Deformation of optical vortex beam by displacement of incident-beam optical axis from spiral phase plate center

Abstract

An optical vortex beam (OVB) is obtained when a Gaussian beam is transmitted through an appropriately designed spiral phase plate (SPP). The OVB can be deformed in its optical intensity and phase distributions by displacements of the optical axis of the incident Gaussian beam from the center of the SPP. This deformation is discussed using theoretical simulations and experimental examples for an eight-segmented SPP. The far-field patterns of the output beams are calculated by using a two-dimensional fast Fourier transform. A dark hole appears at the center of the OVB. When the incident-beam optical axis is shifted from the SPP center, the dark hole is shifted from the output-beam center. It is shown that the shift direction of the dark hole is orthogonal to that of the incident-beam optical axis. The far-field optical intensity distribution of a doughnut shape is changed to a crescent pattern and finally becomes to a Gaussian beam according to the dark hole shift. Predicted crescent patterns are experimentally demonstrated using SPP-integrated photonic-crystal lasers. Obtained results by quantitative discussion will be helpful in constructing a SPP integrated laser as a high-quality chip-scale OVB source.