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Definition: location with minimum beam radius
The beam waist of a laser beam is the location along the propagation direction where the beam radius has a minimum. The waist radius is the beam radius at this location.
A small beam waist (more precisely, a beam waist with small waist radius) can be obtained by focusing a laser beam with a lens which has a high numerical aperture, and making sure that the lens aperture is largely filled by the input beam.
For non-circular beams, the position of the beam waist can be different for different transverse directions. This phenomenon is called astigmatism, and it may be generated or removed e.g. by using cylindrical optics.
See also: laser beams, beam radius, Gaussian beams, Spotlight article 2007-07-11
Since October 2008, the Encyclopedia of Laser Physics and Technology is also available in the form of a two-volume book. Maybe you would enjoy reading it also in that form! The print version has a carefully designed layout and can be considered a must-have for any institute library, laser research group, or laser company.
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