# Research Progress on Router Devices for the OAM Optical Communication
## 3 OAM Channel Switching
The MSI is made up of two strongly modulated long-period fiber gratings (LPFGs), which realize the mode switching between the selected mode pair (OAM\({}_{0}\) and OAM\({}_{2}\)). By applying torsion to the MSI, the propagation constant of the two resonant modes will change, resulting in the control of the phase difference between both modes to switch at multiple channels. The experimental setup and the schematic diagram of the multi-channel MSI can be seen in Figures 3c and 3d, respectively. The majority of the reports discussed above use SLM to realize the function of OAM routing. Although the OAM beam generated by SLM has advantages in terms of conversion efficiency and energy consumption reduction, the switching speed is limited by the material properties of the device itself. In 2017, [PERSON] et al. [124] proposed an OAM-RD that used digital micromirrors instead of SLMs. They applied the device to an optical communication link system based on the data interconnection of OAM selection, as shown in Figure 4a. The optical router uses a binary grating driven by a digital micromirror to break the correspondence between the traditional OAM mode and diffraction order and can not only realize the mutual switching of input OAM modes but also ensure the function of multicast. In general, the scheme achieves the routing function of OAM channel switching well and also utilizes the advantage of a digital micromirror to greatly improve the switching speed, as shown in Table 3. In 2018, an OAM wavelength switch exploiting four OAM modes and four wavelengths was demonstrated by exploiting an integrated OAM multiplexer with four concentric waveguides, which allows for four input ports [125]. In 2021, [PERSON] et al. [126] proposed an optical separation device composed of an SLM and a double-area mirror. They used On-Off Keying (OOK) to modulate four channels to form two inputs, channel A and channel B, each channel with two multiplexed OAM beams. An SLM was utilized to modify the OAM-multiplexed beams in each channel, with one of the beams being transformed into a Gaussian beam. The Gaussian beam was separated by a double-area mirror, thus enabling the switching of the beams carrying different data in the two channels. Figure 4b1 shows the principal architecture of a 2 \(\times\) 2 OAM switcher. The double-area mirror is composed of two different mirrors. One was placed in the central region to reflect the Gaussian beam and the other in the outer region to reflect the OAM beam. Therefore, the separation of the Gaussian beam and OAM beam can be realized only by adjusting the angle between the inner and outer regions of the double-area mirror. As shown in Figure 4b2, two multiplexed beams with OAM modes of \(l_{1}\), \(l_{3}\)s and \(l_{2}\), \(l_{3}\) were applied to input ports A and B, respectively. One beam at each input port was replaced with a Gaussian beam, and the other beam, still carrying the OAM state, passed through the double-area mirror for beam separation. The Gaussian beams extracted from channel A and channel B were introduced into the OAM beams of the other channels, creating mutual coupling. The reconstituted beam was coaxially transmitted and added at the corresponding phase to convert the multiplexed beams of channel A and channel B into the initial topological charge state, thus enabling the data interchange carried by the beam. The proposed method has the advantages of a simple structure, low cost, easy integration, and large angle separation, and can be applied to various OAM reconstruction scenarios, as depicted in Table 3. This facilitates a large-scale flexible OAM network construction. Although the reported works above achieved promising results, a significant breakthrough in OAM switching was reported by [PERSON] et al. in 2023, reporting a 10 OAM \(\times\) 16 wavelengths two-layer switch for 19.2 Tb/s data traffic using an integrated mode multiplexer [127].