Experimental observation of 'pure helical phase' interference using moiré fringes generated from holograms with dislocations

A Laguerre-Gaussian (LG) beam possesses a uniform orbital angular momentum of lhℏ due to its helical phase term exp(-ilφ), where l is the topological charge of the beam. The helical phase or the topological charge of any LG beam can be observed by causing it to interfere with a Gaussian beam or its mirror image. In this paper we propose a novel method of using moiré fringes to study the changes in the helical phase and the topological charge upon interference between any two LG beams of any arbitrary helicity. Using this method we are able to observe the interference purely between the helical phases which can be readily observed using white light illumination as shown in our experimental results. Such pure helical phase interference is not readily observed in conventional interferometry.