# Research Progress on Router Devices for the OAM Optical Communication
## 7 Conclusions  
This paper mainly reviews and summarizes the research achievements of OAM-RDs for OAM-based optical communication. According to different router functions, the router components can be divided into five categories, including OAM channel multicasting, switching, filtering, hopping, and adding/extracting. This review briefly introduced the working principle and development history of different OAM-RDs. With further development of the OAM-based optical communication technology, OAM-RDs play important roles in improving the flexibility and practicability of optical communication systems. OAM multicasting devices can copy the data information of a single channel to multiple channels for better adaptation to the multi-user environment. OAM switching devices can switch the OAM mode value of the input beam to realize effective data switching between different OAM channels. OAM filtering devices can realize the extraction of specific channel information from the multiplexed OAM channels. OAM hopping devices, by changing the OAM modes or frequencies, can achieve channel switching to improve the anti-jamming ability of OAM-based optical communication. Meanwhile, OAM adding/extracting devices, employing the selective addition or extraction of the OAM communication channel, can access the selected channel without interfering with other channels for the intermediate-node users. So far, OAM-RDs are developing towards miniaturization and integration, which coincides with the requirements of devices in today's optical communication. With in-depth research, the developments on the OAM-RDs have achieved a series of important breakthrough results, which play important roles in the OAM-based optical communication system. OAM channel multicasting holds great potential to meet the increasing bandwidth demands in future optical communication networks as it allows a single optical signal to send data through a shared optical network to multiple receivers. However, significant efforts are still needed to send data to ultra-multi-users simultaneously without compromising the bit error rate (BER) of data transmission. Similarly, realizing low-cost channel switching with ultra-fast switching time is a challenge yet to be overcome. The use of multiple SLMs in the switching process increases the difficulty of loading phase design on different SLMs and increases the system complexity and device cost, putting constraints on the high-speed optical communication link system which requires high real-time and economic performance. OAM channel filtering router devices with small sizes and good compatibility with the existing fiber-based OAM applications are one of the future research directions. To achieve efficient channel hopping, the performance of combinational OAM communication schemes should be researched and studied to combine the benefits of both simultaneously. For efficient, reliable, and easily integrable OAM add-drop multiplexer (OADM), deep neural networks must be explored as the traditional rigid construction and beam splitting properties of phase gratings restrict their diffraction efficiency, adaptability, and usefulness. In summary, as a new optical communication technology, OAM-RDs for OAM-based optical communication have been widely explored by researchers, but there are still some challenges to overcome, which are attracting further research. In addition, OAM has developed in the field of optical communication, and it is also a broad application prospect in the field of acoustic communication and traditional RF communication. However, the OAM-RD research in the field of acoustic communication and traditional RF communication is still in its infancy. It is believed that with the continuous studies conducted by worldwide researchers, in the near future, there will be more functions and more optimized and flexible OAM-RDs in various fields to gradually promote the continuous development of wireless communication based on the OAM. Writing Original Draft, B.W. and X.Z. ; review and editing, J.T., B.M., Z.L. and S.A.A.S.