Narrowband reflective generation of higher-order optical vortices in Bragg spun optical fibers

We have studied theoretically reflecting and transmitting properties of Bragg multihelicoidal spun fibers, in which refractive index distribution features l helical branches and possesses an l-fold symmetry in the transverse cross section. It is shown that for a special choice of the profile function modulation, such fibers in certain spectral ranges have the property to change the topological charge of the incoming Gaussian beam in the reflected field by l units. This property could be used for narrowband generation of optical vortices (OVs) from Gaussian beams and for changing the topological charge of incoming OVs.