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Diffraction-limited Beams

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Ask RP Photonics for advice on how to measure the beam quality of your laser source, or how to optimize it, e.g. via advanced resonator design.

Definition: beams with a minimum possible beam divergence for a given waist radius

German: beugungsbegrenzte Strahlen

Category: general optics

How to cite the article

A laser beam is called diffraction-limited if its potential to be focused to small spots is as high as possible for the given wavelength, i.e., if its beam quality is ideal. More precisely, this means that a beam waist with a given beam radius, generated from the beam by focusing e.g. with a curved mirror, is associated with the minimum possible beam divergence. What exactly this means depends on the definition of the beam radius and beam divergence. If second moments are used for the definition of both quantities, the minimum beam parameter product is reached for a Gaussian beam, which has not only a Gaussian intensity profile but also a flat phase front at the beam waist.

A laser which operates on a single transverse resonator mode will usually have a diffraction-limited output, since that single mode is usually the fundamental resonator mode, and the shape of this is usually close to Gaussian. Non-ideal beam quality can be caused by intracavity beam distortions, e.g. in the gain medium, which can either deform the fundamental mode or (more frequently) cause transverse multimode operation. Note that the beam quality of a laser depends not only on the strength of intracavity distortions, but also on certain details of the resonator design; optimization of the latter can be essential for reaching a diffraction-limited output.

For a given optical power, a diffraction-limited beam has the highest brightness.

See also: beam quality, beam parameter product, beam divergence, laser beams, Gaussian beams, brightness

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Dr. Paschotta, author of this encyclopedia, has also published three books in the SPIE Field Guide series:

- Field Guide to Lasers

- Field Guide to Laser Pulse Generation

- Field Guide to Optical Fiber Technology

You can order these books on the SPIE website – just click on one of the images.

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regenerative amplifier

Pulse energy from a regenerative amplifier, when the number of resonator round-trips per amplification cycle is varied. Such instabilities (involving several bifurcations) can be well studied with a numerical model.

This diagram has been made with the RP Fiber Power software.

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