Mode division multiplexing: from photonic integration to optical fiber transmission

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).


Schematic diagram of MDM fiber optic communication system based on photonic integrated circuits and few-mode fiber transmission link.


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.