Abstract

It was found both analytically and experimentally that when a Gaussian laser beam is focused by a thin lens and the waist of the beam is in the plane of the lens and is much smaller than the linear size of the lens, the point of absolute maximum irradiance is located on the axis but somewhere closer to the lens. This effect is referred to as focal shift. In this study, a formula is developed for fast computation of focal shift in focused, apertured Gaussian beams. This formula is a generalization of two previously known focal shift formulas, one for an unapertured Gaussian beam and the other for a plane-wave beam. The proposed formula brings no limitation to the level of beam truncation and gives the focal shift with an accuracy of about 90% or better. Since the point of absolute maximum irradiance in a focused field is near the plane where the field has a minimum spread, the proposed formula provides a convenient means to specify or to create the location of beam waist in a laser focusing system in which the fundamental circular mode of a laser beam is used.

© 1992 Optical Society of America