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Beam Waist

Definition: location with minimum beam radius

Alternative term: beam focus

German: Strahltaille, Fokus

Category: general optics

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URL: https://www.rp-photonics.com/beam_waist.html

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.

beam waist
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.

Questions and Comments from Users

2021-08-03

How to calculate the spot size of a fiber laser operating in the multimode regime?

Answer from the author:

That will depend on how the optical power is distributed over the fiber modes. That in turn can change e.g. you change the bending of the fiber. If you know that power distribution, you can calculate the overall intensity profile by summing up the intensities from all modes, assuming that they are not mutually coherent. (For a typical continuous-wave fiber laser, they are usually not mutually coherent.)

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See also: focus, laser beams, beam radius, Gaussian beams, The Photonics Spotlight 2007-07-11
and other articles in the category general optics

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