➀ Current high-capacity optical communication reaches a single fiber capacity of about 32T in the C+L band. Theoretical research extends to the O, E, S, L, and partial U bands with ultra-wide spectral applications, with some explorations reaching up to 400T. ➁ The total theoretical capacity of a single fiber using all the spectrum is around 580T. ➂ Multi-core fiber-based SDM is considered a feasible technology to further expand single-fiber capacity, and ITU has initiated research on this. ➃ Weakly coupled multi-core fibers with a standard or reduced cladding diameter are the industry trend due to weight and complexity issues. ➄ Inter-core crosstalk is a significant issue in weakly coupled multi-core fibers, reducing effective spectral efficiency. ➅ NICT and Microsoft have collaborated to provide optimal numbers for the number of cores in multi-core fibers, suggesting that 4 cores are the best choice for achieving high spectral efficiency.