RP Photonics logo
RP Photonics
Encyclopedia
Consulting Software Encyclopedia Buyer's Guide

Short address: rpp-con.com

Dr. Paschotta, the founder of RP Photonics, supports your R & D with his deep expertise. Save time and money with efficient support!

Short address: rpp-soft.com

Powerful simulation software for fiber lasers and amplifiers, resonator design, pulse propagation and multilayer coating design.

Short address: rpp-enc.com

The famous Encyclopedia of Laser Physics and Technology provides a wealth of high-quality scientific and technical information.

Short address: rpp-bg.com

In the RP Photonics Buyer's Guide, you easily find suppliers for photo­nics products. As a supp­lier, you can profit from enhanced entries!

Learn on lasers and photonics every day!
VL logo part of the
Virtual
Library

Beam Divergence

<<<  |  >>>  |  Feedback

Buyer's Guide

Use the RP Photonics Buyer's Guide to find suppliers for photonics products! You will hardly find a more convenient resource.

Ask RP Photonics for any advice on laser beams, e.g. on optics to collimate or focus them, or on beam quality and related matters.

Definition: a measure for how fast a laser beam expands far from its focus

German: Strahldivergenz

Category: general optics

How to cite the article; suggest additional literature

The beam divergence of a laser beam is a measure for how fast the beam expands far from the beam waist, i.e., in the so-called far field. A low beam divergence can be important for applications such as pointing or free-space optical communications. Beams with very small divergence, i.e., with approximately constant beam radius over significant propagation distances, are called collimated beams.

beam divergence

Figure 1: The half-angle divergence of a Gaussian laser beam is defined via the asymptotic variation of the beam radius (blue) along the beam direction. Note, however, that the divergence angle in the figure appears much larger than it actually is, since the scaling of the x and y axes is different.

Some amount of divergence is unavoidable due to the general nature of waves (assuming that the light propagates in a homogeneous medium, not e.g. in a waveguide). That amount is larger for tightly focused beams. If a beam has a substantially larger beam divergence than physically possibly, it is said to have a poor beam quality. More details are given below after defining what divergence means quantitatively.

Quantitative Definitions of Beam Divergence

Different quantitative definitions are used in the literature:

As an example, an FWHM beam divergence angle of 30° may be specified for the fast axis of a small edge-emitting laser diode. This corresponds to a 25.4° = 0.44 rad 1/e2 half-angle divergence, and it becomes apparent that for collimating such a beam without truncating it one would require a lens with a fairly high numerical aperture of e.g. 0.6. Highly divergent (or convergent) beams also require carefully designed optics to avoid beam quality degradation by spherical aberrations.

Divergence of Gaussian Beams and Beams with Poor Beam Quality

For a diffraction-limited Gaussian beam, the 1/e2 beam divergence half-angle is λ / (πw0), where λ is the wavelength (in the medium) and w0 the beam radius at the beam waist. This equation is based on the paraxial approximation, and is thus valid only for beams with moderately strong divergence.

A higher beam divergence for a given beam radius, i.e., a higher beam parameter product, is related to an inferior beam quality, which essentially means a lower potential for focusing the beam to a very small spot. If the beam quality is characterized with a certain M2 factor, the divergence half-angle is

beam divergence for non-ideal laser beam

As an example, a 1064-nm beam from a Nd:YAG laser with perfect beam quality (M2 = 1) and a beam radius of 1 mm in the focus has a half-angle divergence of only 0.34 mrad = 0.019°.

Beam Quality Calculations

Center wavelength:
M2 factor: calc
Beam parameter product: calc
Beam waist radius: calc
Divergence half-angle: calc

After you have modified some values, click a "calc" button to recalculate the field left of it.

Spatial Fourier Transforms

It can be helpful to use the spatial Fourier transform of the complex electric field of a laser beam as a function of the transverse coordinates (Fourier optics). Effectively this means that the beam is considered as a superposition of plane waves, and the Fourier transform indicates the amplitudes and phases of all plane-wave components. For propagation in free space, only the phase values change.

The width, measured e.g. as the root-mean-squared (r.m.s.) width, of the spatial Fourier transform can be directly related to the beam divergence. This means that the beam divergence (and in fact the full beam propagation) can be calculated from the transverse complex amplitude profile of the beam at any one position along the beam axis, assuming that the beam propagates in a homogeneous medium (e.g. in air).

Measurement of Beam Divergence

For the measurement of beam divergence, one usually measures the beam caustic, i.e., the beam radius at different positions, using e.g. a beam profiler.

It is also possible to derive the beam divergence from the complex amplitude profile of the beam in a single plane, as described above. Such data can be obtained e.g. with a Shack–Hartmann wavefront sensor.

See also: beam radius, laser beams, collimated beams, beam parameter product, beam quality, beam pointing fluctuations, beam profilers, free-space optical communications, Spotlight article 2007-07-11


Dr. R. Paschotta

This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics Consulting GmbH. Contact this distinguished expert in laser technology, nonlinear optics and fiber optics, and find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, or staff training) and software could become very valuable for your business!

How do you rate this article?

Your general impression: don't know poor satisfactory good excellent
Technical quality: don't know poor satisfactory good excellent
Usefulness: don't know poor satisfactory good excellent
Readability: don't know poor satisfactory good excellent
Comments:

Found any errors? Suggestions for improvements? Do you know a better web page on this topic?

Spam protection: (enter the value of 5 + 8 in this field!)

If you want a response, you may leave your e-mail address in the comments field, or directly send an e-mail.

If you like our website, you may also want to get our newsletters!

arrow

Thin-film Optics Software

RP Coating is a most versatile software for thin-film design. Analyze and optimize a wide range of devices:

coating design

Further features:

Use RP Coating to quickly become a thin-film expert!

– Show all banners –

– Get your own banner! –