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

[narra]

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?

 

[Drakkith]

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.

 

[Gordianus]

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.

 

[Andy Resnick]

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

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.

 

[narra]

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

 

[Andy Resnick]

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.

 

[narra]

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?

 

[Andy Resnick]

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.

 

[Dr Lots-o'watts]

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