A composite square zone plate (CSZP) is proposed to generate two two-arms-cross patterns with equal intensity and arbitrarily designed axial positions. Moreover, the CSZP with the spiral phase has two square optical vortices located at the arbitrary positions. In addition, we find that the same topological charge of twin square optical vortices for the CSZP follows a modulo-4 transmutation rule. The construction method of proposed zone plates is illustrated in detail. We prove numerically and experimentally that the CSZP and spiral-phase CSZP produce two tailorable two-arms-cross patterns with equal intensity and square optical vortices, respectively. The proposed zone plate can be used to generate hollow beams and have potential applications in infrared antennas and laser alignment systems.
Achromatic design for the generation of optical vortices based on radial spiral phase plates
