TOKYO -- NEC Corporation (NEC; TSE: 6701) today announced that it has demonstrated a transmission capacity of 50.9 terabits (Tb) per second on a single optical fiber, over a distance greater than 11,000km. This is the first time 50Tb have been achieved over 10,000km using C+L band erbium-doped fiber amplifiers (EDFA). Such a large capacity over that distance corresponds to a new record breaking capacity distance product of 570 Pb-km (Petabit per second-kilometer). NEC's commitment to pushing the benchmark 50Tb capacity to trans-pacific distances is in line with the current trend of increasing the reach of ultra-high capacity submarine cable segments.

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, the fundamental spectral efficiency limit of optical communications. 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 optimization algorithm. Through this algorithm, NEC obtained an optimized 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, 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 maximize the capacity per fiber pair. In order to deliver a record breaking capacity NEC's researchers developed a patent-pending bi-directional amplifier design that reduces the effective noise figure and the device complexity.

NEC Corp. (Tokyo: 6701)