Energy delivered per area by one pulse, the key number for pulsed laser processing and for optic damage specs. For a Gaussian beam the peak fluence is twice the pulse energy divided by the area defined by the 1/e squared radius. Ablation thresholds of metals for femtosecond pulses are a fraction of a J/cm squared, while good coatings survive tens of J/cm squared at nanosecond durations.
We're running 4 J/cm squared at the target and the coating spec is 10, so the mirrors have margin but don't tighten the focus.
Also heard as
- energy density
Do not mix it up with
- Peak power: Peak power is energy per time; fluence is energy per area, and damage depends on both.
Related terms
Damage threshold
The maximum fluence or power density an optic can take before it is permanently damaged.
Peak power
The highest instantaneous power reached during a laser pulse, roughly pulse energy divided by pulse duration.
Gaussian beam
A beam whose intensity falls off from the center as a bell curve, the shape of an ideal single-mode laser.
Hot spot
A small region of a beam with much higher intensity than the rest, a common cause of optic damage.
Top-hat
Describes a beam with flat, uniform intensity across its width and steep edges, instead of a Gaussian peak.