Exclusive generation of orbital angular momentum modes in parity-time symmetry fiber gratings

Orbital angular momentum (OAM) modes in fibers process great potentials in providing, from a higher dimension, extra communication channels to meet the ever-growing bandwidth requirement for modern information infrastructures. Long period fiber gratings are commonly used to generate these modes in fibers by coupling fundamental modes to degenerate higher order modes. In order for the generation process to be selective for a certain OAM mode, careful control of the state of polarization (SOP) is required for input fundamental modes. Here, we propose a non-conventional long period fiber grating with not only longitudinal index modulation but also azimuthal parity-time symmetry index modulation, and by placing this grating system at the parity-time exceptional point, an OAM mode with its topological charge matched to the parity-time modulation could be exclusively excited by using any fundamental modes with an arbitrary SOP except for the particular SOP with exactly opposite helicity. Our concept demonstrated here to generate OAM modes in fibers may open up promising applications in parity-time OAM-fiber optics.