Fig. 1
From: Active sorting of orbital angular momentum states of light with a cascaded tunable resonator
OAM sorting principle. a Conceptual diagram of the desired OAM sorting system based on a cascaded modular design. As an example, three different OAM states (l1, l2, and l3) enter the system. The first module outputs one state (l1) while rejecting and redirecting the other states (l2 and l3) to the next module, where state l2 is separated from the other state l3 and exits the sorter. The main component of each module here is an optical cavity/resonator tuned to a specific incident OAM state. M1 and M2 are highly reflective mirrors that form a Fabry-Pérot cavity. The resonant OAM state experiences high transmission through the cavity owing to resonant tunneling, while the non-resonant states experience destructive interference within the cavity and are therefore completely reflected. b Measured transmission of various OAM states as a function of the change in length of a Fabry-Pérot cavity. Experimental details are supplied in Supplementary Information Section B. The shift of the transmission peak, as measured using a photodiode, shows a clear dependence on |l|. Some small additional peaks correspond to parasitic cavity modes originating from imperfections in the mirrors, the slight misalignment/asymmetry of the incident beam, and the impurity within the incident OAM state. The insets show the intensity distribution of the transmission and respective fork interference pattern with a reference plane wave. The number of branches at the dislocation in the resulting interference pattern enables an identification of the value of the topological charge (|l|) of the OAM state, validating our claim that the transmitted beam retains its initial state
