# Research Progress on Router Devices for the OAM Optical Communication
## 4 OAM Channel Filtering  
According to different applications, users have different requirements for channels where different OAM beams are located. When users have requirements for specific channels in multiplexed OAM communication links, it becomes increasingly urgent to study  
Figure 4: Principle or experimental setup of OAM-RDs based on OAM channel switching(II): (**a**) schematic of OAM-RD for intra-datacenter optical interconnects [124]; (**b1,b2**) the reconfigurable optical switching based on OAM [126]; (**b1**) dual-area mirror and principle of separating multiplexed beams; and (**b2**) principle of OAM-based optical switching. the filtering technology that filters out useless channels and only extracts specific channels according to the requirements [128; 129; 130; 131; 132; 133]. Using the optical geometric transformation, [PERSON] et al. [134] presented an OAM-RD in 2014 that could implement OAM filtering as can be seen in Figure 5a. The filtering operation works as follows: first, the light from the laser source is collimated and incident on SLM1, which has been pre-loaded with phase holograms designed to produce multiple superimposed OAM modes. The resulting OAM beam is sent into an OAM filter consisting of an SLM to control the light plate and a mode converter made up of two reflective elements used to convert phases. Based on the principle of optical geometric transformation, after the multiplexed OAM beam passes through the mode converter, the original annular distribution of light intensity is converted into a series of strip light spots whose distribution corresponds to the OAM mode value one by one. The light spot is then brought into focus on the focal plane using a convex lens. At various positions, the light spot can be selectively controlled using the SLM. The spot of OAM beam filtering required by the target is blocked and reflected at the same time. Finally, based on the principle of optical geometric inversion, the reflected light spots are recovered from the light spots to the normal annular OAM beam, and the beam splitter is used to separate them. Thus, the filtering operation of the required OAM beam components in the composite OAM beam is completed. The group experimentally demonstrated that the blocking mode and propagation mode output power suppression ratios are greater than 14.5 dB. However, when the mode value interval of the input composite OAM beams is 1, the light spots will overlap following geometric transformation, which results in considerable difficulty in subsequent filtering. This is because of the inherent limitations of optical geometry transformation. In order to further optimize the OAM multicast filter element and change the fixed relationship between diffraction order and OAM mode in the traditional computational hologram, in 2016, [PERSON] et al. [135] proposed a new routing scheme based on OAM-labeled. First, several Gaussian beams are incident at specific incidence angles to reach the Dammann optical vortex grating (DOVX), so as to ensure multiple coaxial OAM beams of different OAM beams are generated at the outgoing end. The receiver incident the OAM beam into a specially designed computer-generated holograph (CGH). By taking the relative root-mean-square error as the evaluation factor of the hologram, the pure phase transmittance function is optimized using the iterative method, and the phase distribution of the CGH is dynamically controlled. Multiple OAM beams are reconverted into Gaussian-like beams by using a CGH, which can then be routed to different output ports. These OAM-labeled beams can be manipulated through direct connection, switching, multicast, and filtering functionalities. These routing functions can be realized by reconstructing CGH.