We report the observation of optical vortices in a laser beam propagating t
hrough the stripe pattern of a cholesteric liquid crystal. The liquid cryst
al is confined in a cell with homogeneous boundary conditions and forms a d
iffraction phase grating. Optical vortices are produced by edge dislocation
s of the cholesteric grating. The vortices show up as spots of zero light i
ntensity in the diffraction maxima. There is one spot in each +1 and -1 dif
fraction maximum and two spots in diffraction maxima +2 and -2. (C) 2000 Op
tical Society of America. OCIS codes: 160.3710, 050.1940, 180.1790, 070.469
0.