Beam Waist

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

Definition: location with minimum beam radius

Alternative term: beam focus

Category: article belongs to category general optics general optics

Page views in 12 months: 7145

DOI: 10.61835/95r Cite the article: BibTex plain text HTML 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.

More to Learn

What is a Beam Width, Beam Size, and a Beam Waist?

Questions and Comments from Users

2021-08-03

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

The author's answer:

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.)

2022-01-16

Does a beam waist occur both inside and outside laser cavity? If so, how do we calculate beam waist inside a laser cavity?

The author's answer:

One or even several beam wastes can occur inside and outside a laser resonator, depending on the used optical elements.

The beam evolution in a laser resonator (e.g. for the fundamental resonator mode) can be calculated it with software based on the ABCD matrix algorithm.

2024-10-01

Does the beam waist stay the same after a lens? I.e., if a beam comes from a single mode fiber (with a 2.5 microns waist) and then after some distance passes through a lens, will the beam waist after the lens be the same as before the lens?

The author's answer:

No, the beam waist radius will generally be very different after a lens.

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