What is more damaging: average power or peak power of a pulsed laser?

In summary, when it comes to optical power, it is important to monitor both the average and peak power in order to avoid damage to components.
  • #1
narra
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When we are considering the how much optical power a component/pin detector can withstand, are we more concerned about keeping an eye on the peak power or can we ignore the peak power so long as the average power is below some threshold value?
 
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  • #2
I'd guess that you have to look at both. If the peak power is only slightly higher, you could probably ignore it depending on what its for, but if there is a large difference you could burn something out during the peak phase even though the average power is low.
 
  • #3
Even though the OP is rather broad, I'd say I've seen more components destroyed by peak power. Femto second lasers that deliver less than 1 mJ can easily vaporize many materials.
 
  • #4
narra said:
When we are considering the how much optical power a component/pin detector can withstand, are we more concerned about keeping an eye on the peak power or can we ignore the peak power so long as the average power is below some threshold value?

There are different mechanisms for damage- thermal, phptchemical, thermomechanical, and nonthermal mechanisms from ultrashort pulses.

http://web.mac.com/mfeit/physics/Michael_D._Feit_files/papers/jap8599.pdf [Broken]

So the answer depends on the relevant mechanism. Pulsed lasers can deposit a large amount of energy in a short time, and so damage thresholds for pulsed sources are generally lower than CW sources.
 
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  • #5
Yes true. If a high peak power is delivered over a short duration on a material that can fairly rapidly dissipate the heat generated then it may not be of much concern, compared to large average power. But speaking more specifically regarding photo-diodes, fibre optic connectors, thin-film filters, prisms, lenses, mirrors and such like. When ever I consult data sheets the power thresholds are sometimes mentioned but never do they specify average or peak power, therefore I wondered if it was generally expected that average or peak power was the concern.

Thanks for your responses.

narra
 
  • #6
narra said:
Yes true. If a high peak power is delivered over a short duration on a material that can fairly rapidly dissipate the heat generated then it may not be of much concern, compared to large average power. But speaking more specifically regarding photo-diodes, fibre optic connectors, thin-film filters, prisms, lenses, mirrors and such like. When ever I consult data sheets the power thresholds are sometimes mentioned but never do they specify average or peak power, therefore I wondered if it was generally expected that average or peak power was the concern.

Thanks for your responses.

narra

Ok- I didn't realize you were specifically talking about optical components.

My understanding is that peak powers are of more concern, but again, it depends:

http://www.semrock.com/laser-damage-threshold.aspx

Note, 'fluence' is an *energy* density, while intensity is a *power* density.
 
  • #7
Useful link. Thanks.

ps How, physically, do you differentiate an energy density versus a power density, just that the latter gives the energy density over time?
 
  • #8
Excellent question- I'm not sure, unless you consider the rate at which the absorbed energy dissipates in the object. Honestly, if you spoke to someone at Semrock I'm sure they would be willing to help.
 
  • #9
narra said:
Useful link. Thanks.

ps How, physically, do you differentiate an energy density versus a power density, just that the latter gives the energy density over time?

If you are working with a cw (continuous wave) laser, you have to consider power density. It's simply the beam power (in W) divided by the surface area on which it is incident. If this ratio is above a certain limit, you'll scrap the material.

If you are working with a pulsed laser, then you must watch for the energy density as well. If the pulse from your laser is 1J, than the surface area of the absorbing material must be, say 1cm square for it not to burn. This defines the energy density threshold.

In the pulsed case, it is assumed that the pulse rate is sufficiently low that the power density is not exceeded.

As for the original question, much depends on the material. A high peak-power pulse can do much damage in no time, but very locally. On the other hand, a high power beam may do no damage if it's on for only a few seconds, but completely melt or ignite the material if left for minutes.

www.gentec-eo.com
 
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1. What is the difference between average power and peak power of a pulsed laser?

Average power refers to the average amount of energy that a laser emits over a certain period of time, while peak power is the maximum amount of energy that is emitted during a single pulse. This means that while average power gives an overall measure of the laser's output, peak power shows the intensity of the laser in a single pulse.

2. Which one is more important to consider when using a pulsed laser?

It depends on the application. If the laser is used for cutting or drilling, peak power may be more important as it determines the intensity of the laser on the material. On the other hand, for applications such as laser surgery or eye treatments, average power may be more crucial as it affects the amount of heat generated and the duration of exposure to the laser.

3. Does the type of material being used impact the damaging effects of average power or peak power?

Yes, different materials may have different thresholds for damage caused by average power or peak power. For example, some materials may be more sensitive to high peak power, while others may be more susceptible to damage from prolonged exposure to average power.

4. Can a laser with high average power but low peak power still cause damage?

Yes, even though the peak power may be low, the prolonged exposure to high average power can still cause damage to materials or tissues. This is because the total energy emitted by the laser can still be significant and cause heating or other effects that may lead to damage.

5. Are there any safety precautions that can be taken to minimize the damaging effects of average power or peak power?

Yes, when using a pulsed laser, it is important to follow safety guidelines and wear appropriate protective gear. Additionally, controlling the exposure time and intensity of the laser can help minimize potential damage. It is also important to understand the properties of the material being used and its threshold for damage from average power or peak power.

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