###### Abstract  
We present a novel insights into orbital angular momentum and energy of a system of  
optical fiber, and the photonic lantern. Finally, the key challenges and prospects for fiber OAM mode systems are discussed.  
Another review paper, focusing on tunable OAM generation, is contributed from Prof. [PERSON]'s group [26]. The authors classify the tunable OAM mode generation methods into three categories, according to the OAM and polarization states. The fiber-based and free-space generation methods are categorized into three types according to the controllable variables, respectively. Last, the pros and cons of each generation method are analyzed and the key challenges for tunable OAM modes are discussed.  
Most fiber-based or free-space OAM beam generators are bulky, slow, and cannot withstand high powers. Prof. [PERSON]'s group design and experimentally demonstrate an ultra-fast, compact chalcogenide-based all-dielectric metasurface beam converter, which has the ability to transform a Hermite-Gaussian (HG) beam into an OAM beam at near infrared wavelength [27]. The topological charge carried by the output OAM beam can be switched between positive and negative values, and the device provides high transmission efficiency.  
For the reception of OAM radio waves, Dr. [PERSON] contributes a communication article using pseudo-Doppler interpolation techniques [28]. The method can be used to receive OAM waves in the far field of an antenna transmitting multiple OAM modes, each carrying a separate data stream at the same RF. The frequency domain method provides a higher signal-to-noise ratio (SNR) than using spatial-domain OAM reception techniques. Moreover, no more than two receiving antennas are necessary to separate any number of OAM modes in principle.  
In OAM communications systems, different OAM beams can carry multiple data channels, boosting the spectral efficiency and capacity significantly. Consequently, the simultaneous processing of OAM beams is necessary, and OAM multiplexing/de-multiplexing devices are key enablers of such systems. Prof. [PERSON] et al. contribute an article on mode-selective photonic lanterns for OAM mode division multiplexing [29]. The authors design a three-mode OAM mode-selective photonic lantern by optimizing the taper length with small mode crosstalk, which employs only a single mode fiber port to selectively generate each OAM mode.  
In a more integrated manner, Prof. [PERSON] and his group explore holographic Silicon metasurfaces for OAM de-multiplexing based on OAM-mode projection [30]. The device uses Pancharatnam-Berry optical elements (PBOEs) and can de-multiplex beams with different polarization and OAM states at the wavelength of 1310 nm. The geometric-phase control is achieved by inducing a spatially-dependent form-birefringence on a silicon substrate, patterned with properly-oriented subwavelength gratings.  
There are two experimental demonstrations for free-space OAM communications systems, one in the 1550 nm optical regime, and the other on 28 GHz RF frequency band. Dr. [PERSON] and Prof. [PERSON] investigate turbulence mitigation methods in free-space optical OAM communications system based on coded modulation [31]. Adaptive optics, channel coding, Huffman coding combined with low-density parity-check (LDPC) coding, and spatial offset are used for turbulence mitigation, achieving a total data transmission capacity of 500 Gbps.  
Finally, Dr. [PERSON] and colleagues evaluate the performance of OAM-based wireless communications systems [32]. To overcome the beam divergence of OAM multiplexing, the authors use a combination of multi-input multi-output (MIMO) and OAM technology, achieving a new milestone in point-to-point transmission rates at 100 Gbps for a 10 m transmission distance.