Mimicking Faraday Rotation to Sort the Orbital Angular Momentum of Light

The efficient separation of the orbital angular momentum (OAM) is essential to both the classical and quantum applications with twisted photons. Here we devise and demonstrate experimentally an efficient method of mimicking the Faraday rotation to sort the OAM based on the OAM-to-polarization coupling effect induced by a modified Mach-Zehnder interferometer. Our device is capable of sorting the OAM of positive and negative numbers, as well as their mixtures. Furthermore, we report the first experimental demonstration to sort optical vortices of noninteger charges. The possibility of working at the photon-count level is also shown using an electron-multiplying CCD camera. Our scheme holds promise for quantum information applications with single-photon entanglement and for high-capacity communication systems with polarization and OAM multiplexing.

Figure 1

The function of our device. (a) Polarization rotation due to the Faraday effect. The rotation angle $\theta$ is proportional to the magnetic field $B$ and the interaction length $d$. (b) Polarization rotation produced by our device. The rotation angle $\theta$ is proportional to the OAM number $\ell$ and the relative orientation $\alpha$ of Dove prisms. (c) Experimental setup for mimicking the Faraday effect to sort the OAM (see the text for more details), where the inset shows the hologram addressed by the SLM to generate a fractional vortex of $M=9/2$.

Figure 2

Experimental results of the even-odd OAM sorter with a single OAM input. The even OAM, e.g., $\ell =-2$, 12, 20, appear at port $A$ while the odd OAM, e.g., $\ell =5$, $-17$, appear at port $B$.

Figure 3

Experimental results of the even-odd OAM sorter with multiple OAM input. The top and bottom panels are the flower patterns with different petal numbers recorded at ports $A$ and $B$, respectively.

Figure 4

Experimental results for sorting the half-integer optical vortices. The top row shows the intensity profiles of the half-integer vortices carried by the modified LG beams. The middle and bottom rows record the intensity profiles of the vortices output from ports $A$ and $B$, respectively.

Figure 5

Experimental results when working at the photon-count level. The left panel shows the intensity simulations of the input OAM mixtures. The middle and right panels display the patterns recorded by EMCCD at ports $A$ and $B$, respectively.