Abstract
This paper overviews the evolution of space-division multiplexing (SDM) technology that will contribute
to high-capacity transmission in future optical communication systems. It focuses on weakly-coupled
multi-core fibers (WC-MCFs), which are expected to be used in the beginning of SDM technologies. The
pursuit of the maximum number of cores accommodated in a single optical fiber led to 32-core MCFs
within a range that was twice the cladding diameter of standard optical fibers. Attention has recently
concentrated on 4-core MCFs with a standard cladding diameter from a practical perspective. This strategy
allows for a maximum four-fold increase in transmission capacity while maintaining the outer diameter
of optical fiber cables. This paper covers the development of 4-core MCFs with such peripheral
technologies as cables and connectivity as well as the results of recent field deployment tests. WC-MCFs
offer a promising path towards achieving future high-capacity optical communication systems.
