# Research Progress on Router Devices for the OAM Optical Communication
## 3 OAM Channel Switching  
For the OAM optical communication system, it might be useful to extend the employment of an OAM-based communications link in a multi-user environment, in which different OAM data channels could be selectively manipulated. One of the OAM-RDs that can realize channel switching plays an important role [113; 114; 115; 116; 117; 118; 119]. In 2013, [PERSON] et al. [120] experimentally demonstrated OAM-based reconfigurable optical switching functions when multiple OAM modes were used by switching \(n\) input modes with arbitrary OAM charges to \(n\) OAM modes with a desired number of charges. The concept of OAM-based reconfigurable optical networking functions can be described in parts \"Charge shift\", \"Charge exchange\", and \"Charge selective manipulation\". \"Charge shift\" can realize the \(m\)-\(l\) shift of all OAM modes by using an SLM with topological charge \(m\)-\(l\) and a mirror. \"Charge exchange\" can reverse the order of OAM modes by using only one SLM, thus realizing the information exchange between the two OAM modes. \"Charge selective manipulation\" combines the functions of \"Charge shift\" and \"Charge exchange\" to easily manipulate the number of OAM modes using two SLMs without affecting the OAM of other modes. By effectively concatenating these three functions, it is possible to reposition any input of \(N\) OAM modes to any desired output state. Then, the team experimentally verified the feasibility of this OAM-based reconfigurable optical networking function system. In multi-pair OAM mode switching, the average conversion efficiency is \(-\)3.6 dB. At a BER of 2 \(\times\) 10\({}^{-3}\) (enhanced FEC (EFEC) threshold), the average OSNR penalties for the channels before and after exchanges are 1.5 dB and 2.4 dB, respectively. This research content has a very important influence on the OAM optical communication industry, and provides a new idea for the subsequent research in the design of router parts, as shown in Table 3  
Low-cost channel switching can also be realized by directing two composite vortex beams with different data and OAM mode values of \(l1\) and \(l2\) on a reflective SLM loaded with an OAM mode value of \(l\)R = \(-(l1+l2)\). This converts the OAM mode values to \(-l2\) and \(-l1\)[121]. Taking the research further, [PERSON] et al. [122] investigated a reconfigurable OAM channel switch with a 2 \(\times\) 2 layout in 2013. The group experimentally demonstrated \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Year** & **Methods** & **Equipment** & \
\begin{tabular}{c} **Strengths/** \\ **Weaknesses** \\ \end{tabular} &
\begin{tabular}{c} **Transmission** \\ **Result** \\ \end{tabular} & **Ref.** \\ \hline
2013 & \begin{tabular}{c} Reconfigurable \\ optical networking \\ functions \\ \end{tabular} & SLM & \
\begin{tabular}{c} Multi-pair data channel \\ exchange/Cumbersome \\ structure \\ \end{tabular} &
\begin{tabular}{c} The average conversion \\ efficiency is \(-\)3.6 dB \\ \end{tabular} & [120] \\ \hline
2013 & Mode overlay & SLM & \
\begin{tabular}{c} Simple structure/ \\ Only switch the single OAM \\ \end{tabular} &
\begin{tabular}{c} When the BER is \\ \(1\times 10^{-9}\), the power loss \\ is less than 2.4 dB \\ \end{tabular} & [121] \\ \hline
2013 &
\begin{tabular}{c} Reconfigurable \\ 2 \(\times\) 2 switch \\ \end{tabular} & SLM & - & Signal-to-noise ratio less \\ \hline
2016 & Space switching & SLM &
\begin{tabular}{c} Adjustable output position/ \\ Complex system and high \\ device cost \\ \end{tabular} & - & [123] \\ \hline
2017 & \begin{tabular}{c} Optical vortex \\ grating \\ \end{tabular} & \
\begin{tabular}{c} Digital \\ micromirror \\ \end{tabular} & Fast switching speed/- &
\begin{tabular}{c} The router has a \\ fast-switching time of \\ 6.9 μs \\ \end{tabular} & [124] \\ \hline
2021 & Optical separation &
\begin{tabular}{c} Reflector and \\ SLM \\ \end{tabular} & Simple structure and low cost/ & - & [126] \\ \hline \hline \end{tabular}
\end{table}