# Research Progress on Router Devices for the OAM Optical Communication
## 2 OAM Channel Multicasting
Figure 1b illustrates the fundamental concept behind the tunable OTDL, which relies on mode-dependent delays to perform the functions of correlation and equalization on a QPSK data signal. Specifically, the input optical beam carrying a QPSK data signal s(t) in the fundamental Gaussian mode is fed into the OTDL system. The input signal in the tunable OTDL system can consist of either pattern symbols [A, B, C, D] or a QPSK signal constellation that is distorted by chromatic dispersion. The OTDL adopts a multi-step approach: first, the system creates the input signal taps by multicasting into different OAM modes (\(l_{1}\), \(l_{2}\),..., \(l_{N}\)), where \(N\in[1\), \(2\),..., \(\infty\)). Second, each of the replicas in the OAM domain is an assigned amplitude and phase coefficient corresponding to a specific tap (\(h1\), \(h2\),..., \(hN\)). Finally, the taps are differentially delayed using a mode-dependent delay subsystem (\(0\), \(T\),..., \((N-1)\)\(T\)), allowing for precise temporal control of the signal, and the taps are combined to provide the output \(\sum_{k=1}^{N}h_{k}\times s(t-(k-1)T)\) in the fundamental mode. Subsequent experiments were conducted to validate the equalization function of the proposed system using a 20 Gbaud QPSK signal as input and subjecting it to 336-1008 ps/nm of chromatic dispersion. The  
Figure 1: Principle or experimental setup of OAM-RDs based on OAM channel multicasting (I); (**a**) experimental setup for OAM-mode multicasting [104], Col: Collimator; (**b**) the concept of tunable OTDL system using the modal-dependent delays [107]; (**c1**,**c2**) the multiplexed vortex state array [108]; (**c1**) depicts the concept of generating a multiplexed vortex state array using a phase-only hologram, and (**c2**) shows the experimental setup, BS, and beam splitter; L1–L4, lenses; CCD1&CCD2, infrared CCD camera; AS, aperture stop. results demonstrate that the system can significantly improve the signal quality, reducing the error vector amplitude (EVM) from 25.4% to 12.9% in the presence of 336 ps/nm chromatic dispersion, as depicted in Table 2.  
In order to further increase the channel utilization rate, in 2022, [PERSON] et al. [108] proposed a multiplexed vortex state array consisting of multiple multiplexed vortex beams, which allows for flexible manipulation of both the spatial position and the corresponding OAM spectrum as desired (Figure 1c1). Then, employing an array as a data-carrier, the research group demonstrated a one-to-many multicasting link through multi-state OAM shift keying, where four various states were utilized to encode a four-bit symbol, achieving high-dimensional encoding; the experimental setup is shown in Figure 1c2. The Gaussian beam generated by a distributed feedback laser diode is converted into a horizontal linear polarization state by a half-wave plate (HWP) and a polarization beam splitter (PBS), and then the multicast OAM is encoded by a specially constructed phase-only liquid-crystal SLM. SLM1 is employed to carry out multicast OAM encoding. By switching holograms loaded on SLM1 continuously, a coded time-varying multiplexed vortex state array sequence is generated, which records the input signals. The array contains three diffraction orders. Regarding every diffraction order as a channel, there are three channels to send information, which are labeled as channel 1 to channel 3. After propagating 1.5 m in free space, the three encoded multiplexed vortex beams are received and decoded by three identical receivers consisting of an SLR camera, a lens, and an infrared CCD camera. In a subsequent demonstration, the research group generated different grayscale images from one transmitter to each of the three receivers.