Experimental observation of optical vortex evolution in a Gaussian beam with an embedded fractional phase step

Laguerre-Gaussian beams may possess an azimuthal phase variation of 2πl, where l is an integer, around the beam axis, resulting in an annular intensity distribution. This azimuthal phase variation and associated vortices can be visualized through the appearance of forked fringes, when interfering the LG beam with its mirror image (a beam of opposite helicity) at an angle. In this paper, we examine the evolution of optical beams with a fractional phase step hosted within a Gaussian beam by experimental analysis of both the phase and intensity distribution. To generate these beams, we introduce differing fractional (non-integer) topological charge variations within a Gaussian beam generated using a spatial light modulator (SLM). We detect the evolution of the vortex from the increase of the fractional phase step by interfering two beams of opposite but equal fractional phase step increment. The interference pattern generated shows evidence of the birth of an additional single extra charge as the fractional phase step increase extends above a half-integer value.