The topological charge (l) is the parameter that defines the amount of orbital angular momentum (OAM) of beams carrying optical vortices. In this article we expand on a robust experimental method to determine the topological charge that we previously developed [B. Khajavi & E. J .Galvez, Opt. Lett. 42, 1516-1519 (2017)]. This method consist of using a pair of confocal lenses and a wedged optical flat as a shearing interferometer. The interference pattern produced by a pure OAM mode directly reveals the magnitude and sign of the topological charge: two conjoined forks with |l| + 1 number of tines pointing away from (towards) one another for l > 0 (l < 0). Superpositions of two modes with the same value of |l| but with the opposite sign produce shearing interference patterns that can be used the infer the value of l and the relative weight of the component modes.
Behzad Khajavi and Enrique J. Galvez, "Determination of the topological charge of complex light beams by shearing interference from a wedged optical flat," Proc. SPIE 10549, Complex Light and Optical Forces XII, 105490M (Presented at SPIE OPTO: January 31, 2018; Published: 22 February 2018); https://doi.org/10.1117/12.2291188.
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