Evaluating the coupling efficiency of OAM beams into ring-core optical fibers.

In optical communications, space-division multiplexing is a promising strategy to augment the fiber network capacity. It relies on modern fiber designs that support the propagation of multiple spatial modes. One of these fibers, the ring-core fiber (RCF), is able to propagate modes that carry orbital angular momentum (OAM), and has been shown to enhance not only classical but also quantum communication systems. Typically, the RCF spatial modes are used as orthogonal transmission channels for data streams that are coupled into the fiber using different free space beams. Free space beams commonly used are Laguerre-Gaussian (LG) and perfect vortex (PV) beams. Here, we study the optimal conditions to multiplex information into ring-core fibers in this scheme. We study the beam coupling efficiency using the overlap between free space beams and RCF bound beams and determine which are the most relevant LG beams to be considered and how their coupling efficiency can be maximized by properly adjusting the beam width with respect to the fiber parameters. Our results show that the coupling efficiency depends upon the OAM value and that this can limit the achievable transmission rates in SDM systems. In this regard, we find optimal coupling configurations for LG beams based on the RCF fiber and beam parameters. Further, we study the PV beam that allows for nearly perfect coupling efficiencies for all spatial modes supported by these fibers. PV beams present higher coupling efficiencies than LG beams and negligible dependence on the OAM value, thus offering an attractive solution to multiplex high counts of OAM channels from free space into a ring-core fiber using a single coupling configuration.