High-Order OAM Mode Generator Using Multi-Cascaded Long-Period Fiber Gratings

We propose and demonstrate the inscription of multi-cascaded long-period fiber gratings (LPFGs) in a few-mode fiber using a CO2-laser. The multi-cascaded LPFGs containing three gratings were fabricated successfully. The first LPFG was used to realize the mode conversion from LP01 mode to LP11 mode, the second one was used to convert LP11 mode to LP21 mode, and the third one achieved the higher-order mode conversion from LP21 mode to LP31 mode. The second-order and third-order orbital angular momentum (OAM) modes can be generated using the cascaded LPFGs with the second and third gratings, respectively. The multi-cascaded LPFGs have the promising potential to be adopted as mode converters and mode scramblers in the mode division multiplexing system thanks to the flexible mode conversion among different higher-order modes.