# Generation of Orbital Angular Momentum Modes Using Fiber Systems
## 4 Advances in Fiber OAM Generation Systems
### Mode Selective Coupler (MSC)-Based OAM Generation Systems
The fabrication of MSC, composed of SMF and the FMF/MMF is easier than the MSC composed of SMF and RCF, while the later MSC is more compatible to the OAM mode transmission system than the MSC composed of FMF/MMF. There have been many works to generate OAM mode based on the two types of MSCs [45; 46; 47; 48]. The experimental setup usually used to generate the OAM mode based on the MSC is shown in Figure 11a The MSC is used to couple the fundamental mode to the higher order CV mode group, and the polarization controllers are used to redistribute the generated random state of \(l\)th-order CV modes to the OAM modes we want to obtain. The polarizer is used to identify the polarization state of the generated OAM modes or to separate the two orthogonal OAM modes. Figure 11: (**a**) A typical experimental setup based on MSC to generate OAM modes; (**b–c**) mode pattern of generated (**b**) LP-OAM, and (**c**) two orthogonal CP-OAM mode, respectively. The authors of this paper have also experimentally demonstrated that arbitrary linearly-polarized OAM mode can be generated by carefully adjusting the PC without changing the polarization state of the input light based on the MSC, composed of a SMF and a TMF. The experimental results are shown in Figure 11b. We can find that the 0\({}^{\circ}\)-, 45\({}^{\circ}\)-, 90\({}^{\circ}\)-, and 135\({}^{\circ}\)-polarized LP-OAM modes with \(\pm\)1 TC are generated successfully. In addition, we have also experimentally demonstrated the generations of \(\sigma^{+}OAM_{\pm 1}\) and \(\sigma^{-}OAM_{\pm 1}\) mode using a single first order CV mode, including \(TM_{01}\), \(TE_{01}\), \(HE_{21}^{open}\), and \(HE_{21}^{odd}\) modes. These two CP-OAM modes can be separated by using a QWP and a polarizer with an angle \(\pm\pi/4\). The results are shown in Figure 11c. [PERSON] et al. have proposed an all-fiber system to generate tunable first order OAM mode based on an all-fiber MSC, composed of an SMF and a TMF [48]. The MSC is used to couple fundamental mode to the first order mode. And they experimentally demonstrate the generation of first order OAM mode, produced by combining \(HE_{21}^{open}\) and \(TE_{01}\) (or \(HE_{21}^{odd}\) and \(TM_{01}\)) with a \(\pi/2\) phase shift, and the topological charge of generated OAM mode can be tuned from \(-1\) to \(+1\) by adjusting the polarizer placed at the end of the FMF. The excitation of first order CP-OAM modes obtained by Equation (7), using an MSC composed of an SMF and an RCF for the first time have been experimentally demonstrated in [47], and the mode purity of excited OAM mode is up to 75%. Compared to the fiber grating-based mode converters mentioned above, the advantages of MSC include low loss, small size, easy fabrication, broad-bandwidth and the controllable coupling efficiency for the pure high-order mode which can be used in certain situations, such as in laser cavities, and so on. However, the MSCs also have disadvantages, for example, one MSC can only convert the fundamental mode to a particular high-order mode and, thus, we can only obtain one OAM mode by one MSC. If we want to multiplex N OAM modes, we need to cascade N MSCs which brings extra loss and complexity.