We demonstrate an experimental scheme that allows the elucidation of the orbital angular momentum discrete spectrum of an arbitrary light signal. The orbital angular momentum spectrum is represented in a Laguerre–Gaussian mode base, and the spectral components are resolved in the frequency domain by exploiting the Doppler frequency shift that is imparted to rotating light beams.
Vasnetsov, M., Juan P. Torres, Petrov, D., Torner, L. Observation of the orbital angular momentum spectrum of a light beam. "Optics letters", Desembre 2003, vol. 28, núm. 23, p. 2285-2287.