# Generation of Orbital Angular Momentum Modes Using Fiber Systems
## 1 Introduction  
Since [PERSON] first demonstrated the orbital angular momentum (OAM) of light as an independent dimension in 1992 [1], beams carrying OAM have attracted increasing interest in various fields. OAM beams are characterized by the phase singularity and helical wavefront. Due to the helical wavefront, the propagating direction (or wavevector) is variant with the azimuthal angle, which is also helical relative to the optical axis. The helical degree is described by so-called \"topological charge (TC)\". OAM beams with different TC can be regarded as several independent dimensions carrying information. These properties make OAM beams different from conventional plane light waves and many unique applications in terms of atom manipulation [2; 3; 4], nanoscale microscopy [5], optical tweezers [6; 7; 8], optical communication [9; 10; 11; 12; 13], and data storage [14; 15] have been realized.  
There are many methods used to generate OAM beams. Such methods can be roughly divided into two categories, spatial and fiber generating methods. Spatial methods are generally assisted by spatial light modulators [16], spiral phase plates [17; 18], diffractive phase holograms [19; 20; 21; 22], metamaterials [23; 24; 25; 26], cylindrical lens pairs [27], q-plates [28; 29], photonics integrated circuitsincluding micro-ring resonators [30], among other devices, as shown in Figure 1. Each spatial method has its own advantages and disadvantages. However, there are two common defections for them, i.e., the high insertion loss and the large volume. Because there exists interface with high refractive index differences in these methods, the conversion processes from incident beams to OAM beams are not modest, thereby leading to a relatively high insertion loss. Meanwhile, the volumes of spatial devices are usually large. This indicates that miniaturization and integration of the space systems is challenging.  
Compared with spatial generating methods, fiber generating methods have shown their certain advantages. A converter applied in fiber methods can be fiber gratings [31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44], mode selective couplers [45; 46; 47; 48], and microstructure optical fibers [49; 50; 51] and photonic lanterns [52; 53]. As a cylindrical waveguide, fiber is a natural beam shaper for OAM beams. Incident beams of any shapes will be converted into the eigenmodes with cylindrical symmetry under the restriction of fiber. Since the conversion process is more modest than that in spatial generating methods, the energy efficiency is higher in fiber generating methods. Moreover, all the devices in fiber generating methods are smaller, which greatly facilitates miniaturization.  
This paper highlights recent advances in fiber OAM generation systems. We start by introducing three types of fiber modes in Section 2, i.e., cylindrical vector (CV), linearly polarized (LP), and OAM modes. Following this, Section 3 briefly describes the basic concepts and theories of OAM beam generation and detection. In Section 4, the recent advances in fiber OAM generation systems are given. A brief discussion of this research and further expectations are presented in Section 5.