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Thread: Optics question regarding lenses and converging light

  1. #1
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    Default Optics question regarding lenses and converging light

    Hello,

    I've been playing with PCX lenses to make a beam expander to reduce the divergence of a laser beam, but why not use a doublet or convex-convex lens and adjust the distance from a laser diode so the light is focused upon a point which is in the far, far distance, for example, 20 miles away? That way you would have no divergence at all for the distance, only a converging beam. Perhaps a lens only a few inches across can't focus a beam to a spot that far away?

    OK, after you stop laughing at my stupid question, can someone please make something which is probably painfully obvious to you known to me? I'm bringing this question to the only place I think I can get an answer on this, so many people are clueless with simple optic theory on the other forums I frequent, and I count myself as one of them! Maybe there is a hobby optics forum somewhere?
    Last edited by Laser57; 09-20-2015 at 21:19. Reason: To simplify the question

  2. #2
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    Quote Originally Posted by Laser57 View Post
    why not use a doublet or convex-convex lens and adjust the distance from a laser diode so the light is focused upon a point which is in the far, far distance, for example, 20 miles away? That way you would have no divergence at all for the distance, only a converging beam.
    The beam is ALWAYS diverging. Even if you focus it to a point, the beam is always trying to get larger, and always will. This isn't noticeable if you focus to a point that is only a few feet away. But if you try to focus to a point that is 500 ft away, you'll notice that you can't get the beam to a tiny spot no matter how you adjust the lens. This is because the beam is diverging the entire way, and by the time it's traveled 500 ft the beam has gotten much bigger regardless of any focusing.

    This page has a diagram that illustrates what I'm talking about. Look at how the beam in the image is clearly focused to a point near the center of the graph, but the actual light diverges away from the focal point and ends up at roughly the same size by the time it reaches the right side of the graph.

    Perhaps a lens only a few inches across can't focus a beam to a spot that far away?
    It has nothing to do with the size of the lens and everything to do with the divergence of the beam.

    Adam

  3. #3
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    Quote Originally Posted by Laser57 View Post
    Hello,

    Perhaps a lens only a few inches across can't focus a beam to a spot that far away?
    I believe this is the limiting factor in what you are trying to do.

    The ability to focus a Gaussian beam to a certain spot size has to do with the FL of the lens system and the input diameter and wavelength. The bigger the input beam the smaller the spot can be for a given distance. So, to focus to small spot at 20 miles would take a huge input diameter (lens). I don't remember the equation. Its a really simple equation. I'm sure you could google it.

    Also, as buffo said you will be fighting divergence due to diffraction effects.( I think thats what he meant)

    I found the equation from newport:
    Last edited by logsquared; 09-21-2015 at 06:40.

  4. #4
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    Thank you, I'm surprised it works that way. Thank you, looks like a beam expander is the only way to reduce the divergence, I was so hoping a converging beam would at least keep the beam tighter for a few miles. The largest lens I have is 12 inches diameter, that's big, but probably not big enough to focus on something 10 miles away. I will be googling for the formula though, have to run to the airport for a trip to a remote part of Alaska today, so off line for awhile.

  5. #5
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    [/QUOTE]

    I guess I should explain the equation. oops... The 10mm in the Lens focal length and the the 1mm in the input beam size.

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