  By exploiting helical gratings (HGs), we propose and simulate flexible
generation, conversion and exchange of fiber guided orbital angular momentum
(OAM) modes. HGs can enable the generation of OAM modes, and the OAM conversion
between two arbitrary modes guided in fibers. A specific HG can exchange the
OAM states of a couple of OAM modes, i.e, OAM exchange. In addition, a Fabry
Perot cavity cascaded with two identical reflective HGs can reflect converted
OAM modes with a comb spectrum. The HGs based generation conversion and
exchange of OAM modes are dependent on helix period, orientation, and the fold
number of helical fringes. The proposed method of generation, conversion, and
exchange of fiber guided OAM modes using HGs is flexible and well compatible
with OAM fibers, featuring a high conversion efficiency close to 100% and a
conversion bandwidth about 10 nm in transmission spectra, while less than 1 nm
in reflection spectra.
