Robust method to probe the topological charge of a Bessel beam by dynamic angular double slits.

Precisely determining the topological charge (TC) of both integral Bessel beams (IBB) and fractional Bessel beams (FBB) is a key issue for their applications. However, most of the common methods could not probe both the IBB and FBB precisely and robustly. In this paper, we present a robust method to probe the topological charge of a Bessel beam by dynamic angular double slits (ADSs). We find that when a Bessel beam vertically illuminates on a dynamic ADS, the information of the TC can be retrieved from its Fourier transform patterns near the optical axis. Even though there is a small misalignment between the center of the beams and the ADS, the global variation tendency is still good enough to obtain the TC. Based on these properties, the dynamic ADS device combining the method of fitting the experimental data can be used to measure the TC of any Bessel light beam precisely and robustly. The error is less than 2% without the misalignment and is less than 6% with a small misalignment for our experimental data. This method paves a new way to measure the TC of vortex beams.