Abstract
Measuring orbital angular momentum (OAM) states of vortex beams is of great importance in diverse applications employing OAM-carrying vortex beams. We present a simple and efficient scheme to measure OAM states (i.e. topological charge values) of vortex beams with annular gratings. The magnitude of the topological charge value is determined by the number of dark fringes after diffraction, and the sign of the topological charge value is distinguished by the orientation of the diffraction pattern. We first theoretically study the diffraction patterns using both annular amplitude and phase gratings. The annular phase grating shows almost 10-dB better diffraction efficiency compared to the annular amplitude grating. We then experimentally demonstrate the OAM states measurement of vortex beams using annular phase grating. The scheme works well even for high-order vortex beams with topological charge value as high as ± 25. We also experimentally show the evolution of diffraction patterns when slightly changing the fractional topological charge value of vortex beam from 0.1 to 1.0. In addition, the proposed scheme shows potential large tolerance of beam alignment during the OAM states measurement of vortex beams.
Highlights
Until now, a wide variety of methods have been proposed to generate optical vortex beams[10,11,12,13,14,15]
The magnitude of topological charge values can be deduced from the number of dark fringes of diffraction patterns
The annular phase grating offers almost 10-dB better diffraction efficiency compared to the annular amplitude grating
Summary
A wide variety of methods have been proposed to generate optical vortex beams[10,11,12,13,14,15]. Through the diffraction intensity pattern after an annular aperture, the measurement of the topological charge value of an optical vortex beam was up to |l| = 919. These well-established schemes achieved impressive operation performance for OAM detection Another simple method that only uses a tilted convex lens was proposed to enable the measurement of the topological charge magnitude and sign[25]. The titled lens method lacks of switchability, reconfigurability, scalability and integration In this scenario, more diverse approaches would still be expected to flexibly detect OAM states of vortex beams including high-order ones. The sign of the topological charge value of vortex beam can still be discriminated as the orientations of diffraction patterns are always different for positive and negative vortex beams.
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