beam waist
- ️Dr. Rüdiger Paschotta
- ️Sun Mar 16 2025
Author: the photonics expert (RP)
Definition: location with minimum beam radius
Alternative term: beam focus
DOI: 10.61835/95r Cite the article: BibTex plain textHTML Link to this page! LinkedIn
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.
Before the beam waist, the generally light converges, i.e., the beam radius gets smaller. After the waist, the light diverges. Note, however, that this behavior may be more complicated depending on the chosen definition of beam radius.
It can happen that a light beam has no real beam waist, e.g. because it starts already diverging when formed at some optical element (for example, a lens).
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.
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