Measuring the Topological Charge of Vortex Beams With Gradually Changing-Period Spiral Spoke Grating

A method to detect of the topological charges (TCs) for vortex beams by using gradually changing-period spiral spoke grating (GCPSSG) is designed and experimentally demonstrated. The value of TC can be measured due to the fact that the diffraction spot will appear a bright high intensity when the TC is equal to the diffractive grating spoke number, and otherwise the central intensity remains doughnut-like structure. Meanwhile, the rotated direction of the twist fringes is opposite with the grating, therefore the sign of TC can be distinguished. Moreover, in addition to discussing the light beam illuminates the geometric center of the grating, the beam misalignment condition is also considered. In this case, the magnitude and sign of TC can be simultaneously determined by the number and orientation of the bright spots, respectively. The detection of TCs up to ± 160 is realized with this scheme. The experimental results indicate that this scheme is robust and effective because the GCPSSG shows good tolerance to environment vibration and beam misalignment for the TC diagnostics, which is anticipated to be useful for optical communication.