Optical vortices in a Laguerre-Gaussian LG01 beam

We have investigated experimentally the evolution of the intensity distribution of an initial off-axis vortex with topological charge 1 in a Laguerre - Gaussian LG(1)(0) background beam. It was found that the non-generic dislocation surface of the Laguerre - Gaussian beam destroyed and new vortices are created during propagation in agreement with calculations. The innermost vortex moves to the centre of the beam, in contrast to the other vortices, which propagate to the periphery. The motion of the innermost vortex is explained by a model employing phase and intensity gradients where the changes of the background beam during propagation are taken into account.