# Lasers and Focusing Lenses

1. Jul 26, 2012

### muffins321

I'm doing this for a home project over the summer.

So lets say I took a small laser and passed the beam through some focusing lenses (or evena single lense). Is it possible for the beam to expand?
Also, if the laser is powerful enough to burn through paper, will the focusing lenses be damaged?

Thank You very much to anybody who answers.

2. Jul 26, 2012

### Redbelly98

Staff Emeritus
Welcome to PF.
Yes. After coming to a focus, the laser beam will start expanding beyond that point. Or, if it happens to be a negative lens -- i.e. a concave one shaped like )( -- then the laser beam will start expanding right after the lens.
Probably not, especially if the beam's focused spot is not located right at one of the lenses.
You're welcome.

3. Jul 26, 2012

### muffins321

Wow, thanks so much, it's really going to help. If it's not any trouble, I'd like to ask a few more questions.

So, the beam will expand, not refract right?

Also, it will when it expands, it won't take on a flash-light type role, where the light is shining everywhere right?

4. Jul 26, 2012

### davenn

there is refraction happening within the lens regardless of if its a concave lens like redbelly98 showed, which will diverge ( spread out ) the beam, or if its a convex lens that is used to focus a beam

yes after going through a concave lens the laser light beam will be spread out into a more diffuce bright glow rather than a narrow beam.

cheers
Dave

5. Jul 26, 2012

### muffins321

Okay, thanks, the link also helped. :D

The goal of my project is to pass a laser beam through a concave lens to spread out the beam and increasing it in size, but then also sending it through a convex lens so that it focuses. Both of the lenses will be circles, so my plan is to have the straight beam turn into a more circular beam by the time it passes through both of the lenses (so it will be like a tubelight with out the actual tube). Although, I do think I will need more than two lenses.

Not sure if this will work, hopefully it will. Do you think it will work in the end, after a lot of tweaking, or is this plan hopeless? :D I won't care if you jsut crush my plan right before my eyes, I don't like wasting my time doing something that won't work in the end.

6. Jul 26, 2012

### davenn

the shape of the beam out of the lens will be the same shape it was when it entered the lens
Think about the way a refractor telescope (the one with lenses) or binoculars work....
if they changed the shape of the light beam from an object, we would never see that object the way it looks without the lens. we see the same shape, just magnified.....

look a little way down that wiki page to the heading .... Imaging properties to see what I mean

Dave

Last edited: Jul 26, 2012
7. Jul 26, 2012

### DaveC426913

Whether or not you succeed or fail, please be make yourself aware that even small lasers can cause permanent eye damage, up to and including blindness.

Are you sure your skills are up to it?

Last edited: Jul 26, 2012
8. Jul 27, 2012

### muffins321

Oh, so I need to find another way to expand it...

And yes, my skills are up to it, I have protective goggles.

9. Jul 27, 2012

### Khashishi

You need a rather strong lens (short focal length) to expand the laser much. You might have better luck with a drop of water to function as the lens.

10. Jul 27, 2012

### Drakkith

Staff Emeritus
I don't understand what you mean by changing the "shape" of the beam. The beam is circular, or rather cylindrical if you think of a small section of the beam path. At least I think it is. Wouldn't simply expanding the beam work for your purposes?

11. Jul 27, 2012

### dmriser

It sounds like you want to expand the beam, this can be done using 2 lenses, both "focusing" lenses. You'll need to find the focal length of each lens.

You can get a good estimate of the focal length of a lens by holding the lens over a table directly underneath a florescent light bulb. Changing the height above the table should change the clarity of the image you see on the table. When you can see the image clearly, measure the distance. This is roughly the focal length.

Next, you'll want to pass the laser beam through the shorter focal length lens first. The beam will focus to a point and begin to diverge afterwards. When the beam travels 2 focal distances, it will be at its original size (beam diameter). You'll want to place the longer focal length lens at exactly its focal length after the point when the beam is focused to the tightest spot (1 focal length of the first lens).

As the beam travels beyond 2f it expands to a size larger than its original diameter, when you place the second lens you will collimate the beam (reduce divergence as much as possible). Thus, the placement of the second lens is more critical. You should play with the distance, while observing the spot size at a point far away. It should exit the lens and stay roughly the same size.

Good luck.

12. Jul 27, 2012

### Integral

Staff Emeritus
Good post.

In addition, to get a reasonalbe measure of the focal length the distance to lamp or over head light needs to be greater then 10x the focal distance. Note that you will be focusing an image of the source.

13. Jul 30, 2012

### Redbelly98

Staff Emeritus
In fact, muffins321 could just use binoculars (or a telescope, or a monocular) to expand the beam. Send the beam straight into the eyepiece, and an expanded beam will come out the other end. Do not look into the binoculars/telescope/monocular while doing this!

14. Jul 31, 2012

### muffins321

really? That's great! :D
But the shape will be a circle right? not something ugly or malformed? :D

15. Jul 31, 2012

### Drakkith

Staff Emeritus
In the absence of severe aberrations it should be the same shape leaving as it was when it entered. I don't think you will have a problem as long as you aren't using the cheapest of the cheap equipment.

16. Jul 31, 2012

### muffins321

Okay, thanks. :D
Now all I have to worry about is making sure the beam still has enough intenisty to brun someone when it coems out of the lenses.

17. Aug 2, 2012

### sugeet

all laser beams are zernike polynomial functions(mostly),
however the basic TEM00 mode is guassian, these beams, have a beam waist, where the are the thinnest, or where the spot size is least, correspondingly they have what is called a railegh range, within which the beam does not diverge appreciably, if you force it to converge by a lens, you reduce the rayleigh length, further to burn the paper, you need high energy focussed, but you are also limited by your laser energy, you need to keep it long at the beam waist to actually burn it, if you are working with an ordinary low power laser

Last edited by a moderator: Aug 4, 2012
18. Aug 4, 2012

### sugeet

sorry its hermite polynomial functions not zernike

19. Aug 4, 2012

### Redbelly98

Staff Emeritus
Laser beams can be expressed as the sum of $\text{(hermite polynomial)} \cdot \text{(gaussian)}$ terms. But I don't see the relevance to this discussion -- the OP just wants to expand a beam, and we don't even know whether it's TEM00.