The exponential growth of emerging big data services and global internet traffic has driven traditional single-mode optical fiber communication systems toward their capacity limits. Mode division multiplexing (MDM) technology has emerged as a promising solution to expand communication capacity by enabling multi-mode parallel transmission within a single fiber core and utilizing spatial dimensions. However, variations in the intensity and phase distributions of different modes present significant challenges: the precise manipulation of modes and accurate separation and conversion of mode channels. This precision is essential for achieving high-performance communication systems. Consequently, research on mode manipulation and its applications in MDM-based optical fiber communication holds substantial importance for addressing the growing demand for communication capacity and advancing optical fiber communication technology.

Mode manipulation and MDM-based optical fiber communication research has demonstrated significant advancements in recent years. Scientists have explored the fundamental principles of mode manipulation, encompassing effective refractive index matching, mode coupling, and optical field wavefront control. Multiple mode multiplexing/demultiplexing technologies have emerged, including planar waveguide-based, fiber-based, and free-space approaches. Planar waveguide-based technologies achieve high integration and miniaturization through waveguide construction on substrates. Shanghai Jiao Tong University demonstrated 4-channel multiplexing/demultiplexing using a silicon-based waveguide with a multimode interferometer structure. Zhejiang University developed a compact 4-mode multiplexer/demultiplexer with low insertion loss and high crosstalk suppression utilizing multimode micro-ring waveguides. Fiber-based technologies, employing fibers as the control medium, provide direct integration with few-mode fibers. Long-period fiber gratings, mode-selective couplers, and photonic lanterns represent key implementations. Shanghai University and Peking University have achieved significant advances in mode conversion and multiplexing/demultiplexing using these methods. Free-space-based technologies facilitate manipulation of optical field amplitude, phase, and polarization in free space. Multi-plane light conversion (MPLC) technology has been widely implemented for precise mode conversion control. Researchers at Cailabs in France achieved multiplexing/demultiplexing of multiple LP modes with high mode purity and low inter-modal crosstalk. Our research team has contributed significant advancements, developing a high-precision, non-destructive characterization technique for fiber microstructures, enabling 3D reconstruction of multi-core and few-mode fiber structures. We implemented mode multiplexing/demultiplexing technologies based on MPLC and fiber coupling, achieving high-efficiency mode conversion. Additionally, we proposed innovative few-mode fiber designs and pumping schemes to address gain imbalance in few-mode fiber amplifiers, supporting long-haul MDM transmission systems.

Mode manipulation, a fundamental approach in MDM-based optical fiber communication systems, has demonstrated substantial research progress through the development of mode multiplexing/demultiplexing techniques and devices, offering innovative solutions for capacity expansion in fiber-optic communication systems. Our research team has contributed significantly in three key areas: 1) high-precision characterization of fiber microstructures, 2) advanced mode manipulation technologies, and 3) development of few-mode fiber amplifiers. As technology advances, mode manipulation applications are expanding beyond telecommunications into emerging fields such as optical imaging and photonic computing systems, while requiring more precise multidimensional parameter coordination and crosstalk suppression. Future research directions will emphasize exploring novel materials with exceptional optoelectronic properties, designing advanced photonic structures with subwavelength precision, and developing innovative methodologies for intelligent mode control. These developments are anticipated to advance MDM technology toward ultra-high-capacity optical networks while fostering interdisciplinary innovations in photonic information processing.