# Generation of Orbital Angular Momentum Modes Using Fiber Systems
## 5 Discussions and Perspectives  
As a new property of light that has been discovered relatively recently, OAM is playing an important role in various areas. There are two types of methods to generate OAM beams, spatial and fiber generating methods. Spatial generating methods have advantages in terms of flexible design and easily manipulation, but miniaturization is difficult. Compared with spatial methods, as natural azimuthal periodic beam-shapers, different types of fibers provide another way to generate OAM beams. Due to the miniaturization and low insertion loss, fiber methods occupy a place in OAM generation. CV modes, OAM modes and LP modes are three types of fiber modes. Each possesses its own unique properties. Any electric field in the fiber is able to be decomposed into the superposition of one group of them, such as \(E=\hat{x}OAM_{-l}=0.5 EH_{l-1,m}^{reen}+0.5 iEH_{l-1,m}^{odd}+0.5 HE_{l+1,m}^{ even}-0.5 iH_{l+1,m}^{odd}=\hat{x}LP_{l,m}^{even}-i\hat{x}LP_{l,m}^{odd}\). Mathematically, they operate as vector bases describing the electric field in the fiber. A particular higher order spatial mode consists of four degenerated modes. The four degenerated modes can be CV modes, OAM modes or LP modes. They are able to completely describe \(l\)th order electric field in fiber, in their own forms. There exist transformation relations among these three modes. The so-called \"OAM generation\" is equivalent to adjusting the amplitudes and phases of the four degenerated modes in a particular order and simplifying the expression in OAM mode bases using physical methods. Taking \(\hat{\sigma}^{+}OAM_{-l}\) as an example, if simultaneously calculating the equivalent CV modes, we obtain the typical expression similar to that found in almost all the studies, namely, \(\hat{\sigma}^{+}OAM_{-1}=TM_{01}+iTE_{01}\). Equations (2)-(4) give all of these transformation relations among CV modes, OAM modes and LP modes. As for the detection of OAM beams, researchers have aimed  
Figure 14: Mode pattern of generated OAM modes and the multiplexing of OAM modes using the MSPL. Reprinted with permission from [53], copyright 2018 The Optical Society. Figure 13: Schematic of operating principle of OAM generating based on the MSPL. Reprinted with permission from [52], copyright 2018 The Optical Society. to detect the TC, that is, the helical degree of the OAM beams. Because the physical nature is the phase detection, the interference method is mostly used. The TC of an OAM beam can be recognized from the interference patterns. The common interference patterns are vortex shape or fork-like shape, depending on the specific interference condition, namely, type of reference beam, divergence, tilted degree, and phase difference. A number of devices can generate OAM beams in a fiber system, including the optical fiber grating, the mode selective coupler, the microstructure optical fiber and the photonics lantern. Among these devices, the optical fiber grating and the mode selective coupler are relatively mature. However, their defection is also obvious. They can just couple the fundamental mode to one particular higher order mode. The microstructure optical fiber seems to be better than the former. It has more freedom in design and may be potential to realize some unique functions that other devices can't access. However, the fabrication of microstructure fiber is still a significant problem. Many microstructural optical fibers only exist in theory and are difficult to fabricate. Photonic lantern is the most potential OAM generating device so far. PL can be regarded as an enhanced mode selective coupler, which is able to distribute core modes into different higher order modes in distinguishing channel. This brings a significant benefit in terms of demultiplexing. Additionally, if reversing the structure of the PL (which requires a re-design but not a direct reverse the demultiplexing PL), different higher order modes can merge into a single core. Multiplexing can also be realized.