Determining topological charge based on an improved Fizeau interferometer

We propose a new method to determine topological charge by using an improved Fizeau interferometer. This interferometer is very easy to realize, as well as interference fringes are very distinct. Phases of vortex, Hermite-Gaussian, and elliptical vortex beams are experimentally verified using this method. It provides a convenient way to determine the sign and magnitude of topological charge. This method may have some potential applications in space optical communication.