Mode division multiplexing: from photonic integration to optical fiber transmission
Schematic diagram of MDM fiber optic communication system based on photonic integrated circuits and few-mode fiber transmission link.
The co-propagation of multiple orthogonal modes with different field distributions and propagation paths in one multimode waveguide are called mode division multiplexing (MDM), which was first, to the best of our knowledge, brought up 40 years ago (as early as 1982) and now is revived again due to the "Moore's law" demand of optical communication capacity and photonic integration.
Unlike the "Moore's law" of conventional microelectronics, the density of photonic integrations is limited by the intrinsic property of lightwave. In order to break the ceiling for PIC integration density, the development of PICs will eventually step into spatial division multiplexing (SDM), including MDM to increase signaling density in the single waveguide and 2.5/3D integration for advanced spatial stacking.
As for optical fiber communication, the capacity crunch of the conventional single mode fiber (SMF) is almost approached. SDM, including most importantly MDM, is prospected to be the next solution for maintaining this "Moore's law" trend of optical fiber transmission.
Linear polarization (LP) MDM is a more natural upgrade solution with respect to the current SMF solution, which can be divided into strongly coupled MDM and weakly coupled MDM. Currently, weakly coupled MDM is rather attractive for short reach applications, in which multiple-input multiple-output (MIMO)-less MDM for reduced cost and stable transmission for robust field applications is highly demanded. While, for long-haul optical transmission, strongly coupled MDM would be attractive. Besides, orbital angular momentum (OAM) MDM is also an attractive solution with high scalability, which can be expected to be highly useful for the ultra-large capacity links.
The research group led by Prof. Jiangbing Du from Shanghai Jiao Tong University, provided a review of the cutting-edge progresses of MDM technology for the scenarios from photonic integrated interconnection to optical fiber communication, including their recent works of MDM low-noise amplification, FMF fiber design, MDM Silicon photonic devices, and so on. The review paper is published in Chinese Optics Letters, Volume 19, No. 9, 2021 (J. Du, et al., Mode division multiplexing: from photonic integration to optical fiber transmission).
For MDM photonic integration, multimode interfaces for chip-to-fiber coupling, multimode passive devices (including bends, crossings, power splitters, mode MUXs, and so on), multimode active devices like switches, are the essential components for on-chip multimode signal processing. Till now, the highest number of on-chip MDM channels is 12, and the challenge for further higher-order mode multiplexing will be how to relax the fabrication tolerance of coupling efficiency as mode order increases. However, other multimode passive devices, like multimode interfaces, bends, and crossings, still have difficulty supporting more than six modes with high performances. These devices with higher-order mode operation, lower insertion loss, more compact footprint, and even more universal for design are the crucial trends for the future research. Besides, MDM switching networks with optimized routing architecture and compactness for more mode channels are the key challenges to realize large-scale MDM on-chip interconnection. For currently achievable MDM supporting 2-6 modes, hybrid multiplexing with WDM is a reliable method to enhance the total capacity of photonic integrated interconnection and also a long-term trend for on-chip multiplexing.
For MDM optical fiber transmission, a number of research progresses about FMFs, all-fiber mode MUXs, fiber amplifiers for FM systems, and MDM fiber transmission links, are carried out in recent years. A novel inverse design method for weakly coupled FMFs with high accuracy, high efficiency, and low complexity for fast and reusable fiber designs is also introduced. As for FM amplifiers, which play an essential role for long-haul MDM transmission, distributed Raman amplifiers (DRAs) show the advantages of maintaining the mode dependent gain (MDG) for each mode, providing more flexible and customizable designs for FM amplification. MDM transmission with larger capacity and longer distance is the main target all the time. MDM combined with multi-core fibers and WDM shows powerful prospects for next-generation fiber communication with ultra-high capacity.
The implementation requirements need to be considered for MDM in order to make it practical for field applications. As for photonic integration, MDM devices are quite reliable based on the standard fabrication. The key problem would be the limitation of the optical interface between the PIC and optical fiber, where significant mismatch can be expected. As for optical transmission, MDM can be applied in two scenarios, one is a brand-new transmission system with everything compatible with MDM, the other one is the subsystem holding SMF input/output to compatible with single-mode system and showing highly efficient MDM functionality at the same time. Due to the stability issue, short-reach applications with weak coupling fibers for MIMO-less interconnection are currently more practical. Generally, one can expect practical applications of MDM with PIC transceivers and active optical cables based on weak coupling FMFs and FM amplifier subsystems.
To conclude, MDM technology shows a dominant position to overcome the capacity crunch of optical communications, no matter for chip-scale, short-reach interconnection or longhaul transmission. There are still some challenges for MDM devices and systems (for both integrated photonics and fiber optics) to achieve more reliable properties. The core problem includes but is not limited to the methodologies to increase the mode channels and reduce the system cost per channel, improving the compatibility between single-mode and MDM systems at the same time. Further heroic researches and achievements are expectant to be made in the future to lead this area for maintaining the "Moore's law" trend of optical communication.
