A Focusing Diffractive Optical Element for Flat-Top Beam Shaping Resilient to Etching Depth Errors

Diffractive optical elements (DOEs) have become core components for converting Gaussian beams into flat-top beams due to their advantages of flexible design, compact size, and precise control over light field distribution. However, during the fabrication of multi-step DOEs, some processes such as ion beam etching tend to produce etching depth errors, which cause deviations of the surface micro–nano phase structures from the designed values and thus severely degrade the beam shaping performance. This paper proposes a focusing DOE for flat-top beam shaping, which combines the focusing phase with the phase optimized by the weighted constraint iterative algorithm to establish a phase distribution resilient to etching depth errors. Thus, the proposed focusing DOE exhibits significantly improved robustness to etching depth errors and effectively reduces the structural complexity of laser optical systems. Our experimental results show that the designed DOE can stably convert the incident Gaussian beam into flat-top beams within the etching depth error range of ±30 nm, with both the flat-top beam uniformity and diffraction efficiency above 95%, and the maximum tolerable etching depth error reaches ±90 nm.