Experimental observation of optical vortex evolution in a Gaussian beam   with an embedded fractional phase step
  We detect the vortex evolution from the increase of the fractional phase step by interfering two beams of opposite but equal fractional 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 
W.M. Lee, X.-C. Yuan and K.Dholakia