# Laser beam with 5 millimeter diameter

1. Jul 28, 2013

### sml2010

Hi

Please help me to know : if i have a laser beam with 5 millimeter diameter that reduced to 0.5 millimeter diameter with a lens, Can i reduced the 0.5 millimeter diameter beam to 0.05 or less that 0.5 with another the same lens?

https://www.physicsforums.com/attachment.php?attachmentid=60563&stc=1&d=1375041919

What distance do you suggest between these two lenses?

I am waiting for your help.

Thank you

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2. Jul 28, 2013

### Cthugha

Wait. It is not that easy. By just placing one lens, you will focus the beam. That means you will get a very small spot at a single position and afterwards it will spread again. Do you want a small spot at some position or do you want a small collimated beam?

If you want a collimated beam, you need to build something like a telescope. You place the first lens will focus the beam at the focal distance of the first lens. Now you place the second lens in a distance to the focal spot which equals the focal length of the second lens. If you do it right, you will again get a collimated beam with a different diameter. The magnification (or demagnification) is then simply given by focal length of lens 1 divided by the focal length of lens 2.

So you want a lens with long focal length placed first and another lens with short focal length (placed at a distance equal to the sum of the focal lengths away from each other) to get a small diameter collimated beam starting from a larger diameter collimated beam.

3. Jul 28, 2013

### sml2010

hi dear Cthugha

I only need a very small spot at a single position, the laser beam in my focus distance heated big zone (near 1.2 mm) but i want have a laser beam with 0.2 mm heated zone.
I am looking for a method to have a small spot beam to have a smaller heated zone.
Do you know any other method to get smaller spot beam?

thank you

4. Jul 29, 2013

### Cthugha

The minimal spot size you can get, can be calculated via Gaussian optics (https://en.wikipedia.org/wiki/Gaussian_beam).

In a nutshell, there are two main ways to get a small spot:

1) Use a length with short focal length. The shorter your focal length is, the smaller the spot can be. If you need really small spots, a microscope objective may help. However, this is typically only an issue if you want spots sizes on the order of less than ten micrometers. 0.2 mm does not sound too bad. What wavelength are you working with? Shorter wavelengths make it easier to get small spots.

2) Use a broad collimated beam. What a lens does is essentially a Fourier transform. To get a small spot, you need to have a lot of different momenta present. You can achieve this by having a large collimated beam before the lens. So if using a short focal length lens alone is not sufficient, building a telescope to get a broader collimated beam before focusing it down might help.