Multi-order orbital angular momentum mode generators based on integrated long-period fiber gratings.

We propose integrated long-period fiber gratings (LPFGs) fabricated by a CO laser to realize a multi-channel and multi-order orbital angular momentum (OAM) mode generator. The integrated LPFG is inscribed on multiple surfaces of the few-mode fiber (FMF) by rotating the fiber in the opposite direction at an angle θ. By controlling the rotation angle, the number of integrated LPFGs can be set. The selected rotation angle is 43 , which can integrate up to nine LPFGs, i.e., realizing that the number of channels for first-order orbital angular momentum (OAM) mode conversion is nine. The integrated LPFGs fabricated in this method allow a flexible design of channel spacing. In addition, the flexible selection of the integrated grating period achieves the simultaneous generation of multi-channel second-order and third-order OAM mode conversion. The multi-channel and multi-order OAM mode generators have important application in optical communication multiplexing systems and OAM sensing.