Technical Evolution and Current Progress of Space-Division Multiplexing in Submarine Optical Communications (Invited)

Driven by the booming global digital economy and explosive growth of intercontinental data traffic, transoceanic submarine communication systems face an urgent demand for exponential capacity expansion. However, the capacity of conventional single-mode fiber system has gradually approached the nonlinear Shannon limit, resulting in insurmountable bottlenecks for traditional capacity-scaling approaches. Space division multiplexing (SDM) has emerged as a transformative technology to overcome this limitation by introducing multiple spatial channels, while reducing the cost per bit for submarine systems. Nevertheless, the stringent constraints of deep-sea environments, including limited spatial enclosure, remote power feeding and long-term reliability requirements, present critical challenges for SDM technology. This paper provides a thorough review of the technical evolution, current progress and future development trends of SDM for submarine optical communications. We systematically present the core technical approaches of SDM transmission fibers, SDM-optical amplifiers, and spatial division multiplexers. Furthermore, we summarize key experimental and system-level advances in SDM communication systems targeting submarine applications. Finally, we conclude that SDM has formed a complete technical ecosystem for submarine applications, and discuss some future directions towards high integration, energy efficiency, hybrid core/mode-division architectures, and industrial standardization. This review aims to provide a systematic reference for the development of next-generation submarine optical communication systems.