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Thread: Optics question regarding divergence when using a cylinder lens

  1. #1
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    Default Optics question regarding divergence when using a cylinder lens

    My question is in regard to what I'm going to call divergence but I don't know if that really applies in this case. For example, if I have a 1.5 mRad 532nm laser and shoot it through a round cylinder lens to produce a line, what happens to the divergence in the opposite plane of the line? Is it the same, depending upon the divergence of the beam shooting through it? I've wondered if the divergence isn't reduced, when looking at a line lens as a kind of beam expander, except in only one plane. Perhaps that idea is way off from reality, but hoping someone might know.

  2. #2
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    When the cylinder expands the beam it does not change the beam dimensions or the divergence in the plane perpendicular to the line. But, your eye, just like a CCD, can saturate and when the intensity of the spot get's greater the spot appears to grow larger. The cylinder spreads out the light, the intensity is reduced and so the line looks thinner. Try viewing the original spot without the lens,but look through a filter or dense sun glasses. It will look smaller.

  3. #3
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    Thank you, that explains how it looks and I agree, when I have my eye protection on I can see the dot is very small and with a line, much thinner to the eye due to the reduced intensity and from that being able to see it. I was wondering from a theoretical respect whether the line might not spread out with as much divergence as the beam of the laser normally spreads. The amount of spread a line has must have an amount of divergence too, I was wondering if there is a way to reduce it? With a normal beam we can use beam expanders to reduce divergence, but with a line, how can I reduce its divergence? Perhaps impossible.

  4. #4
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    This is not at all impossible. To thin the line you need to reduce the divergence in the plane perpendicular to the line. Whether you use the cylinder or not, the axis perpendicular to it can be treated as you treat any beam. You can use beam expansion with either an anamorphic prism pair or a cylinder pair. Or, you can use a pair of spherical lenses to expand the beam in both plains (you're going to spread one of them even further with the single cylinder, so who cares). Another issues with many Chinese (low budget lasers) is that they are not perfectly focused from the factory. Using a lens pair to expand the beam allows you to add a small amount of positive or negative correction as well. This is almost like getting something for nothing where even a 1:1 power telescope can still improve the divergence.

  5. #5
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    Thank you, I was looking for a way to keep the line as thin as possible for as long a distance as I can, hoping the mere spreading of the beam using a round cylinder lens could do so in some kind of inverse relationship from that alone. From this answer, it appears the mRad of the beam going into a cylinder lens determines the spreading of the line in the opposite plane, unless adding a pair of corrective optics. Although, I'm not confident I understood you fully.

  6. #6
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    You've got it. You will have to add optics, but the optics are simple and easy to set up. If you go that way, post about it and specific recommendations can be given.

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