Dynamic interferometry measurement of orbital angular momentum of light.

We present a dynamic interferometry to measure the orbital angular momentum (OAM) of beams. An opaque screen with two nonparallel air slits is employed that can be regarded as the Young's double-pinhole interference. When the OAM beams with an annular intensity distribution vertically incident, the far-field interference patterns depend on the phase difference of the light in the two pinholes. We scan the angle between the two slits, and the output intensity at center changes alternatively between darkness and brightness. Utilizing this characteristic, we can measure the OAM of light. This scheme is very simple and low-cost. In addition, it can measure very large topological charge of OAM beams due to continuous scanning.