模分复用技术：从集成光子芯片到光纤传输
模分复用集成光通信传输系统的示意图
上海交通大学杜江兵研究员、何祖源教授团队在Chinese Optics Letters, 2021年第19卷第9期上(J. Du, et al., Mode division multiplexing: from photonic integration to optical fiber transmission)发表了一篇综述文章，综述了近期模分复用技术从光子集成互连到光纤通信应用的研究进展。
模分复用（MDM）技术，是指具有不同路径和模场分布、携带不同信息的多个正交模式，在同一多模光波导中共同传播的技术。模分复用概念的首次提出可以追溯到约40年前。近年来，由于光纤通信容量与集成光子领域的摩尔定律演进在日益增长的需求面前逐渐接近瓶颈，模分复用技术又重新受到众多关注。
线偏振模式的模分复用系统（LP-MDM）是相对单模系统来说更容易平稳过渡到多模系统的解决方案，LP-MDM包括弱耦合MDM和强耦合MDM，其中弱耦合MDM无需多入多出（MIMO）器件具有低成本的优势，更适用于短距互连场景，而对于长距离通信，强耦合MDM具有更低的非线性，且MIMO复杂度也可进一步降低，是更合适的选择。此外，轨道角动量（OAM）MDM具有更高的可扩展性，可用于超大容量传输链路。
与传统微电子领域的摩尔定律不同，光子集成芯片（PIC）的器件密度受限于光波的本征特性，即波长尺寸无法做到像电子器件一般紧凑密集。因此，PIC的集成密度存在难以突破的瓶颈。为了打破这个瓶颈，在PIC中采用MDM技术能够实现在一个多模波导中传播多个模式进而提升信道密度。
MDM光子集成芯片中，片上光波导进行多模信号的产生、调制、交换与处理的关键器件包括：芯片与光纤耦合的多模光接口、多模无源器件（如弯曲、交叉、功率分束器、模式复用器等）和多模有源器件（如光开关）。目前，片上MDM的模式通道数最多为12个，而如何降低更高阶模式复用效率对加工误差的敏感性是更高阶模式复用技术得以应用的关键挑战。多模无源器件如：光接口、弯曲和交叉，还较难实现支持多于6个模式的高性能器件。更多模式数、更低损耗和模间串扰、更普适的多模设计，仍是多模器件今后的研究趋势和难点。此外，具有优化的交换网络结构和紧凑的系统尺寸的多模光开关，也是大规模MDM光子集成互连的关键挑战之一。研究发现，将2到6个模式的MDM系统与波分复用相结合可以有效提升整个通信系统的传输容量，这也是未来MDM的发展趋势。
近50年来，光纤通信在经过掺铒光纤放大器、波分复用技术、先进调制编码、数字信号处理等技术的升级革新后，传统单模光纤的传输容量已逐渐逼近极限，空分复用（也包括最关键的模分复用）已成为维持光纤通信容量继续沿摩尔定律演进最具竞争力的解决方案。
近年来，关于MDM光纤传输系统的研究，在少模光纤、全光纤模式复用器、少模光纤放大器以及MDM光纤传输链路等领域都取得了众多成果。研究发现，一种逆向设计的方法，具有高准确率、高效率、低复杂度、计算速度快和可重复利用的优点，可用来设计弱耦合少模光纤（FMF）。此外，少模光纤放大器在长距离MDM传输中具有举足轻重的地位，分布式拉曼放大器（DRA）可降低模式差分增益、提供更灵活、可定制的少模放大设计。更大容量和更长距离一直是MDM传输不变的目标，将多芯光纤和波分复用技术与MDM结合，是面向下一代超大容量光纤通信强有力的技术路线。
MDM系统的具体实施需根据实际应用进行适配。对于光子集成芯片，MDM器件的实现仍旧依赖传统工艺流程的稳定性能，而最关键的问题在于芯片与光纤之间的光耦合接口，其受集成光波导与光纤之间显著的模场失配限制，较难实现高效耦合。对于光纤传输系统，MDM的配置有两种方式：一是采用一个全新的完全匹配MDM的多模传输系统，二是维持单模光纤的输入输出同时能实现多模功能的子系统，从而更好地与传统单模系统进行匹配。考虑到稳定性的问题，采用弱耦合少模光纤的无MIMO短距传输系统是目前来看更可行性的方案。因此，将集成光收发模块与基于弱耦合少模光纤和少模光纤放大器的有源光缆相结合，可能会成为未来MDM光通信系统应用的趋势。
综上所述，不论是芯片级短距互连，还是长距通信，MDM技术都已经成为光纤通信系统克服容量瓶颈的关键技术方案。对于集成光子芯片和光纤传输系统来说，MDM器件及系统在实际应用之前仍面临着众多挑战。譬如：如何增加模式复用通道数，降低每个模式通道的平均成本，并提高MDM与单模系统的兼容性。未来，还需要更多关于MDM的突破性研究，来引领光纤通信容量的发展沿着摩尔定律继续前进。