Mode Conversion and Orbital Angular Momentum Transfer Among Multiple Modes by Helical Gratings

The coupling feature of the helical gratings (HGs) has been studied on flexible conversion between two orbital angular momentum (OAM) modes by both the transverse and longitudinal modulation of the HGs. Apart from one-to-one OAM exchange, the HGs can achieve OAM transfer among multiple modes and provided that both the helix and phase matching conditions are well satisfied for successive coupling. It is contributed from the fold number of modulation fringes of the HGs and the difference multiplication of the propagation constants between two of the successive modes. Based on the coupled mode theory, we investigate the mode conversion and OAM transfer among three modes using a three-fold HG in a ring-core fiber, and numerically simulate it by a transmission matrix. Our work about the multiple modes transferring can be regarded as an extension of mode coupling for the optical gratings, and might flourish the mode conversion, as well as OAM manipulation in optics fields.