# Research Progress on Router Devices for the OAM Optical Communication
## 2 OAM Channel Multicasting
The results showed that the images received by each receiver were basically consistent with the transmitted signals, and the total bit error rate(BER) of the three channels was only 4.16 \(\times\) 10\({}^{-4}\), indicating successful multicasting of various signals, as presented in Table 2.  
Although the aforementioned methods achieved promising results, the overall complexity of the spatial frequency distribution increases considerably with an increase in the number of multiplexing channels. This adversely affects the hologram generation for multiplexed vortex beams, resulting in an increase in the OAM spectrum error. Therefore, a tradeoff is made between the bit error rate (BER) and the number of multiplexed channels. To address the above problem, in 2023, [PERSON] et al. [109] reported a unique concept of OAM neural communication employing a network model based on a U-net \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Year** & **Method** & **Input** & **Output** & **Equipment** & **Strengths/ Weaknesses** & **Transmission Result** & **Ref.** \\ \hline
2013 & \begin{tabular}{c} Sliced phase \\ patterns \\ \end{tabular} & Single OAM & \
\begin{tabular}{c} Multiple \\ same-mode- \\ spacing \\ OAMs \\ \end{tabular} & SLM &
\begin{tabular}{c} Low loss \\ efficiency/ \\ Low integration \\ \end{tabular} & Channel crosstalk \(<\) & [105] \\ \hline
2015 & \begin{tabular}{c} Adaptive \\ correction \\ \end{tabular} & \begin{tabular}{c} Gaussian \\ beam \\ \end{tabular} & SLM & \
\begin{tabular}{c} Multiple \\ adjustable/ \\ Limited mode \\ \end{tabular} & Maximum power \\  & & & OAM &
\begin{tabular}{c} Limited mode \\ selection \\ \end{tabular} & [106] \\ \hline
2022 & \begin{tabular}{c} Optical \\ tapped- \\ delay-lines \\ \end{tabular} & \begin{tabular}{c} Gaussian \\ beam \\ \end{tabular} & \
\begin{tabular}{c} Multiple \\ adjustable \\ OAM \\ \end{tabular} & SLM &
\begin{tabular}{c} Reduced error \\ vector magnitude \\ (EVM) \\ \end{tabular} & Error-vector \\  & & & & & & \\ \hline
2022 & \begin{tabular}{c} Multiplexed \\ vortex state \\ array \\ \end{tabular} & \begin{tabular}{c} Multiplex \\ OAM \\ \end{tabular} & \begin{tabular}{c} Multiple \\ multiplexed \\ OAM \\ \end{tabular} & SLM & \
\begin{tabular}{c} High channel \\ utilization/- \\ \end{tabular} &
\begin{tabular}{c} The mean square \\ error is 1.06 \(\times\) 10\({}^{-3}\) \\ \end{tabular} & [108] \\ \hline
2023 & \begin{tabular}{c} 16-ary shift \\ keying \\ \end{tabular} & \begin{tabular}{c} Multiplex \\ OAM \\ \end{tabular} & \begin{tabular}{c} Multiple \\ multiplexed \\ OAM \\ \end{tabular} & SLM & \
\begin{tabular}{c} -/Ultra-high \\ precision, 1-to-40 \\ multicasting \\ \end{tabular} &
\begin{tabular}{c} The mean square \\ error is 0.02, BER of 0 \\ \end{tabular} & [109] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of channel multicasting routers used for the OAM-based optical communications. neural network (Figure 2a) to transform the complex amplitude transmission functions into phase-only holograms (Figure 2b) [109]. The phase-only hologram predicted by their proposed network model generated a high-precision OAM spectrum for OAM multicasting with OAM-SK using only a few training datasets. At the transmitter, the information is sent to multiple users simultaneously when a beam of light passes through the hologram. Based on OAM-SK, the data at each user are encoded, and four different OAM states are selected to represent a 4-bit symbol, as can be seen in Figure 2a. Gray is used for a 0 weight OAM state, whereas OAM state 1 is represented by white. Decoding is performed at the receiver using a well-designed multiplexed OAM phase plate. A gray-scale algorithm measures the relative power of the four regions and translates it into the corresponding data. By switching holograms in the time sequence, real-time OAM optical communication can be achieved because, once trained, the network model can realize the noniterative calculation of all input signals. The hardware refresh time indicates the switching interval.