fluence (original) (raw)

Author: the photonics expert (RP)

Definition: optical energy per unit area

Alternative term: radiant exposure

Category: article belongs to category general optics general optics

Related: optical intensitygain saturationlaser gain mediasaturable absorberslaser-induced damage

Units: J/m2, J/cm2

Formula symbol: ($F$)

Page views in 12 months: 4479

DOI: 10.61835/j0u Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

Definition of Optical Fluence

In general physics, the fluence is defined as the time-integrated flux of some radiation or particle stream. Specifically in optics, the fluence ($F$) e.g. of a laser pulse is the optical energy delivered per unit area. Its most common units are J / cm2 (joules per square centimeter).

In radiometry, the term radiant exposure is used instead of fluence.

In the same way as an optical intensity, the fluence is a position-dependent value. For a laser beam, the fluence is often highest on the beam axis and lower at positions somewhat away from that axis. For continuous-wave beams, the term fluence is meaningful only in combination with some irradiation time.

In some cases, one is interested in the peak fluence, which is the highest fluence value occurring within the laser beam profile. For a Gaussian beam, the peak fluence is the total optical energy divided by ($\pi w^2 / 2$), where ($w$) is the Gaussian beam radius.

From the time-dependent optical intensity, one can obtain the fluence by temporal integration over the full pulse duration.

Common Uses of Fluence Values

If an intense short or ultrashort pulse saturates the gain e.g. of a laser crystal or active fiber, the pulse duration is often far below the upper-state lifetime. The local degree of saturation then depends only on the pulse fluence, and not on the temporal distribution of the intensity. An important property of any laser gain medium is its saturation fluence.

For slow saturable absorbers, essentially the same remarks apply as for gain media.

In the context of laser-induced damage by laser pulses, one often specifies the damage threshold of a material as a fluence. This does not mean, however, that the damage threshold is independent of the pulse duration; usually, the critical fluence value rises for increasing pulse durations.

Frequently Asked Questions

What is optical fluence?

Optical fluence is the optical energy delivered per unit area, for example by a laser pulse. Its most common unit is joules per square centimeter (J/cm2). In radiometry, the same quantity is called radiant exposure.

Fluence is the time-integrated optical intensity. It can be calculated by integrating the intensity over the full duration of a pulse. For a continuous-wave beam, the fluence is the intensity multiplied by the exposure time.

What is peak fluence?

The peak fluence is the highest fluence value occurring within the spatial profile of a laser beam. For a Gaussian beam, the peak fluence is the total pulse energy divided by the effective beam area of ($\pi w^2 / 2$), where ($w$) is the Gaussian beam radius.

Why is fluence a useful quantity in laser technology?

Fluence is a key parameter for many processes. For short pulses interacting with a laser gain medium, the degree of gain saturation depends on the fluence. Also, the laser-induced damage threshold of materials is often specified as a critical fluence.

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