Optical vortex arrays in a multimode silicon waveguide

The immense potential of optical vortex arrays (OVAs) has been validated in applications such as particle manipulation, optical imaging, metrology, quantum, and optical communication. Photonic integrated technology has provided a powerful platform for the manipulation of vortex beams, which has the advantages of compact footprint, high controllability, and flexibility. However, since vortex beams are not eigenmodes of rectangular optical waveguides, the on-chip generation of in-plane OVAs has not been reported yet. Here, we propose a controlled method to generate in-plane OVAs by superimposing polarization-multiplexed modes in multimode silicon waveguides. Based on the proposed method, two distinct types of OVAs can be flexibly generated over an ultra-wide optical bandwidth. One exhibits opposite topological charge directions between adjacent vortices (vortex-antivortex), which are achieved by superimposing the fundamental transverse magnetic (TM₀) mode with the i-order transverse electric (TE_i) mode. The other shows consistent topological charge directions between neighboring vortices, which is realized by superimposing the i-order transverse magnetic (TM_i) mode with the TE_i mode. By controlling the phase of modes and waveguide width, the topological charge, transmission state, and quantity of vortex beams can be flexibly adjusted.