# Silicon Integrated Nanophotonic Devices for On-Chip Multi-Mode Interconnets
## 6 Conclusions and Perspectives  
In summary, we have reviewed the recent progresses on silicon integrated nanophotonic devices for on-chip multi-mode interconnects. In the first section, we reviewed high-dimensional (de)multiplexers, including mode (de)multiplexers, hybrid (de)multiplexers and DPWA-based MDMs. Mode (de)multiplexers were successfully developed based on cascaded ADCs with low losses and low crosstalk, whose mode capacity can be expanded further by introducing a hybrid scheme involving WDMs, PDMs or DPWAs. In the second section, a review of multi-mode routing devices was carried out, including multi-mode bends, multi-mode crossings and multi-mode splitters. Low losses, low crosstalk and compact footprints have all been proven for these devices, enabling arbitrarily routed MDM circuits and complex multi-mode networks. In the third section, we gave a brief review of the inverse design method for multi-mode photonic devices. Based on this method, the component library can be standardized for MDMs. In the fourth section, we reviewed reconfigurable MDM devices, including mode switches and mode ROAMD. The mode channels can be dynamically deployed by using these devices, leading to an efficient and flexible MDM network.  
So far, silicon MDMs have been applied in actual communication systems, especially for short-reach optical links, where on-chip MDM devices are employed to excite higher-order modes (e.g., LP\({}_{01}\), LP\({}_{11}\)) in few-mode fibers [253; 254; 255] or multi-core fibers [256; 257] to boost their capacity. However, the practical applications of monolithic on-chip communications are still limited. Several key improvements would enable the widespread use of practical on-chip MDM interconnects. The mode (de)multiplexers and multi-mode routing devices are cornerstones in the MDM system, so, first and foremost, we must improve the performance of these basic components, especially when a larger capacity is desired. There might be two possible ways to proceed. One approach is to adopt a hybridized scheme involving all the three dimensions, i.e., wavelengths, polarizations and modes, as proposed in Section 2.2. Moreover, the inverse design method appears poised to reshape the landscape of multi-mode photonics due to its ability to explore the full parameter space, which could enable mode manipulation at a higher level of freedom. Another emerging trend lies in reconfigurable multi-mode photonics. As discussed in Section 5, the mode switching processes always happen in the single-mode realm; as a result, the multi-mode reconfigurable devices are actually combinations of mode (de)multiplexers and single-mode switches, leading to quite a large footprint (a few millimeters). The device size could be effectively shrunk if higher-order modes can be directly switched without demultiplexing. Phase-change material (PCM) could potentially realize such direct switching since it can provide an extremely large index variation range [258; 259; 260; 261; 262]. Finally, it would be both interesting and impactful to build practical on-chip interconnects that fully integrate all the required components (e.g., LDs, PDS, MDMs) with micro-processors. To achieve this, Si-III/V hybrid integration technology can be exploited to combine silicon MDM devices with other high-performance active components [263; 264; 265; 266]. In conclusion, this novel silicon MDM technology has significantly motivated high-capacity optical communications, paving the way for the future of on-chip interconnects. We believe that multi-mode photonics will become more and more important in a wide range of applications.  
**Funding:** This research was funded by the National Key Research and Development Program of China (2019 YFB2203603), the National Natural Science Foundation of China (61922070, 11861121002, 61961146003) and Zhejiang Provincial Natural Science Foundation (LZ18F050001, LD19F050001).  
**Conflicts of Interest:** The authors declare no conflict of interest.