Mathematically, since the coupling process is linear, the input and output modes are related by a linear matrix. Techniques invented for MIMO wireless communication for channel matrix estimation can be straightforwardly adopted for MDM signals and the input signal can be recovered by inverting the channel matrix. The challenge is that the computational complexity for this MIMO equalization is proportional to the number of the elements of the channel matrix. Therefore, the complexity of MDM MIMO equalization scales unfavorably as the square of the number of MDM channels.
CREOL faculty members Rodrigo Amezcua-Correa, Guifang Li and Axel Schülzgen have designed and fabricated a special fiber to reduce the MDM MIMO equalization complexity using a divide-and-conquer approach. The few-mode multicore fiber (FM-MCF) divides the SDM channels into multiple uncoupled cores. By doing so, not all SDM channels are coupled to each other, thus reducing the number of MIMO matrix element by a factor equal to the number of cores. In order to ensure core-to-core isolation, air holes are used to confine the modes of each of the cores.
This fiber has been used in a record-setting SDM transmission experiment, in collaboration with COBRA Research Institute, Eindhoven University of Technology, and the results was published in the October 2014 issue of Nature Photonics. They demonstrated transmission over a 1 km FM-MCF, employing 7 fewmode cores, each supporting the LP01 and two degenerate LP11 modes, which results in 7 cores × 3 modes × 2 polarizations = 42 simultaneously transmitted spatial channels. This corresponds to 21 conventional SMF transmission channels. The air-holes minimize inter-core crosstalk and reduce the required MIMO equalizer complexity from 42×42 to 7×6×6, and hence reduce power consumption. A single-carrier spectral efficiency of 102 b/s/Hz is achieved by encoding 24.3 GBaud 32 quadrature amplitude modulation (QAM), allowing for 5.103 Tb/s per carrier gross (4 Tb/s per carrier net) data rate spatial superchannels. Combining the spatial dimension with 50 wavelength channels on a 50 GHz International Telecommunication Union (ITU) grid, a gross total capacity of 255 Tb/s (200 Tb/s net) is demonstrated, further indicating the viability of combining few-mode and multicore transmission techniques in a single fiber for achieving ultra-high capacity.