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 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.
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