Efficient mid-Infrared vortex beam generation using optical waveguides integrated with micro-spiral phase plates

Vortex beam generation from a mid-infrared photonic chip was numerically demonstrated by engineering the phase front of a waveguide mode. Multi-level micro-spiral phase plates (SPPs) consisting of N segments were patterned on the waveguide facet. The micro-SPP introduced an annular phase change by 2 pi that converted the Hermite-Gaussian (HG) waveguide mode into Laguerre-Gaussian (LG) mode at topological charge vertical bar 1 vertical bar = 1. Using a 3D finite difference time domain simulation, we resolved the near-field optical phase map next to the micro-SPP and its far-field beam profile projected from the waveguide surface. The vortex beam formation was spectrally dependent. A uniform and symmetric donut-shaped pattern corresponding to an ideal vortex beam was created at lambda = 3.1-3.3 mu m. In addition, the HG -> LG mode conversion efficiency was improved by increasing the number of segments, N. Thus, our micro-SPP enabled present photonic circuits to create vortex beams, which is critical for the development of optical interconnection and broadband communication.