Vortex beam array generated by a volume compound fork grating in lithium niobite

Abstract

Here, we report a generation of vortex beam array based on a micro volume compound fork grating fabricated by a femtosecond laser direct writing technique in lithium niobite crystal. The grating is able to convert a Gaussian laser beam into an array of large number of vortex beams with high purity and high diffraction efficiency. The intensity distribution and the topological charges of the vortex beams are investigated, indicating there are multiple pairs of vortex beams with opposite topological charges in the array. Interference synthesizing method is also studied to superpose the opposite order vortex beams to produce integer-order vector-vortex beams, in addition to the generation of the fraction-order vector-vortex beam. The theoretical calculation and the experiment polarization measurement results indicate that the beams produced have spatial anisotropic polarization distribution. The micro structure of the compound fork grating has potential applications in generating vector-vortex beams in integrated optics field.