Precision measurement of the topological charge of a fractional vortex beam based on angular-grating-diffraction OAM spectrum

Vortex light beams carrying fractional vortices have shown promising applications in many fields such as optical communications, optical encryption, and quantum information processing. Accurate detection of the topological charge of a fractional vortex phase is essential for these applications. In this paper, a simple yet effective method for measuring the fractional topological charge is proposed, which is based on the detection of the orbital angular momentum (OAM) spectrum of a fractional vortex beam diffracted by an angular grating. When the fractional vortex beam passes through an angular grating, the OAM spectrum broadens, increasing the number of detectable OAM components. By applying nonlinear least squares fitting to the broadened spectrum, the topological charge can be detected with high precision. Experimental results demonstrate the detection range for fractional topological charges is from -6 to 6, with a resolution of 0.01 and an error of less than 0.005. Our protocol offers significant potential for high-capacity information transfer.