Focal shift in Laguerre–Gaussian beams

Abstract

The shift in the plane of best focus away from the geometrical focalplane is calculated for a Laguerre-Gaussian beam. It is found thatthe plane of best focus is closer to the focusing lens than predictedby geometrical optics by an amount dependent not only on the Fresnelnumber of the field at the exit plane of the lens but also on themode order (7 References).