Determining the Signed High-Order Topological Charge With an Improved Mach–Zehnder Interferometer

Although the topological charge of a beam carrying an optical vortex is an important parameter, only low-order topological charge can be determined with most existing methods. We improve the Mach–Zehnder interferometer to measure the magnitude of the topological charge of Laguerre–Gaussian beam from the bright interference petals produced by the beam and its conjugate beam, and a plane mirror is used on one arm of the interferometer to produce the conjugate beam. Furthermore, the sign of the topological charge is determined by passing the beam through a triangular aperture and analyzing the diffraction field. We demonstrate that the magnitude and sign of the topological charge up to ±120 can be determined with this method.