Mode-division multiplexers (MDMUXs) play a pivotal role in enabling the manipulation of an arbitrary optical state within few-mode fibers, offering extensive utility in the fields of mode-division multiplexing and structured optical field engineering. The exploration of MDMUXs employing cascaded resonant couplers has garnered significant attention owing to their scalability, exceptional integration capabilities, and the anticipated low insertion loss. In this work, we present the successful realization of high-quality orbital angular momentum MDMUX corresponding to topological charges 0, ±1, and ±2, achieved through the utilization of cascaded fused-biconical tapered couplers. Notably, the measured insertion losses at 1550 nm exhibit remarkable minimal values: 0.31, 0.10, and 0.64 dB, respectively. Furthermore, the 80% efficiency bandwidths exceed 106, 174, and 174 nm for these respective modes. The MDMUX is composed of precision-manufactured high-quality mode selective couplers (MSCs). Utilizing a proposed supermode propagation method based on mode composition analysis, we precisely describe the operational characteristics of MSCs. Building upon this comprehensive understanding, we embark on a pioneering analysis elucidating the influence of MSC cascading order on the performance of MDMUXs. Our theoretical investigation substantiates that when constructing MDMUXs, MSCs should adhere to a specific cascading sequence.

Orbital angular momentum (OAM), described by an azimuthal phase term exp ( jlθ ) , has unbound orthogonal states with different topological charges l. Therefore, with the explosive growth of global communication capacity, especially for short-distance optical interconnects, light-carrying OAM has proved its great potential to improve transmission capacity and spectral efficiency in the space-division multiplexing system due to its orthogonality, security, and compatibility with other techniques. Meanwhile, 100-m free-space optical interconnects become an alternative solution for the “last mile” problem and provide interbuilding communication. We experimentally demonstrate a 260-m secure optical interconnect using OAM multiplexing and 16-ary quadrature amplitude modulation (16-QAM) signals. We study the beam wandering, power fluctuation, channel cross talk, bit-error-rate performance, and link security. Additionally, we also investigate the link performance for 1-to-9 multicasting at the range of 260 m. Considering that the power distribution may be affected by atmospheric turbulence, we introduce an offline feedback process to make it flexibly controllable.

为更好地解决少模光纤在传输中由于模式耦合过强而导致的信号串扰问题，对弱耦合光子晶体光纤中的线偏振（LP）模式以及矢量模的传输特性进行了研究，设计了一种可传输20种矢量模的双包层光子晶体光纤。通过有限元法模拟光纤参数对相邻LP模式间最小有效折射率差的影响，优化结构参数，使光纤支持稳定传输6种LP模式并满足弱耦合要求。最后分析了不同模式的有效模场面积、弯曲损耗。结果表明：各模式之间的最小有效折射率差达到1.12×10⁻⁴，表明模式间的串扰可忽略。基模有效模场面积达到了1040 μm²，且其相应的非线性系数低至1.07×10⁻¹⁰。此外，在弯曲半径为38 mm时，各模式弯曲损耗最大仅为5.65×10⁻⁸ dB/km。与主流的单模光纤及少模单包层相比，该结构具有大模场面积，低模间串扰及更强的抗弯曲能力，丰富了空分复用技术的开发思路。在大数据、虚拟现实、网络传输容量等新兴业务以及光纤传感方面提供了有益的参考方案。

Broadband mode converters are essential devices for space-division and wavelength-division multiplexing systems. There are great challenges in the generation of higher-order modes above the third order with low loss and high mode purity employing all-fiber devices. In this paper, an all-fiber LP₄₁ mode converter is proposed and fabricated by tapering a nine-core single-mode fiber bundle. Experimental results indicate that this all-fiber LP₄₁ mode converter is low-loss, high-purity, and ultrabroadband. The insertion loss is less than 0.4 dB. The purity of odd LP₄₁ at 1310 nm is 95.09%, and the operating bandwidth exceeds 280 nm.

Space division multiplexing (SDM) is promising to enhance capacity limits of optical networks. Among implementation options, few-mode fibres (FMFs) offer high efficiency gains in terms of integratability and throughput per volume. However, to achieve low insertion loss and low crosstalk, the beam launching should match the fiber modes precisely. We propose an all-optical data-driven technique based on multiplane light conversion (MPLC) and neural networks (NNs). By using a phase-only spatial light modulator (SLM), spatially separated input beams are transformed independently to coaxial output modes. Compared to conventional offline calculation of SLM phase masks, we employ an intelligent two-stage approach that considers knowledge of the experimental environment significantly reducing misalignment. First, a single-layer NN called Model-NN learns the beam propagation through the setup and provides a digital twin of the apparatus. Second, another single-layer NN called Actor-NN controls the model. As a result, SLM phase masks are predicted and employed in the experiment to shape an input beam to a target output. We show results on a single-passage configuration with intensity-only shaping. We achieve a correlation between experiment and network prediction of 0.65. Using programmable optical elements, our method allows the implementation of aberration correction and distortion compensation techniques, which enables secure high-capacity long-reach FMF-based communication systems by adaptive mode multiplexing devices.

Recently, transmitting diverse signals in different cores of a multicore fiber (MCF) has greatly improved the communication capacity of a single fiber. In such an MCF-based communication system, mux/demux devices with broad bandwidth are of great significance. In this work, we design and fabricate a 19-channel mux/demux device based on femtosecond laser direct writing. The fabricated mux/demux device possesses an average insertion loss of 0.88 dB and intercore crosstalk of no more than - 29.1 dB. Moreover, the fabricated mux/demux device features a broad bandwidth across the C+L band. Such a mux/demux device enables low-loss 19-core fiber (de)multiplexing over the whole C+L band, showing a convincing potential value in wavelength-space division multiplexing applications. In addition, a 19-core fiber fan-in/fan-out system is also established based on a pair of mux/demux devices in this work.