As the cornerstone of the modern information society, optical fiber communications serve as critical infrastructures supporting the development of next-generation information technologies such as the Internet, 5G communications, big data, cloud computing, and artificial intelligence. It has been designated as a strategically prioritized industry in China. However, with global data traffic growing exponentially, the transmission capacity of traditional single-mode fiber is rapidly approaching the Shannon limit, and the “capacity crisis” has become a major challenge restricting the development of future communications. Against this backdrop, the development of revolutionary new optical fiber communication technologies has become a strategic frontier fiercely contested by academia and industry worldwide. Few-mode fiber, with its unique spatial mode multiplexing characteristics, is regarded as the most promising technical pathway to break through current communication bottlenecks. Its core advantages are reflected in three key aspects: First, by utilizing multiple orthogonal spatial modes for mode-division multiplexing, it can significantly enhance the transmission capacity of a single fiber. Second, compared to alternative solutions like multi-core fiber, few-mode fiber achieves capacity expansion solely by increasing the number of transmission modes, giving it a distinct advantage in energy efficiency. Most importantly, it maintains strong compatibility with existing single-mode fiber infrastructure, ensuring relatively low deployment complexity. Due to the strengths, few-mode fiber technology is considered a key enabler for 6G communications, widely recognized as the most industrially viable solution for achieving ultra-high-capacity transmission at Pbit/s or even Ebit/s scales.

This paper reviews the research progress on mode multiplexers/demultiplexers, few-mode fibers, few-mode fiber amplifiers, and the unique equalization techniques specific to few-mode fiber communication systems. First, the advancements in mode division multiplexing/demultiplexing devices are introduced. The devices are primarily categorized into three types: free-space mode multiplexers/demultiplexers (Figure 4), all-fiber mode multiplexers/demultiplexers (Figure 8), and optical waveguide-based mode multiplexers/demultiplexers (Table 1). The working principles and performance parameters of each type are discussed in detail. Next, the types of few-mode fibers and their impairment measurement techniques are reviewed. The parameter characteristics and application scenarios of different few-mode fibers are presented, along with the measurement methods and performance evaluations of impairments unique to few-mode fibers (Table 2). Subsequently, the mechanisms and optimization approaches of few-mode erbium-doped fiber amplifiers and few-mode Raman fiber amplifiers are elaborated, focusing on aspects such as gain coefficients and differential mode gain. Finally, the multiple-input multiple-output (MIMO) equalization methods for few-mode fiber communication systems are summarized, including data-aided equalization methods and blind equalization methods without data assistance (Figures 34 and 36), along with some representative transmission experiments in few-mode fiber communication systems.

Few-mode fiber communication systems, as a crucial development direction for next-generation optical communication technologies, have attracted extensive attention from both academia and industry due to the significant potential in enhancing communication capacity. This review provides a detailed discussion on the research progress of key technologies in few-mode fiber communication systems. Although substantial advancements have been achieved in the key technologies, numerous challenges remain for their practical implementation. With continuous breakthroughs in core technologies and improvements in system integration capabilities, few-mode fiber communication systems are expected to achieve more efficient, stable, and cost-effective optical transmission in the future, which will provide strong support for meeting the ever-growing global demand for communication capacity.