# Generation of Orbital Angular Momentum Modes Using Fiber Systems
## 4 Advances in Fiber OAM Generation Systems
### OAM Generation Based on Photonic Lantern  
The photonic lantern (PL) is a low-loss optical device that connects several single-mode cores or few-mode cores to a single multimode core. Early interest in PLs were based on their original application in astronomical instrumentation [75; 76], but recently its application in optical fiber communication has been attracting increased attention from researchers. The PL can be a spatial multiplexer for space-division multiplexing because one PL can multiplex \(N\) modes where \(N\) is the number of SMF used to fabricate the PL, which in principle allows the capacity of the communication system to be multiplied by \(N\)[77; 78; 79; 80; 81; 82; 83; 84; 85; 86; 87; 88; 89].  
There are five types of PLs that have been reported to date [80; 81; 82; 83; 84; 85; 86; 87; 88; 89]. The first type of PLs is fabricated by inserting \(N\) separate SMFs into a surrounding glass cane which is usually the PCFs made from glass with a pattern of air holes in the cladding [80; 81]. Then the resulting glass body is heated and drawn down to form a taper transition to an MMF port. The MMF core is formed by the fused mass of SMFs, with the reduced-index cladding formed by the case glass. This method has the advantages of easy control of the arrangement of SMFs by designing the arrangement of air holes of the cane. However, the SMFs are accommodated in separate compartments. The second type of PLs is fabricated by inserting a bundled N SMFs together into a capillary which has a lower RI than that of the SMF cladding [82; 83]. Then the capillary is heated and drawn on a tapering machine to form the MMF port, which is more practical. However, the arrangement of SMFs with this method is more difficult than that of the first type of PLs, and with the number of SMFs increasing, the difficulty of fabricating PLs also increases. The third type of PLs is fabricated by inserting an MCF into a capillary which has a lower RI than that of the MCF cladding [84; 85]. Then the capillary is heated and drawn on a tapering machine to form the MMF port. This method has the advantages, for example, several kilometers of the MSC can be drawn at once and this method needs only one MCF compared to the first and second type of PLs needing \(N\) SMFs. However, the MCF used in this method needs to be specially fabricated. The fourth type of PLs is fabricated by collapsing several holes in the multi-core PCF [86; 87]. This is achieved by heating the PCF without significant stretching the fiber. Then the surface tension causes the holes to shrink and collapse completely which in effect makes the cores bigger until they merge to form a multimode core. The last type of PLs is fabricated by the direct laser writing technique on an integrated waveguide chip for locally modifying the structure of a substrate material in three dimensions [88; 89; 90]. There have been several studies regarding generating or multiplexing several OAM modes based on the mode-selective PL (MSPL) [52; 53].  
The generation of \(OAM_{\pm 1}=LP_{11}^{a}\pm iLP_{11}^{b}\), \(OAM_{\pm 2}=LP_{21}^{a}\pm iL_{21}^{b}\), and the mixed OAM mode \(OAM_{+1}+OAM_{-2}\) and \(OAM_{-1}+OAM_{+2}\) are experimentally demonstrated by using the five-mode MSPL whose ERI profile is arranged to a ring shape in [52], as shown in Figure 13.  
An all-fiber OAM multiplexer based on a six mode MSPL and a mode PC (MPC) that can multiplex both OAM modes of \(-l\) and \(+l\) up to the second order have been proposed in [53]. The generation of \(OAM_{\pm 1}\), \(OAM_{\pm 2}\), the mixed modes of arbitrary two OAM modes in these two OAM mode groups and the mixed mode of all four OAM modes have been experimentally demonstrated, as shown in Figure 14.  
Compared to the MSC and the LPFGs, the MSPL can multiplex \(N\) modes at the same time and, thus, it can be used to multiplex \(N\) OAM mode at the same time. However, with the number of SMF to fabricate the PL increasing, the difficulty of fabricating it also increases at the same time, and the insertion loss will also increase.