Generation of Optical Vortices by Linear Phase Ramps

Generation of optical vortices using linear phase ramps is experimentally demonstrated. When two regions of a wavefront have opposite phase gradients then along the line of phase discontinuity vortices can be generated. It is shown that vortices can evolve during propagation even with the unequal magnitude of tilt in the two regions of the wavefront. The number of vortices and their location depend upon the magnitude of tilt. vortex generation is experimentally realized by encoding phase mask on spatial light modulator and their presence is detected interferometrically. Numerical simulation has been performed to calculate the diffracted intensity distribution from the phase mask, and presence of vortices in the diffracted field is detected by computational techniques. 1. Introduction Optical fields possessing phase singularities or wavefront dislocations have received great deal of attention in recent years because of their fascinating properties and potential applications. The study of phenomena associated with phase singularities has generated a new branch of physical optics called singular optics, which reveals their basic properties and possible applications. Phase singularities also known as optical vortices are the points where the real and imaginary part of wave function vanishes. At phase singular points amplitude is zero and the phase is indeterminate. As a result, the wave front acquires the shape of helicoids, causing the precession of the Poynting vector. Nye and Berry showed that vortices in scalar field are strongly connected with the phase discontinuities or singularities [1]. Near the dislocation centers the phase gradients line is found to form closed vortex-like structures. Optical vortices can appear spontaneously or can be created in one of several ways, such as by the manipulation of laser cavity [2], transforming Gaussian beams into helical beams [3], computer generated holograms [4], cylindrical lenses [5], and spiral phase plates [6]. In recent years several important applications of optical vortices have been demonstrated. For example vortices have been used in optical tweezers [7], wave guiding [8], astronomy as coronagraph [9], optical testing [10], and so forth. In this paper, experimental and simulation studies are performed to show the formation of vortices along the line of phase discontinuity which arises due to the different phase gradients in the wavefront. To explain the concept of vortex evolution we first discussed a simple case in which the two parts of the wavefront are given equal and opposite phase variations.