# Limiting Divergence Of Lasers

I have read that you can limit the divergence of a laser by sending it backwards through a telescope. I have not yet been able to do this with my telescope, but seem to have some success using less powerfull lenses. Would anyone be willing to help me conduct this expiriment. The problem I am having when using the telescope is I don't notice any beam comming out of the scope at all. What are some possibilities to what I could be doing wrong? When I use my other less powerfull 'telescopes' the beam stays very focused and small at a longer distance. From what I have read this technique only works with a NeHe laser, and I have one of those. You can read more about this here: http://members.misty.com/don/laserdon.html

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Integral
Staff Emeritus
Gold Member
What you are talking about is also called a beam expander. The idea is to shine your laser beam through a lens, now ideally a laser produces a parallel beam of light so it will be focused to a point at the focal length of the lens. Now if you place a lens with a longer focal length, such that its focal plane conincides with the focal plane of the first lens the beam will emerge from the the second lens as a larger parallel beam. The amount of expansion will be determined by the ratio of the focal lengths of the lens.

Now if you have a divergent laser beam, the beam will be focused to a point a bit beyond the focal plane of the first lens, if you adjust the location of the second lens so the point of focus of the beam lies in the focal plane of the second lens you will get an expanded Parallel beam out.

The best way to do this is to locate the first lens then measure the diameter of the beam out of the second lens, adjust the position of the second until you get a parallel beam out.

It is not clear to me why your telescope does not work (you are shining the beam through the eye piece?)

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I got it working now by removing the eye piece. Is it important I keep the eyepiece on for this? The hole is just so tiny on the eye piece I can't get the laser aligned corectly through it. Thanks.

Integral
Staff Emeritus