Multiple beam interference in spiral phase plates

Optical transmission through a spiral phase plate is analyzed by treating the device as a Fabry-Perot etalon with an azimuthally-varying thickness. The transmitted beam is calculated to contain a coherent superposition of optical vortices with different winding numbers. This yields an intensity profile with a periodic modulation as a function of azimuthal angle, which is verified experimentally. Applications in atom optics and matter wave interferometry will be discussed.