light beams (original) (raw)

Author: the photonics expert (RP)

Definition: light which propagates essentially in one direction and has a limited spatial extension in directions perpendicular to its beam direction

Category: article belongs to category general optics general optics

Related: beam propertiesnumerical beam propagationlightlaser beamsGaussian beamsmultimode beams

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DOI: 10.61835/kk3 Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What are Light Beams?

A light beam is light which propagates essentially in one direction (directional light) and at the same time has a limited spatial extension in directions perpendicular to its beam direction. Often, it has a roughly circular cross-section but no precise spatial limitation.

A plane wave also propagates in a certain direction but does not have the limited transverse spatial extension, and is therefore not regarded as a light beam.

Light beams are occasionally encountered in nature, but play many important roles in technical optics and photonics — in particular, in the form of laser beams. Those are most suitable for applications like free-space optical communications, imaging or material processing. Many optical techniques and devices work only with light beams, not with light of other forms.

Light beams are generally considered in homogeneous transparent media — for example, as free-space light beams in air or vacuum. However, their character may be preserved e.g. when hitting a mirror or a prism which essentially just sends the light into a new direction. They may also be manipulated with other types of optical elements — for example, with lenses for refocusing, optical filters for modifying their spectral characteristics, optical modulators for modulating properties like optical power and polarization, and beam splitters for splitting them into two or more beams.

A light beam can be seen by an eye outside its covered volume only if part of the light is scattered towards that eye — for example, by dust particles on density fluctuations in air. Generally, a light detector can only detect light entering it, but not just passing it.

In contrast to light beams, light rays are a more abstract concept used within ray optics, also called geometrical optics. They have no transverse extension. Light beams are sometimes described as bundles of rays.

Properties of Light Beams

Specific properties of light beams are spatial properties, in addition to other properties of light which are encountered also for other forms of light. Examples of the latter are optical frequency and optical bandwidth, optical power and polarization. Some of those properties may change along a beam; for example, the optical power may be gradually reduced by absorption or scattering in the medium.

A light beam with circular cross-section may be characterized by a beam radius which generally changes during propagation. Over long enough distances, it must increase due to diffraction, since light is a wave phenomenon (→ wave optics). However, the beam radius may initially decrease before reaching a beam focus (or beam waist) where it has its minimum value. The increase in beam radius in the far field is associated with the beam divergence. A light beam can actually have two different focus positions e.g. for x and y direction; this is called astigmatism.

The beam waist radius and divergence angle together determine the beam quality of a beam, which is essentially a measure of how well it can be focused and is related to spatial coherence.

A special kind of light beams are Gaussian beams, having a Gaussian amplitude profile with flat or parabolic phase profile. In contrast to those, multimode beams have more complicated spatial properties.

Calculation of Beam Propagation

For various types of beams, propagation in homogeneous optical media can be calculated with analytical means. Most prominent are Gaussian beams, which can also easily be propagated in optical systems with optical elements of many types; one essentially needs to propagate only a complex ($q$) parameter from which the full beam profiles are obtained.

The most versatile methods are those of numerical beam propagation, where an arbitrary initial beam profile can be propagated through an in principle arbitrary medium, described e.g. with a three-dimensional refractive index profile — although approximations are often used in practice, which require restrictions like a weak index contrast.

Generation of Light Beams

Light beams are often generated with lasers (or with similar devices such as a master oscillator power amplifier), and are then called laser beams.

Non-laser light sources (e.g. incandescent lamps or gas discharge lamps) usually emit light in a spatial form which is not a beam. However, one can form light beams from such light. For example, one may place the filament of an incandescent lamp in the focus of a parabolic mirror to collimate the light (or at least some substantial fraction of its total power), i.e., send it in essentially one direction, which e.g. allows for more controlled illumination of some object.

A remarkable difference between laser beams and non-laser beams is that the former usually exhibit a far higher beam quality. Besides, many other properties of laser light differ much from light made with other sources; see the article on laser light for details.

Frequently Asked Questions

What is a light beam?

A light beam is light which propagates mainly in one direction while having a limited spatial extent in the transverse directions. Unlike a plane wave, it does not extend infinitely sideways.

What is the difference between a light beam and a light ray?

A light beam is a physical entity with a finite width, subject to wave effects like diffraction. A light ray is an abstract concept from geometrical optics, representing an infinitesimally thin line indicating the path of light.

Can one see a laser beam from the side?

A light beam is only visible from the side if some of its light is scattered towards the observer's eye, for example by dust particles. Without such scattering, the beam is invisible unless it directly enters an eye.

What causes a light beam to spread out as it travels?

A light beam naturally spreads out over distance due to diffraction, which is a fundamental property of wave phenomena. This spreading is described by the beam's beam divergence.

What determines how tightly a light beam can be focused?

The ability to focus a light beam to a small spot is determined by its beam quality. Beams with high quality, such as Gaussian beams, can be focused more tightly than multimode beams.

How are light beams generated?

Light beams are often generated by lasers, which naturally produce directional laser beams. They can also be formed from non-laser sources like lamps by using optical elements, such as a parabolic mirror, to collimate the emitted light.

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