Study of fractional optical vortex beam in the near-field

The near-field amplitude and phase characteristics of fractional optical vortex beam from a spiral phase plate fabricated on the tip of an optical fiber is reported here. A near-field scanning optical microscope is used to measure the intensity and phase structure of the fractional optical vortex beam as a function of propagation distance, Z and fractionality, l. The measured intensity-phase structure of the fractional optical vortex beam as a function of Z, from near- to far-field and l are complex and dominated by diffraction-related smoothening of the structure. Theoretical simulations carried out using the angular spectrum method matches well with the experimentally measured behavior. The trajectory of singular point in the vortex beam, as its charge is changed from l = - 1 to +1 in fractional steps, is simulated and measured using an interferometer setup. Such studies enable us to understand the complex nature of the fractional optical vortex beam in the near-field, its modifications during propagation to far-field, and as a function of fractionality.