Fig. 3
From: Active sorting of orbital angular momentum states of light with a cascaded tunable resonator
Modular design of an OAM sorter. a Experimental setup of the OAM sorter. A half-wave plate (HWP) is used to rotate the linear polarization axis of the laser beam prior to spatial filtering to facilitate the alignment with the optical axis of the SLM. An isolator is used to prevent unwanted reflected light from entering the frequency-stabilized laser cavity. The superimposed OAM states are prepared by an SLM loaded with specially designed patterns. A reference beam is generated from one of the ports of a non-polarizing beam-splitter (BS) for interference with the output OAM states in the latter part of the experiment. A lens (L1) is used to focus the incident beam to the first cavity (FP1) with the size of the focal spot matched to the cavity mode. The transmitted signal from each FP cavity is monitored using either a photodiode or a CCD camera. While only two FP cavities (enclosed in the dashed box) are used in this experiment, the high sensitivity and signal-to-noise ratio of the cavities allow a number of cavities to be cascaded to maximize the number of OAM states being sorted. b Optical circulator consisting of a polarizing beam-splitter (PBS) and a quarter-wave plate (QWP). The incident light is p-polarized (linear polarization). Light reflected from the FP cavity passes the QWP twice, transforms into the orthogonal s-polarization state, and therefore exits the PBS from a different port. RCP: right-handed circular polarization; LCP: left-handed circular polarization. c–g Intensity distributions and fork-like interference patterns when the pair of FP cavities are each transmitting a different OAM state. These images were taken at the two output ports when both cavities were tuned on resonance for one of the two OAM states. To capture these images, the first cavity was tuned on resonance to transmit |l = +1〉 and the second cavity was tuned to transmit |l = + 2〉 (and vice versa). We have also shown that the output OAM states can be swapped between the two ports dynamically by varying the cavity lengths (Configuration 1 vs. Configuration 2)
