# Research Progress on Router Devices for the OAM Optical Communication
## 2 OAM Channel Multicasting  
In OAM-based optical communication networks, the transmission efficiency of OAM communication links can be greatly expanded by the selective manipulation of different data channels. One such OAM router device (OAM-RD) realizes OAM multicasting by dispersing a single channel into multiple channels and then transmitting data information from a single channel to multiple channels. These channels are orthogonal to each other and can serve different users in multi-user communication so as to better adapt to the multi-user environment. At present, multicast technology is widely used in traditional optical communication and OAM optical communication [95; 96; 97; 98; 99; 100; 101; 102]. For an OAM optical communication system, the OAM-RD, which can realize the OAM multicasting, plays an important role [103]. In 2012, [PERSON] et al. proposed a method of spatial mode multicasting in OAM mode spatial division multiplexing [104]. As depicted in Figure 1a, a quadrature-phase-shift-keyed (QPSK) optical signal is transmitted from a single-mode fiber to a fiber collimator. During the OAM mode multicasting stage, when an OAM beam with input topological charge 1 encounters an angular-amplitude aperture with n-order rotational symmetry, it can be assigned to a multi-OAM beam with equally spaced OAM topological charges [105]. For coherent detection, one of the multicast channels is demultiplexed and then coupled to a single-mode fiber. First, the Gaussian beam emitted from the collimator is transformed into an OAM beam using a spatial light modulator (SLM) loaded with a helical phase distribution to generate a single OAM channel. And then, the input single OAM channel is converted into a superposition of multiple OAM channels using a specially designed slicing phase pattern on SLM-2. SLM-3 is utilized for demultiplexing one of the multicasting channels. After demultiplexing, the beam passes through a pinhole and then couples into a single-mode fiber for coherent detection. Subsequently, this research group demonstrated through experiments that their proposed method can produce up to eight multicast modes with uniform power and less than \(-20\) dB crosstalk, and the power loss induced by the multicasting is only \(0.3\) dB, as shown in Table 2. The significant advantage \begin{table}
\begin{tabular}{c c c c} \hline
**OAM-RD** & **Schematic Diagram** & **Function** & **Reference** \\ \hline Multicasting & Input & Extended communication link & [95; 96; 97; 98; 99; 100; 101; 102] \\ \hline Switching & Input & Output & Exchanging data & [113; 114; 115; 116; 117; 118; 119; 120; 121; 122; 123; 124; 125; 126; 127] \\ \hline Filtering & Input & Output & Extracting specific channel & [128; 129; 130] \\ \hline Hopping & & OAM mode & & \\  & Input Output & \(l_{1}\) & \(l_{2}\) & \(l_{3}\) \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\ \hline \end{tabular}
\end{table} 
Table 1: Schematic diagram of OAM router function and previous works related to OAM-RD. of this scheme is that it can generate the required OAM modes while maintaining a low power loss. However, multiple optical devices are needed in the experimental process, and the device integration is relatively low. In real-world applications, beam power requirements may vary for different users. To address this issue and to improve the overall transmission efficiency, in 2015, [PERSON] et al. [106] proposed a multicast scheme with adjustable power by employing a specially designed complex phase pattern to enable the generation of composite beams with multiple OAM modes superimposed with adjustable success rates. In 2022, [PERSON] et al. [107] presented a novel approach to creating tunable optical tapped-delay-lines (OTDLs) based on the OAM modal domain. This method leverages OAM mode multicasting to generate the necessary taps, thereby enabling flexible and efficient tunability.