A small beam waist (more precisely, a beam waist with small waist radius), also called a tight beam focus, can be obtained by focusing a laser beam with a lens which has a short focal length and most importantly a high numerical aperture, and making sure that the lens aperture is largely filled by the input beam. A high beam quality is also an important precondition for tight beam focusing.
For non-circular beams, the longitudinal 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 lenses.
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