Acoustic vortex (AV) beam is triggering the significant research interest in information and communication sciences due to its infinite and mutual orthogonal orbital angular momentums (OAMs). Therefore, measuring the topological charges of an AV beams become a task with great significance. In this work, we present a Fraunhofer diffraction (FD) pattern of an AV beam that can be used to quantitatively detect the OAMs of AV beams. We both theoretically and numerically investigate the FD patterns of AV beams passing through a multipoint interferometer (MPI). It is demonstrated that the topological charges of the AV beams can be determined from the interference intensity patterns. The proposed method may pave the way to the practical applications of AV beams.
* Project supported by the National Natural Science Foundation of China (Grant Nos. 61775050 and 11804073), the Natural Science Foundation of Anhui Province, China (Grant Nos. 1808085MF188 and 1808085QA21), and the Fundamental Research Funds for the Central Universities, China (Grant No. PA2019GDZC0098).
Cite this article:
Chao-Fan Gong(龚超凡), Jing-Jing Li(李晶晶), Kai Guo(郭凯), Hong-Ping Zhou(周红平)†, and Zhong-Yi Guo(郭忠义)‡ Measuring orbital angular momentum of acoustic vortices based on Fraunhofer’s diffraction 2020 Chin. Phys. B 29 104301
Fig. 1.
Geometry and notation of generic MPI consisting of N points, uniformly distributed over a circle of radius a. The points are indicated by white dots and the angular coordinate of the n-th point is αn = 2π nN−1.
Fig. 2.
Simulated far-field intensity patterns behind multipoint interferometer of N points illuminated by AV beam with topological charge l.
Fig. 3.
Simulated far-field intensity patterns behind MPI with N points illuminated by AV beam with topological charge l, with patterns for l = |m| and l = – |m| being mirrored in x axis.
Fig. 4.
(a) Displacement of MPI with respect to propagation axis of impinging beam with d for N = 6, and (b) MPI axis tilted with respect to the propagation axis of the impinging beam with θ for N = 6.
Fig. 5.
Far-field intensity patterns behind a multipoint interferometer for N = 5 illuminated by AV with l = 2 displacement and tilt of the multipoint interferometer with respect to the propagation axis of impinging beam with (a) d = 0 mm with θ from 0° to 20°, (b) d = 0.3 mm with θ from 0° to 20°, (c) d = 0.6 mm with θ from 0° to 20°, and (d) d = 0.9 mm with θ from 0 ° to 20°. A displacement and a tilt of the multipoint interferometer with respect to the propagation axis of the impinging beam result in distorted and blurred interference patterns.
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