# Expansion of Laser Beams

So, here I am again, and I have now built a strong laser which can set things on fire. Yay me. :D

But, the second step comes in..how am I supposed to expand the laser beam? The beam is a 0.2 cm circle, and I need to get it to at least 2.5 cm. Can somebody explain how I can do this? (Try not to bring water unless it's neccesary)

Thank you to anybody who answers. :D

jtbell
Mentor
Use two lenses, first one to make the initially-parallel beam diverge, then one to converge the now-diverging beam back into a parallel state.

So you're saying that I should expand the beam by diverging it, and then converging the expanded beam to make it smaller but also focus it?

Also, do you have any suggestions as to what the power of the lenses would be? I know I'm gonig to have to get a convex and a concave lens, but I'm not too sure about the powers.

Muffins, I explained the expansion process in detail in your previous thread, but please be careful. Lasers can be a great tool for experiment and educational demonstration, but only under safe conditions. I hope that you are aware of the danger and are taking all necessary precautions.

Here's a schematic.
http://electron6.phys.utk.edu/optics421/Laboratories/images/expander.gif
The Keplerian method is what I described to you in the previous thread.

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Yes, I am taking all the precautions. I am going to make sure no one is on the same floor, I have protective goggles, and I am going to wear fireproof clothing...I love Science, but there's no need to go blind or lose a limb for it. :D

Thank you for the diagram, it helps me so much. Just one final question: The beam will be as (or nearly as) focused at the end as it was in the beginning (before passing through the lenses), right?

I'm not sure if I understand your question.

The beam will be roughly collimated as it exits the laser (it will always diverge a bit, depending on the quality of your laser), and if you do a good job, it will be roughly collimated when it exits the second lens. In this case the laser would be focused at 1 focal distance from the first lens, and only there.

I see. All I want is for the beam to still be a beam when it comes out of the second lens, not for it to act like a flash light and send light everywhere, because I still want it to have the burning laser qualities that it had before it entered the lenses. So in the end, I still want to the expanded laser beam to be able to set things on fire (don't worry, that part of the experiment will be done outside, far away from anyone).

Basically, what I'm saying is that the beam will still be a circle when it comes out of the lenses, just expanded, right? And retaining the qualities of setting things on fire due to its immense concentration?

I'm guessing you're using a diode laser, so the beam will probably be more elliptical after expansion, the shape might look rather ugly. Anyway, the expanded beam will not have the same intensity as the original beam. You don't add any photons to the beam by expanding it.. The highest beam intensity will occur when the spot size is as small as possible. The spot size you can achieve is a function of several parameters, but mostly the focal length of the lens you use. A shorter focal length lens will converge more rapidly, but it will also have a smaller spot size.

Here's a tool for you to use to calculate this effect.
http://www.calctool.org/CALC/phys/optics/f_NA

http://www.sintecoptronics.com/ref/FocusedBeamDiameter.pdf

I am using a diode laser, you guessed right :D. The shape may pose a problem then...I guess I'll have to find a way for it to not look ugly (any ideas).

I suspected it would not have the same power, so I guess I'll have to make the beam extremely powerful so that it will still be able to burn things when it comes out of the lenses. Although when you say it won't have the same same intensity, do you mean the it will be half or a third of the intensity (on the terms the diameter will go from 0.2 cm to 2.5 cm)?

The links really helped though, thank you. :D

nasu
Gold Member
If the diameter increases from 0.2 cm to 2.5 cm, this is a factor of 12.5. The area will increase by a factor of about 150.
So your intensity (power per unit area) decreases by the same factor after expansion (at least).
If you want to use the laser to burn or melt materials, expanding the beam is a quite weird thing to do.

I see. All I want is for the beam to still be a beam when it comes out of the second lens, not for it to act like a flash light and send light everywhere, because I still want it to have the burning laser qualities that it had before it entered the lenses.

So in the end, I still want to the expanded laser beam to be able to set things on fire (don't worry, that part of the experiment will be done outside, far away from anyone).
No, you've got things wrong.

One, if you use a lens to expand the beam it is going to keep expanding like a flash light beam. That will happen anyway even if you don't use a lens - the laser beam expands.

Two. And here is I think where you've really got things wrong. If you think making the beam bigger with a lens will make your beam more powerful, you're wrong. You're limited to the amount of energy that comes out of the laser in the first place. Making the beam bigger will just spread the energy over a bigger area - making it weaker. If you want to burn stuff - focus the beam of the laser to make it smaller. A bit like burning twigs with a magnifying glass and the sun.

DaveC426913
Gold Member
krd is correct. What makes a laser (or sunlight) capable of burning thing is the fact that the energy is focused on as small an area as possible.

Expanding the laser beam does exactly the opposite.

jtbell
Mentor
So you're saying that I should expand the beam by diverging it, and then converging the expanded beam to make it smaller but also focus it?
No, the converging lens stops the diverging beam from diverging further and makes it parallel again.

I didn't realise diode lasers were strong enough to actually burn things. It's making me curious- if you need it to ignite something, why do you want a thicker beam? Why not just focus it as much as possible?

So why is it that when you tilt the plates of a laser to different angles you see the different normal modes projected on the screen?