NEC Corporation has announced that it has demonstrated a transmission capacity of 50.9 Terabits per second on a single optical fibre, over a distance greater than 11,000km. According to the company, this is the first time 50Tb has been achieved over 10,000km using C+L band Erbium-Doped Fiber Amplifiers (EDFAs). Such a capacity over that distance corresponds to a record-breaking capacity distance product of 570 Pb-km (Petabit per second-kilometer).
Achieving such a high capacity, even with extremely wide bandwidth EDFAs, requires efficient use of the bandwidth at a level that is close to the Shannon limit. NEC notes there is more than one way to design modulation formats that can get close to the Shannon limit in the linear regime. However, most of them do not fare well when operated in the nonlinear regime, where they suffer more performance gap towards the nonlinear Shannon limit.
In order to overcome this issue, NEC’s researchers developed a multilevel, linear and nonlinear constellation optimisation algorithm. Through this algorithm, NEC obtained an optimised 32QAM (opt32) constellation that achieves close to Shannon capacity but, more importantly, has a higher nonlinear capacity limit, which is especially more relevant to submarine transmission. Moreover, says NEC, this new modulation format is much easier to implement, as it does not require any iterative decoding or non-uniform coding. As such, opt32 modulation allowed NEC researchers to achieve an unprecedented spectral efficiency of 6.14 b/s/Hz over a trans-Pacific distance.
Also in line with current trends, C+L amplification is used to maximise the capacity per fibre pair. In order to achieve their results, NEC’s researchers developed a patent-pending bi-directional amplifier design that reduces the effective noise figure and the device complexity.