The relationship between laser fluence and pulse duration
Tuesday, August 23, 2022
If you’ve read our quick guide on calculating laser fluence, you know that fluence is a laser’s energy divided by the area illuminated.
Laser fluence is a useful metric to know how concentrated the energy is spatially. It tells us nothing, however, about how fast that energy is delivered. This is where pulse duration comes into play.
How slow is too slow?
Laser material processing is a balance of heat-in vs heat-out.
The laser inputs energy into the system very locally. This heat then gets dissipated throughout the bulk material around the target.
If you shorten the pulse duration, you leave less time for heat to diffuse, resulting in a more intense buildup right on target. But how short is short enough?
The speed at which heat diffusion happens is dictated by phonon relaxation time (also known as thermal relaxation time). This is what sets the minimum threshold for “how fast” you need to input energy.
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How fast is too fast?
Our water-cooled high-power detectors provide the highest power handling on the market, but even them have a limit, known as the damage threshold. In fact, all optics will eventually degrade if the energy (fluence) is too high and delivered too quickly (overly short pulse duration).
An increase in fluence and a decrease in pulse duration both translate to increased power per area. For this reason, damage thresholds are often listed as a unit of W/cm2.
Take note that these damage thresholds exist for all optical components, including power and energy detectors, lenses, mirrors, beam splitters, etc.
How fast is just right?
So far, we’ve seen that if the pulse duration is too slow, the heat will dissipate and the process won’t be able to take place.
We’ve also seen that there is a risk of breakage if the fluence is too high or the pulse duration too short.
It may seem like the optimal solution is to supply as high a fluence and as short a pulse as possible (without breaking anything). Well… sorry to disappoint, but we can find a counter-example in laser paint removal.
For a high fluence, the efficiency of laser paint removal actually decreases when pulses are made shorter. The hypothesized explanation is that matter that is ejected at the start of the pulse blocks subsequent light. Which makes it more efficient to increase the pulse duration while maintaining the fluence unchanged. In this way, the ejected matter blocks a smaller portion of the total fluence.
There are other cases where a high fluence yields the best efficiencies with short pulse durations, but the opposite becomes true for a lower fluence.
Just goes to show that there is no substitute for experience.
Explore the relationship between laser fluence and pulse duration with our calculator
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Take advantage of our experience
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