Simultaneous Detection of Modal Composition and Wavelength of OAM Fields Using a Hexagonal Vortex Filter

The present study investigated the possibilities of transmitting information using different wavelengths (Near-IR) and different orbital angular momentum (OAM) modes simultaneously and applying modal decomposition using a passive hexagonal vortex filter (HVF) fabricated using a lithography procedure. The HVF was designed by a modulo-$2 \pi$ phase addition of helical phases with three different orders (1, 2, and 3) with unique linear-phase functions in a hexagonal formation (0, $\pi /4$ and $-\pi /4)$. When a light field composed of multiple OAM modes is incident on the HVF, they are spatially mapped to unique lateral locations in the detector plane. Any change in OAM mode will result in a variation in the intensity distribution at the different locations but no change in the lateral mapping locations. However, when there is a change in wavelength, the linear phase varies and maps the modes at different lateral locations from the center of the detector. In this way, it is possible to measure the modal composition of the light field and its wavelength simultaneously and independently. We believe that the proposed approach will introduce a new and efficient dimension — wavelength, for free-space optical communication (FSOC) applications resulting in enhancement of information bandwidth.