Quasi-diffraction-free beams

A diffraction-free beam is obtained by the superposing of plane waves whose wave vectors make an angle with the propagation axis. These plane waves ar e realized with point sources that are distributed uniformly around a circl e and an infinitely large aperture lens. After the field passes through the lens it has nondiffracting properties and is described by the zero-order B essel function. Relaxing these conditions makes the beam diffraction free w ithin only a limited region. The beam generated from such a geometry is ref erred to as a quasi-diffraction-free beam. The effects of the width of the annular source on the beam spread are discussed and compared with those for a Gaussian beam. Approximate expressions for quasi-diffraction-free beams are also obtained. (C) 2001 Optical Society of America.