# Research Progress on Router Devices for the OAM Optical Communication
## 6 OAM Channel Adding/Extracting
In the upper path, the reflected beam is first reflected by M1 and then passes through DP2, after which it is split into two beams by BS2. One beam reaches port A of BS2 while the other is reflected towards port B. Conversely, in the lower path, the transmitted beam first passes through DP1 and is then reflected by M2 before being split by BS2. One of the resulting beams is directed towards port A of the BS2, while the other reaches port B. The proposed method ensures that the topological charge interval between adjacent OAM beams is at least equal to 2 at both output ports of the MZI. As a result, the distance between the converted light spots \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Year** & **Methods** & **Equipment** & **Strengths/ Weaknesses** & **Transmission Result** & **Ref.** \\ \hline \multirow{3}{*}{2013} & Reconfigurable & \multirow{3}{*}{SLM} & Operate channels & The power loss of this & \multirow{3}{*}{[152]} \\  & multiplexers & & flexibly/Only one channel & & scheme is less than 2 dB & \\  & multiplexers & & at a time & & and the channel error rate & is only 2.0 \(\times\) 10\({}^{-3}\). \\ \hline \multirow{3}{*}{2019} & Geometric & \multirow{3}{*}{SLM} & Realize the addition and & \multirow{3}{*}{The energy purity of the measured OAM is about 95\%.} & \multirow{3}{*}{[153]} \\  & transformation & & & & multiple SLMs & 95\%. \\ \hline \multirow{3}{*}{2019} & \multirow{3}{*}{OAM analyzer} & Interferometer, Dove & \multirow{3}{*}{Separate the odd and even
\begin{tabular}{} \end{tabular} } & The crosstalk values are & \multirow{3}{*}{[154]} \\  & & prisms & OAM beams/- & & sort the odd and even \\ \cline{1-1}  & & & OAM beams & OAM beams & \\ \hline \multirow{3}{*}{2021} & OAM add-drop & diffractive deep & Diffraction efficiency and & Signal-to-noise ratio & \multirow{3}{*}{[155]} \\  & neural & mode purity are very high/ & penalties of \textasci{-}1 dB at a BER & & \\ \cline{1-1}  & network & - & of 3.8 \(\times\) 10\({}^{-3}\) & & \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Summary of channel adding/extracting routers used for the OAM-based optical communications. corresponding to these OAM modes will be greater than the width of each individual spot, effectively avoiding any overlapping and enabling high-density OAM mode analysis with enhanced discrimination resolution, as presented in Table 6. This feature addresses a key challenge in the OAM mode analysis, where previously the ability to analyze high-density OAM modes was effectively constrained. Generally, phase gratings are employed in the construction of the OAM add-drop multiplexer (OADM). However, the rigid construction and beam-splitting properties of phase gratings restrict their diffraction efficiency, adaptability, and usefulness. In recent years, deep neural networks have emerged as a promising technique for information processing in the OAM mode. These networks have the capacity to theoretically approximate functional relationships in any input-output domain. In 2021, [PERSON] et al. [155] introduced an OAM add-drop multiplexer (OADM) utilizing an optical diffractive deep neural network (ODNN). By leveraging the powerful data-fitting capability of deep neural networks and the intricate light-field manipulation abilities of multilayer diffraction screens, a five-layer ODNN was constructed. This ODNN enables precise control over the spatial positioning of vortex beams, facilitating the selective coupling and separation of OAM modes. The specific structure of the OAM add-drop multiplexer (OADM) using the optical diffractive deep neural network (ODNN) is depicted in Figure 7c. The input plane of the diffraction neural network is divided into several ports: (1) in port: this port receives the multiplexed OAM channels; (2) drop port: it extracts the selected OAM channel from the multiplexed channels; (3) add port: this port uploads an OAM channel carrying a new data stream; (4) through port: it outputs the re-multiplexed OAM channels.