A multi-pinhole interferometer for characterizing optical vortices Jonathan Preston, City College of New York John Noe and Marty Cohen, Laser Teaching Center, Stony Brook University We have investigated a simple interferometric method for determining the topological charge l of an optical vortex recently described by Berkout and Beijersbergen [1]. The method utilizes a multi-pinhole interferometer (MPI), a series of small circular apertures equally-spaced around the circumference of a circle. An optical vortex beam incident on the apertures creates an interference pattern that with an appropriate number of apertures is characteristic of l. The method can be applied to vortices of arbitrary size and eliminates the need for a reference plane wave or a complete sampling of the vortex light field. MPIs with 5, 6 and 9 apertures were fabricated by drilling 170 micron diameter holes into 225 micron thick Al disks 25.4 mm in diameter. In the first two devices the apertures were on a circle with radius r = 1.75 mm; for the 9 aperture device we used r = 2.0 mm to better match the diameter of the l = 3 and 4 LG modes. These dimensions were dictated by the available collimating lenses and aperture fabrication methods, the overall table length, and profile measurements on the collimated LG modes. The interference patterns were recorded directly on an Electrim EDC1000N CCD camera at distances from 127 to 610 mm. A long opaque tube attached to the front of the camera sufficed to block room light. The expected patterns were simulated by evaluating Eq. 2 in Ref. [1] in Mathematica. We also created several tools with Mathematica to facilitate the use and fabrication of the MPI. LG modes up to order l = 4 were created using an open cavity He-Ne laser followed by an astigmatic mode converter [2]. Only the 9 aperture MPI is able to distinguish topological charge up to l=4. The previous work was limited to charges up to 2 and no more than 7 apertures [1]. All of the patterns we obtained with 5 or 6 apertures were a good visual match to those previously reported or predicted [1]. The 9-aperture patterns we observed resemble Bessel functions, as anticipated [1]. One unanticipated result was that the interference patterns were apparent even when the diffraction patterns of the apertures had only very slight overlap. We also observed that the interference patterns not from pure LG modes are qualitatively different from those that are from pure modes. We thank Jeff Slechta for fabricating the MPI devices and Giovanni Milione for helpful discussions. JK would like to thank the Physics REU program and the Laser Teaching Center for financial support. REFERENCES [1] G. C. G. Berkhout and M. W. Beijersbergen, Phys. Rev. Letters {101}, 100801 (2008). [2] M. Beijersbergen et al., Opt. Commun. {96}, 123-132 (1992).