# Research Progress on Router Devices for the OAM Optical Communication
## 6 OAM Channel Adding/Extracting  
Selective channel adding or extracting is also an important part of optical communication links [150; 151]. In 2013, [PERSON] et al. [152] reported an OAM mode division multiplexer that efficiently added or extracted a single OAM beam. Figure 7a gives the working mechanism of the proposed experimental setup. The desired OAM beam is transformed into a Gaussian beam while the mode values of other OAM beams remain unchanged. Afterward, using an SLM loaded with two different region grating phases, a composite beam is separated. To filter out the center Gaussian beam of the coaxial composite beam, the inner grating changes the diffraction direction of the Gaussian beam, while the outer grating does not change the diffraction direction of the OAM beam, enabling the OAM beam to reflect along its normal path and thus extract the target OAM beam channel. For a 100 Gbit/s QPSK data channel, this scheme incurs a power loss of less than 2 dB, and the BER of the channel is only 2.0 \(\times\) 10\({}^{-3}\), as can be seen in Table 6. Although this OAM-RD can flexibly manipulate a single OAM channel, controlling the extraction or addition of multiple OAM channels is still a challenge due to the power loss caused by crosstalk between channels. Figure 7: Principle or experimental setup of OAM-RD for OAM channel adding/extracting: (**a**) concept of OAM channel add/drop multiplexing [152]; (**b**) the schematic of the modified MZI [154], BS: beam splitter, M: mirror, and DP: Dove prism; (**c**) concept of OADM for OAM add-drop multiplexing [155]. To add or extract multiple OAM channels, in 2019, [PERSON] et al. (2019) developed a geometric transformation-based OAM mode division multiplexer that successfully realized the extraction and addition of multiple OAM channels. First, a series of narrow light spots corresponding to the OAM mode values are obtained by using two SLMs loaded with special phases and a Fourier lens to transform the incoming OAM multiplexed beam based on the principle of optical geometric transformation. Then, the target OAM beam to be extracted is transmitted by setting the reflection phase on the SLM, while the other OAM beams remain reflected. Finally, the reflected light path passes through the previous optical components to complete the optical geometric inverse transformation and restore the original OAM beam, thereby achieving the extraction of multiple OAM beams in the composite beam. When it is necessary to add multiple OAM beams, the desired OAM beams to be added are first transformed into narrow light spots at the focal plane position through optical geometric transformation. Then, the SLM is adjusted to transmit the target OAM beam to be added while reflecting other OAM beams. After the reverse transmission along the same optical path, the added beam and the original beam are restored into a new OAM multiplexed beam at the output end through a beam splitter and optical geometric inverse transformation, thus completing the OAM beam adding operation. This scheme can not only realize the adding and extracting of OAM beams simultaneously but also further duplicate the other OAM modes in the composite OAM beam without affecting them, reducing the spot size to maintain the high energy purity of the OAM beam, as depicted in Table 6.  
In 2019, [PERSON] et al. (2019) introduced a high-density OAM analyzer comprising two OAM mode converters and a modified Mach-Zehnder interferometer (MZI). This MZI, which includes a pair of Dove prisms in addition to the standardized MZI configuration, effectively avoids the inherent overlap of adjacent OAM modes within the mode converter. In this scheme, the multiplexing OAM beam is incident into the modified MZI, which can sort and separate the even and odd OAM modes into different ports, respectively. The schematic of the modified MZI is shown in Figure 7b: when the input beam is incident to the beam splitter of BS1, the beam is divided into two beams.