# Research Progress on Router Devices for the OAM Optical Communication
## 3 OAM Channel Switching

Table 3: Summary of channel-switching routers used for the OAM-based optical communications. that multiplexed OAM beams can be selectively re-sent to different outputs by using multiple SLMs to first split the separated beams in space, followed by rerouting the separated beams. A schematic of the 2 \(\times\) 2 OAM switcher that can either pass or redirect each input OAM beam can be seen in Figure 3a. Initially, the multiplexed input beam is converted into a Gaussian-like beam. At this time, the mode values of other OAM beams also change accordingly. After that, the OAM beam and the Gaussian beam are separated in space by employing a specially designed beam control phase mode, and the coaxial recombination of the Gaussian beam and OAM beam on another path is realized. Finally, the composite beam's mode up-conversion is accomplished using the programmable SLM. In order to complete the mode up-conversion of the composite beam and the switching of the dual OAM channels, the center and peripheral phases of the composite beam correspond to distinct OAM modes. This allows the Gaussian beam to be restored to an appropriate OAM beam. The research group applied two multiplexed beams with OAM modes of \(l\) = +4, \(-4\), and \(l\) = +2, \(-6\), respectively, to input ports A and B. After system switching, the two multiplexed beams could complete the arbitrary switching of the OAM mode, and the signal-to-noise ratio was less than 2.5 dB, as presented in Table 3.  
In addition to channel switching between the different OAM modes, the researchers then focused on spatial location switching. In 2016, [PERSON] et al. [123] proposed a reconfigurable OAM mode-switching and spatial-switching scheme. By using multiple SLMs, this scheme realized the arbitrary control of the output position of the beam space while regulating the mode value of the input OAM beam. Thus, the tasks of OAM mode switching, space switching, joint OAM mode, and space switching can be completed in synch with each other. The experimental settings are shown in Figure 3b. Four channels transmit four different OAM beams generated by SLM1-SLM4 to specific locations in the spatial domain, which are then collected by three unpolarized beam splitters, BS1-BS3. BS1 collects the beams from SLM1 and SLM2 and splits them horizontally, whereas BS2 collects the beams from SLM3 and SLM4 and splits them horizontally. SLM5 emulates a 4 \(\times\) 4 switching node. The four input ports of the switching node are linked to the four OAM channels after BS3, and the four output ports are projected onto a camera for monitoring. Thus, 4 \(\times\) 4 OAM  
Figure 3: Principle or experimental setup of OAM-RDs based on OAM channel switching(I) (**a**) Functional block diagram of the 2 \(\times\) 2 OAM-based switch [122]; (**b**) experimental setup of reconfigurable 4 \(\times\) 4 OAM mode switching, space switching, and joint OAM mode and space switching [123]; (**c**) the schematic diagram of multi-channel MSI [118]; (**d**) experimental setup for the mode generation, dynamic switching, and detection between OAM modes. OC, 50:50 optical coupler; PC, polarization controller; H, holder; R, rotator; Obj, objective; ATT, attenuator; BS, beam splitter; Col, collimator; SLM, spatial light modulator; CCD, charge-coupled device [118]. mode switching, space switching, and joint OAM mode and space switching are realized. This scheme further expands the utilization range of the optical networks and improves the flexibility of network control, as illustrated in Table 3. However, the use of multiple SLMs in the process increases the difficulty of loading phase designs on different SLMs and increases the system complexity and device cost to a certain extent. This has certain constraints on the high-speed optical communication link system, which requires high real-time and economic performance. Very recently, [PERSON] et al. [118] proposed a unique concept of multi-channel (OAM) mode switching based on an in-fiber mode selective interferometer (MSI) formed in a four-mode fiber.