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Thread: Beam pinhole profile question

  1. #1
    Join Date
    Aug 2009
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    Auburn, Washington
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    Laser Warning Beam pinhole profile question

    I'm debating this with a good friend....

    Say you have something like a 650nm that puts out something like a square or rectangle beam by nature. Once you have a good collimator lens and have the beam as good as it can get - and lets say for the sake of argument it is solid 6mm X 4mm beam, if you put a centered 3mm round pinhole right before the scanners, would you have a consistent round beam after the scanners? Not counting divergence and scanner mirror angles. Just a round beam for the most part?

    I say it would tend to stay round. The debate is that it is the nature of the square beam at the laser that will make it go back towards square after the scanners. Not being an expert I can't say which is correct, but it just seems to me that clipping off everything but the center of the beam into a 3mm circle, it would stay that way for the most part.

    Just thought I would get some feedback before I try the pinhole trick and then find out somehow the beam isn't what I expected.

    Gene

  2. #2
    Join Date
    Jul 2009
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    The Netherlands
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    Default

    very interesting, i have been thinking this also.
    But then some other people say it would go back to it's old form.
    But if you make a 6X4 mm beam into a round 3mm how does the beam "know" it's old shape ? Is this something optical i don't understand ?

    Looking forward to some answers of the more scientifically persons here.
    I think it would stay round.
    I didn't fail !
    I just found out 10,000 ways that didn't work.

  3. #3
    Join Date
    Jan 2008
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    Stockholm, Sweden
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    Default

    Well, there are two basic parameters that determines the shape and size of a beam at a distance: Initial size and divergence. If you pass the beam through a pinhole you will change the initial size, but not the divergence (unless the hole is very small, in which case you get an increase in divergence through diffraction). If the divergence is uniform the beam should remain in the shape of the pinhole. That is rarely the case with beams from laser diodes though - their divergence is not the same along their axis, so the beam will change its shape over a distance.

  4. #4
    Join Date
    Jul 2005
    Location
    Tampa Florida
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    502

    Default If it helps use it

    Over a long distance you will probably get elipse back due to the divergence difference for X and Y of the diode but if it is only a couple meters to the screen for a lightshow a simple "pinhole" may help. Also consider if you clip the beam you will get diffraction and may end up with concentric rings in the far feild. Cool physics demonstration but...Phil 142laser
    Phil Bergeron( AKA 142laser)

  5. #5
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    Aug 2009
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    Auburn, Washington
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    Now that it is explained, it makes sense. The law of gravity is a good idea. The laws of physics when it comes to lasers is not that great of an idea. Dammit!

    Gene

  6. #6
    Join Date
    Dec 2008
    Location
    Vezon, Belgium
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    Default

    you can avoid most of the diffraction effects by focusing your beam into a pinhole say 100 microns or less, then recollimate it

    it's called spatial filtering and is relatively easy to do if I'm not wrong

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