As the transmission capacity of existing single-mode optical fiber communication systems gradually approaches its theoretical limit - nonlinear Shannon limit, space division multiplexing (SDM) capacity expansion technology using the cross-sectional spatial dimension of optical fibers to carry multi-core and/or multi-mode channels has been studied extensively in recent years. However, with the increase of the number of fiber cores and the number of channels per core in SDM fiber, the coupling crosstalk between many channels leads to the square law increase of the complexity of MIMO compensation algorithm, and the cost and power consumption of DSP module seriously limit the capacity expansion of SDM technology. The complexity of MIMO can be reduced by increasing the number of cores in a single fiber while maintaining a large core spacing. However, for fibers with a diameter greater than 200 microns, it is difficult to pull and fuse, and the deployment life is in doubt due to stress and other factors. The mutual restriction of these factors makes the scalability of SDM's communication capacity also face a bottleneck. In addition, the parallel transmission and separation (multiplexing and demultiplexing) of multi-space/mode channels in multi-core and low-mode fibers involves complex off-axis mode conversion, which is also a problem to be solved.

  By taking advantage of the OAM mode in annular fiber core fiber, which can transmit ultra-low crosstalk packets and the number of each group of modes is fixed at 4, combined with the characteristic that OAM mode emission only needs angular phase regulation, a 7-channel mode conversion phase plate with off-axis phase compensation is designed. In the self-developed 7-ring core OAM fiber, low crosstalk emission and transmission between fiber cores and mode channel groups within the fiber cores are simultaneously realized.

  This new scheme requires only a fixed-scale 4 × 4 MIMO algorithm at the receiver to compensate for crosstalk among the four degenerate modes within the pattern set, and the number of multiplexed channels can be expanded by increasing the number of pattern sets per core rather than the number of cores. The total fiber diameter is less than 200 microns, thus solving the main contradiction facing SDM technology capacity expansion.

 By combining the space division multiplexing technology with the mode division multiplexing technology, the transmission capacity of single fiber can be doubled, which can become a direction of the development of optical communication in the future.  But at present, it is a challenge for the fusion technology and device adaptation of multi-core fiber, and the manufacturers have not unified the production standard of multi-core fiber completely.  Shinho Fiber Communication has developed a special Fiber splicer that supports up to 8 Fiber cores, which can be adapted for outdoor engineering.  Shinho will continue to pay attention to the development of related technologies and provide the latest optical fiber processing solutions, and grow together with optical communication.