The beam waist (or beam focus) 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 that location.
Figure 1: The beam waist is the location where the beam radius is smallest.
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.
Simple relations between the beam waist radius and the beam divergence, for example, exist for Gaussian beams.
光电产品选型平台