Diffractive optical elements (DOEs) are widely used to realize special diffraction fields today, but the size of the effective Fresnel diffraction field of the DOEs with plane wave incidence is limited by the wavelength of the incident beam, sampling interval of the DOE, and distance between the DOE and the output plane. In this paper, a method is proposed to extend the size of the effective Fresnel diffraction field with an introduced intermediate plane and two-step diffraction calculation. Zero padding is used on the DOE plane, the sampling interval on the intermediate plane is correspondingly decreased, and the size of the Fresnel diffraction field on the output plane is finally extended. The accompanying aliasing is eliminated by placing a low-pass filter on the intermediate plane. Both numerical simulations and experimental results show the validity of the proposed method to extend the size of the effective Fresnel diffraction field of the DOEs with plane wave incidence.

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Axial multifocus beams (AMBs) potentially offer convenience in scientific and industrial applications. However, the general approach used for forming AMBs based on the Fresnel zone plate limits the conceivable applications. In this paper, we introduce a custom-made method to generate AMBs by the point source method to obtain initial phase distribution and a modified Gerchberg-Saxton algorithm to iteratively optimize the phase distribution. Meanwhile, the sidelobe is also constrained in the design simultaneously. Here, foci numbers, positions, and intensity distributions of the AMBs have been designed by binary optical elements, which can be controlled readily and effectively. Both numerical simulations and experiment results show the validity of the proposed method.

In this letter, we propose a method for designing super-resolution hollow beams (SHBs) with long depth of focus (DOF) using an M-ring 0-π pure-phase binary optical element (BOE) and a focusing lens when a linearly polarized Bessel–Gaussian beam incident. We also introduce a custom-made method for SHBs with long DOF to iteratively obtain optimal 0-π phase distribution. Moreover, it is shown that the hollow beams can provide great flexibility by controlling spot size and DOF through adjustment of the BOE parameters, including different ring numbers and positions, and also have significant potential for particle trapping.

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