NEC Corp. (TSE: 6701) says it will use the post-deadline session of the Conference on Lasers and Electro-Optics (CLEO) 2017, held this week at the San Jose Convention Center, to discuss a demonstration of 50.9-Tbps on a single optical fiber over a distance greater than 11,000 km via a new modulation algorithm and C+L band erbium-doped fiber amplifiers (EDFAs). The figure corresponds to what NEC asserts is a record-breaking capacity/distance product of 570 Pb-km.
As NEC notes, research is ongoing in several locations to push capacities toward the Shannon Limit, the fundamental spectral efficiency limit of optical communications. Such research has investigated a variety of modulation formats, most of which fare well in the linear domain but not so well outside of it.
NEC's approach involves a multilevel, linear and nonlinear constellation optimization algorithm that produced a 32QAM (opt32) constellation. The format achieves capacities close to the Shannon Limit with a higher nonlinear capacity limit than other approaches. The transmission format is comparatively easier to implement than other approaches as well, NEC says, as it does not require iterative decoding or non-uniform coding.
The NEC researchers used the opt32 modulation format to achieve spectral efficiency of 6.14 b/s/Hz over a trans-pacific distance. As such, NEC sees the technology as particularly useful to long-distance submarine network applications. Along these lines, the company paired the new transmission format with C+L amplification to maximize the capacity per fiber pair. The EDFA leveraged a patent-pending bi-directional design that NEC asserts reduces the effective noise figure and the device complexity.
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