Multiple-beam interference in a spiral phase plate
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 where the orientation rotates as a function of the laser frequency. These effects are quantified experimentally and theoretically.