Laser Output Modeling

[mecheng2011]

Hi guys, I'm new to the forum, I'm a mechanical engineer but running a project a little outside my comfort zone which is to do with optics.

I am trying to calculate the power density of a pulsed Nd:YAG laser but am getting in a muddle with the Average and Peak powers. The project is to model the surface temperature increase on a Silicon Carbide tile, when the laser is focussed to a spot of 0.53mm diameter.

I have used equations found online (http://www.newport.com/images/webdocuments-en/images/20063.pdf [Broken]) and am coming out with a value of 6.165e9 W/cm^2, which seems a bit excessive.

If you could let me know if I am on the right track, or where I am going wrong, it would be much appreciated.

Thanks

Alex

The laser details are:


Repetition rate: 20Hz
Pulse Energy: <300mJ @ 1.06um
Beam Divergence: <1mRad
Pulse Duration: 22ns

 

[Dr Lots-o'watts]

Hi mecheng2011,

Power density is simply (average power)/area.

Total energy in 1 second is 20 x 0.3 J = 6 J, so power is 6 W.
Area in cm2 is pi x (0.0265)squared = 0.002205 cm2.

Power density is thus 6W/0.002205cm2 = 2721 W/cm2.

That Newport link is good but you don't have to bother with peak power to get power density in W/cm2.

Can I ask what's the application?

 

[mecheng2011]

Thanks. I was just getting confused as I am trying to model a single pulse, and I felt that maybe it should be somewhat higher as this is a Class 4 laser.

It is for a dissertation, I am basically designing a tool to convert non-visible light from Class 4 lasers (of the specification I wrote below) into something visible on a screen by using an IR camera. I am specifically trying to model this in Ansys, by using a section of target material, and applying a heat flux as a spot on it. I am trying to model for 1 pulse, and 1 second of pulse fire, and trying to see if the target heats up enough to be visible, or heats up too much and damages the material for both. I've been getting some ridiculous values, and my model seemed correct so assumed I must have made an error somewhere else- hence the issue of power density!

Perhaps from your experience, what would you expect, if using a ceramic material such as Alumina. Would you expect it to damage the surface quickly? I am very out of my field here with optics and lasers!

Thanks for your help

 

[Dr Lots-o'watts]

I readily see why it's class 4: http://en.wikipedia.org/wiki/Laser_safety#Class_4

The amount of variables determining the result of laser-matter interactions is impressive, and it does indeed occupy much of my professionnal time. The best way I can help you further is to refer you to my homepage, if ever your project involves experimental work.

 

[RocketSci5KN]

The trouble with alumina is that it conducts heat very poorly. You might consider other materials with better thermal conductivity and/or actively cool the target (assuming it does get too hot). If you intend to use only a small time window, this may not be an issue. Keep in mind the safety issues of reflected IR light off the sample.