Damage threshold of short pulses....

[fog37]

TL;DR Summary

fluence vs intensity for damage threshold of ultrashort pulses

Hello,

A femtosecond optical pulse is characterized by a maximum peak intensity $I(W/m^2)$, a pulse duration $\Delta {t}$ and a repetition rate $R (Hz)$.

The damage threshold of an optical material is often expressed using either the maximum fluence $(J/cm^2)$ or the maximum intensity $(W/cm^2)$. Which parameter is more appropriate in the case of a sequence of short pulses? Fluence (energy density) seems to be the more critical parameter in the case of a train of short pulses...I think that a material would be locally damaged if a lot of energy, i.e. $J$, was deposited over a small area $cm^2$ in a very amount short time $(s)$. This leads me to think that maximum peak intensity should be the parameter to worry about instead...

 

[DaveE]

I think you are kind of defining "short pulse" in your question. Materials can be damaged by too much energy being absorbed over a "long" time frame, like heating. They can also be damaged "instantly" by high peak energy regardless of duration, like ionization or disassociation. The role of repetition rate is to either cause multiple instances of "short" pulse damage, or to increase the amount of energy delivered (e.g. as heat) in longer pulses. There are non-linear (threshold) effects involved.

Both parameters are important and represent different processes. Kind of like being hit by one bullet or being hit by 100 90 MPH base balls; either scenario might hurt you, but the damage done has a different signature.

 

[DaveE]

This article may be of interest:
https://www.techbriefs.com/component/content/article/tb/supplements/pit/features/articles/33554

 

[crashcat]

I started to write a response but I am slowly realizing I don't understand LIDT as much as I thought.

Here's a nice link anyway. A good high level overview:
Edmunds LIDT Overview

 

[fog37]

Thank you crashcat. I have read the infos at the link.

• Well, in the case of a continuous beam (CW) laser, I think it is clear that, given two laser sources, the one with the higher the rated intensity $I (W/m^2)$ will cause the more the damage. As far as the entity of the damage, the exposure time and beam spot size clearly matter: the longer the exposure and the smaller the beam spot the worst the damage. Also, for the same intensity, laser sources different wavelengths may cause different level of damage.
• Pulses: when dealing with a train of pulses, we need to consider, the pulse duration, peak intensity $I_{max}$, and repetition rate $R$. As far as the damage threshold, I feel like things get tricky. The pulse energy is defined also as the ratio between the average intensity and pulse rate $R$: $$\frac {I_{average}}{R}$$
• The LIDT for pulsed lasers is specified as a fluence with units of $J/cm^2$. In most cases, the LIDT fluence value will increase as the pulse duration increases because, the same energy, I guess, is spread over a longer time interval.
• The graph below: if a pulsed laser emits $1 W$, i.e. 1 joule of energy in $1s$, the more pulses are sent over a second, i.e the higher $R$, the less the pulse energy is contained in individual pulse.
• 
• In summary, I believe that, to cause a lot of damage, a pulsed source should a) emit many pulses per second (high repetition rate $R$), each pulse should have a lot of energy, and the pulse duration should be small so the pulse peak intensity is high...