Abstract
Orbital angular momentum (OAM) is an inherent characteristic of photons, parallel with the linear momentum and spin angular momentum (SAM) of light. It has also been demonstrated in electron beams and, more recently, neutron beams. OAM can be described by discrete states known as the OAM states. Commonly, one can measure the OAM state by interference with a reference planewave. Recently, some new technologies (e.g. time-resolved two-photon photoemission microscopy or plasmon resonances) have been introduced to determine OAM states as well. However, the separation of different OAM states has remained a major challenge. In particular, for OAM states propagating in optical fibers, the direct realization of fiber-integrated OAM state detection using a single device is important, and it constitutes a crucial link in advancing OAM mode division multiplexing for optical fiber communications. Here, we report a fiber integrated OAM sorter using mainly a surprisingly simple yet highly efficient optical half lens. Our approach is based on a half-lens with broken symmetry that was integrated onto the fiber facet to obtain dispersed focal points for different OAM states. We believe that the device will soon become a commonly used OAM sorting technique due to the number of unparalleled features it offers.