) photonic chip based supercontinuum light source.

• The benefits of this approach are many: With tens of cores (>30) the capacity is increased by the same factor, and hence the joint spectral efficiency of the HC-SM-MCF is greatly increased. The HC-SM-MCF we propose has such  low crosstalk that transmission above 1000 km is easily attainable without the need for MIMO processing and with fibre diameters that ensure the mechanical fibre reliability. With a MC-EDFA enabling long-haul transmission, the pump power can be distributed among the cores yielding a considerable collective increase in energy-efficiency of the transmission link. The use of a single supercontinuum source additionally leads to a massive energy saving, as many individual WDM lasers, including cooling, are substituted by a single source. In addition, using a photonic chip based supercontinuum source allows for extending the spectral range, as a useful supercontinuum bandwidth from such a chip can extend over hundreds of nanometers. 

• We present a proof-of-principle transmission experiment where we demonstrate the first photonic chip based frequency comb, relying on spectral broadening of a mode-locked laser comb in an AlGaAs photonic chip, with a sufficient comb output power to support several hundred Tbit/s optical data. We use a 33.6 nm wide part of the generated comb spectrum to carry 80× 40 Gbaud WDM channels and imprint the channels with 16-QAM and use polarization division multiplexing (PDM). The high comb OSNR allows us to distribute the 80 WDM PDM channels over 30 spatial channels, and we demonstrate successful 9.6 km transmission in a heterogeneous 30-core fibre reaching a total of 661 Tbit/s after FEC overhead subtraction.

• The suggested system is scalable beyond the demonstrated bit rate, as the transmitter can utilize a broader spectral range and use higher order modulation, and the fibre and EDFA can have a higher core-count.

Feasibility demonstration: 661 Tbit/s data transmission using a single optical chip as the transmitter

A central part of our proposal was demonstrated and presented as a prestigious postdeadline paper at the international Conference on Lasers and Electro-Optics (CLEO) 2016, in San Jose, California, USA,  5-9 June 2016. The experiment constituted a demonstration of the highest bit rate generated on the light from a single chip-based light source and simultaneously transmitted through a multi-core fibre. A total data rate of 661 Tbit/s was achieved. Or in other words twice the current total worldwide internet traffic was carried by the light from a single chip, which in a dramatic way demonstrates the power of optical technologies. 
The figure above shows the principle.  

To reach this record, the PHOTONMAP team optimised several optical parameters: time, frequency, polarisation, quadrature (complex modulation) and space (multi-core fibre). 40 Gbaud (40 GHz pulse train) data symbols were generated and multiplexed in frequency by adding 80 wavelength channels. All these channels were then polarisation multiplexed and each pulse carried 16 QAM quadrature modulation allowing for 4 bits per pulse. Finally, a special optical fibre developed by Fujikura in Japan counting 30 cores within a fibre diameter of 228 microns and reaching 9.6 km was used. Each core could carry a copy of the generated optical signal. The total throughput was thus 661 Tbit/s.