Generation of three equal-intensity foci based on a modified composite zone plate

In this paper a modified composite zone plate (MCZP) based on the Thue-Morse and Fresnel zone plates is proposed. The focusing properties of the MCZPs are studied theoretically and experimentally. Along the propagation direction the MCZP generates three main foci with approximately equal intensity. The point spread functions (PSFs) at the separate focal planes of the three foci have almost the same ratio between the energy in the diffraction limit region of the PSF plane and the energy in the entire PSF plane. The MCZPs have potential applications in the fields of three-dimensional optical tweezers and multi-plane lithography.