Designing diffractive arbitrary shaped top-hat beam in non-paraxial systems

Abstract

Beam shaping techniques with diffractive optical elements have garnered considerable attention for laser material processing and microscopy because of their high efficiency of light utilization. Particularly, the design of top-hat beams with several shapes including circular and rectangular is required to facilitate a high-throughput system for line scanning and surface peeling applications. In this study, we propose a diffractive beam shaping method for the generation of a tophat beam with arbitrary shapes under tight focusing conditions. We implemented the iterative Fourier-transform algorithm (IFTA) with an error function in the form of a Gaussian distribution in the input laser beam to calculate an optimized phase distribution for generating a top-hat beam with arbitrary shape. This phase distribution was generated with a phase-only spatial light modulator and relayed with an optical system to the pupil of an objective lens with a numerical aperture of 0.75. The point spread function under the focal spot was observed with a microscopic imaging system placed opposite to the beam focusing optics. We experimentally demonstrate that the size of the focused top-hat beam is twice the size of the airy disk under tight focusing conditions. Further, we measure the profile of the generated beams. The proposed method with a spatial light modulator offers an adaptive control on the uniformity of the generated distribution, which fluctuates according to the effect of slightly different laser conditions on the diameter and profile of the input beam.