定义：光束半径最小的位置。

激光光束的束腰指的是光束传播方向上光束半径最小的地方。这个地方的光束半径成为束腰半径。 

图 1 束腰指的是光束半径最小的地方。

通过一个数值孔径较大的透镜聚焦可以获得较小的束腰半径。对于不是圆形的光束，对于不同截向方向来说束腰的位置是不一样的，这个被称为像散效应。 

Definition: location with minimum beam radius

Alternative term: beam focus

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.