By Geoff Bennett
Director, Solutions and Technology
In a keynote presentation at OFC 2014 in San Francisco, a challenge was laid down to the dense wavelength-division multiplexing industry: to close the Atlantic with 16 quadrature amplitude modulation (16QAM). What that technobabble actually means is this: following the successful deployment of the first generation of coherent 100 gigabits per second (Gb/s) subsea transmission in 2010, by 2014 internet content providers (ICPs) – companies such as Apple, Facebook, Google, Microsoft and Amazon – were eager for a further doubling of subsea capacity. That capacity increase could be delivered in several ways, but increasing the number of bits carried in each modulation symbol by moving to 16QAM was an obvious approach. At that time, however, the idea of running 16QAM over trans-Atlantic distances was a major challenge, because contemporary field trials of 16QAM prototypes were struggling to achieve reach of 600 kilometers (km). Other options for increasing subsea capacity included expanding the amplifier spectrum and moving to a flexible transmission grid – both of which have been exploited in recent years in solutions such as Infinera’s FlexILS open line system.
The reasons why ICPs need subsea capacity vary. For a given ICP, the goal may be to provide a consistent quality of experience for users regardless of where they or their contacts are – assuming local internet access isn’t the bottleneck. One consequence of that is that user content may have to be cached across data centers all around the world. In 2014 the amount of trans-Atlantic capacity used by ICPs on their private network links exceeded public internet traffic for the first time, and in 2017 private network traffic exceeded global internet capacity – all in a market that’s growing at a compound annual growth rate of 46 percent (source: Telegeography).
In a public webinar hosted by Light Reading, Dr. Steve Grubb, Global Optical Architect for Facebook, and two of Infinera’s Distinguished Engineers, Dr. Jeff Rahn and Dr. Pierre Mertz, described a recent subsea trial on the 6,600-km MAREA trans-Atlantic cable. In this record-breaking trial, Infinera’s fourth-generation Infinite Capacity Engine (ICE4) advanced coherent technology achieved new records for trans-Atlantic real-time spectral efficiency, namely:
- 21 bits per second per hertz (b/s/Hz) over 6,644 km, translating to a fiber capacity of 26.2 terabits per second (Tb/s)
- Using a loopback on the same cable, 4.46 b/s/Hz was achieved over 13,210 km for 18.6 Tb/s of fiber capacity
Details of the trial were presented within the webinar, a free-to-view recording of which can be found here. There were three fundamental reasons for the record-breaking results:
- MAREA is an extremely advanced and high-quality subsea cable. It uses large area fiber with an area of 150 square microns to deliver extremely low non-linear penalties. Coupled with unusually close amplifier spacing of 55 km, this delivered a very favorable environment for high-order modulation such as 16QAM.
- ICE4 is the first commercial coherent technology to make use of Nyquist subcarriers, in which the light from each laser is split into multiple parallel signals. The result is that high-data-rate signals experience unusually low non-linear penalties, and channels can be spaced more closely together.
- ICE4 is based on Infinera’s unique large-scale photonic integrated circuit (PIC) technology. A PIC-based implementation allows tighter channel spacing thanks to the use of a common wavelength locker circuit. In addition, multi-carrier operation within the coherent signal processor means that advanced forward error correction gain sharing can be used.