Analyses of measurement of the asymmetric orbital angular momentum spectrum with a difference approach

In recent years, vortex beams have gradually become a hot topic of research due to their unique properties of the orbital angular momentum (OAM) [1-4]. In this paper, a differential method for the measurement of asymmetric OAM spectrum is proposed by using an improved Mach-Zehnder interferometer. The number of mirrors in such an interferometer is odd and even for the two optical arms, respectively. In addition, a phase modulator is added to one of the optical arms to tune the phase difference. The comparison between experimental results and numerical simulations shows that our method is feasible for measuring asymmetric OAM spectra, which cannot be easily achieved using normal methods. In addition, we propose a previously unconsidered encoding dimension for the optical communication of vortex light: the shape of the orbital angular momentum spectrum. The method introduced in this paper can be applied to further application scenarios and can measure both symmetric and asymmetric spectra.