Space division multiplexing (SDM) is a critical technology that can increase the capacity of existing single-mode fiber optic communication systems by tens of times. It is worth studying as an effective means to overcome the capacity bottleneck of traditional single-mode fiber optic communication systems. This overview introduces the key technologies and research progress of the multiplexer/demultiplexer, fiber amplifier, few-mode fiber, and optical transmission system integrated device in strong coupling communication systems with few-mode fibers. This overview also introduces some of the more classic or latest experiments in strong coupling communication systems with few-mode fibers, and discusses the future research directions of few-mode optical transmission system.