Diffractive optical elements for the formation of laser beams with intensity distributions in the form of ring light traps with zero total orbital angular momentum are considered and studied by means of a computational experiment. The beams are formed as a superposition of two vortex beams with the same magnitude but different sign orbital angular momentum by encoding multilevel diffractive optical elements (256 quantization levels) in polar angle. The considered elements form an intensity distribution in the form of spots located in the same places where the light rings are located in the initial vortex beams. Such beams can possess all the properties of vortex beams when propagating in the atmosphere, for example, increased stability to aerosol noise.
Vortex beams are currently used in areas such as optical communication, optical measurement optical micromanipulation and many other applications. There are several prospective ways to generate vortex beams such as: by using special gratings [1,2], spiral phase plates[3], vortex zone plate [4]. Bessel and Gauss-Laguerre beams [5,6] are also considered as Vortex beams. Generation of Bessel beams by vortex axicons were considered in [6]. Possibility of combining the structures and zones topological charge of axicon in the same element was shown. Desired order of Bessel beams can be generated by a large variability of phase diffractive optical elements. In [7] method of forming a simple vortex beams by using a new type of diffractive optical elements, was presented. Diffractive optical element is a lens vortex with a topological charge zones, like the vortex in axicon [8]. In this paper, we have generated vortex beams by the method described in [7], but in addition the lens partitioned into two areas. Each area has different focal length. The proposed element structure can significantly extend focal region with the generated vortex beam that allows rotating microscopic objects in the threedimensional layer.
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