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Limiting Divergence Of Lasers

  1. Jun 23, 2004 #1
    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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  3. Jun 23, 2004 #2


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    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?)
    Last edited: Jun 23, 2004
  4. Jun 24, 2004 #3
    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.
  5. Jun 24, 2004 #4


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    Yes, the eyepiece is critical! This requires 2 lenses to work, the first converges the beam to a point, the 2nd captures the now divergent beam and restores the parallel nature of the beam.

    You need to know the focal length of both lenses and both lenses need to be convex. To learn the focal length of your eyepiece, take it outside and try to burn an ant! That spot you see is actually an image of the sun, When the spot is smallest (burns best!) it is in the focal plane of the lens so the distance from the center of the lens to the image is your focal length. You can measure from the end of the lens. Do the same with your objective lens, now for a beam expander to work the lenses must be positioned so the focal lengths are at the same point or so the distance between the lenses is F1+F2.

    This alignment is critical and the beam must be coaxial, these alignments can be difficult to make. The pros spends 10s of thousands of dollars on alignment fixtures for this very reason.
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