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Thread: Lens vs. Prisim

  1. #1
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    Default Lens vs. Prisim

    This may have been covered before on the threads that taut the competing methods for reducing/expanding beams to fit scanners or reduce divergence. But, I can't find a comparison of far field performance when used to modify a COMPOUND beam. Within a few hundred mm of the diodes a tightly stacked rectangle of 445 nm beams is an overlapping mix of diverging beamlets in a converging stack. Using even relatively low power (1:2) expanding or reducing lenses, aberrations still limit the far field combined pattern. Keeping this simple, I am only talking about 1 dimensional cylinders vs a single pair of anamorphic prisms. The best I can do with 6 diodes llllll is 6x5mm. The outermost beams have a lot of coma ie one edge of the vertical line flares vs the other edge and the beam becomes displaced. By slightly realigning the beams I can cause the flare to sweep across the stack reducing but, not eliminating its appearance. Because it has not been convenient to me I have not tried to readjust the individual diode focus to compensate for these aberrations nor have I purchased a prism pair...yet. Should I?

  2. #2
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    I have found that when using cylindrical telescopes small adjustments to the collimator make a huge difference in the far field spot. Also, I have found the longer the focal length of the telescope lenses the better the spot. I found this to be true for the ca$io and the LOC reds.

    I should mention the better the collimator the better the telescope will work. Aspheres seem to be the best.

    Prisms are way easier to work with but the losses get high with greater magnifications.

  3. #3
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    I wouldn't waste your time with stacked diodes and prism pairs -> I found it gave very poor far field.
    If you want a pair of prims to loan + experiment with, I could lend you a set.

    The best thing I have found is not to add any optics to the "compound beam" as you put it.
    The best I could manage was 8x diodes with 4.5x4.5mm at aperture, 1.1mRad, H+V Pol.

    What are you trying to achieve exactly?
    e.g. beam size / shape to fit on scanners etc. At what divergence is acceptable?

    Cheers,
    Dan
    - There is no such word as "can't" -
    - 60% of the time it works every time -

  4. #4
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    Your stacking density is about the same as mine 4 vs 6 diodes =4.5mm vs 6mm. I am building a 6+6 and the large Eye Magic scanners will accept a 9mm x9mm input beam. Reducing the far field divergence is the goal. I am impressed with your raw far field divergence as mine is currently 1.5mrad using O-like lenses.

  5. #5
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    Dan,
    Rather than editing, I'll just add this here. Firstly, sorry about not thanking you for your offer to lend some prisms,that was very kind, however your suggestions are more valuable. Your poor results with that method discourages me and so lets talk about results with the cylinders.
    logsquared,
    What kind of focal length are you talking about when you say "longer"?
    Looking at this more carefully Dan actually has a slightly lower stacking density, implying an initially broader fast axis stripe and this might explain his better divergence. And maybe better route than using a telescope.
    Dan,
    What collimator lens are you using?
    Thanks

  6. #6
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    A 90mm and a -30mm FL worked much better than a 30mm and a -10mm. Also the higher the magnification the worse the spot became. Looking at my notes it looks like two positive lenses worked better that a positive followed by a negative. This is opposite of what i found with the LOC's where a Galilean worked better than a Keplerian. The tests were done with O-like on the 445's and aixiz glass on the LOC's.

    If you are getting 6mm with 6 diodes llllll and 1.5 mRad you're doing pretty good. At 1W I measured the beam at aperture with a O-like to be %98 power at 2mm and the FA was %98 at 4.5mm the divergence is 1.9ish.

    So the best I can see and still retain power would be 12mm wide for 6 llllll and, because the aperture size increased, aprox. 1.1 mRad with out correction.

  7. #7
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    Quote Originally Posted by planters View Post
    Dan,
    Rather than editing, I'll just add this here. Firstly, sorry about not thanking you for your offer to lend some prisms,that was very kind, however your suggestions are more valuable. Your poor results with that method discourages me and so lets talk about results with the cylinders.
    No probs :-)

    Quote Originally Posted by planters View Post
    Looking at this more carefully Dan actually has a slightly lower stacking density, implying an initially broader fast axis stripe and this might explain his better divergence. And maybe better route than using a telescope.
    Unless you need to squeeze down the beam to fit on your galvos (which I dont think you need to do in this case) I wouldnt go for a telescope. They are best when you have a really low divergence to play with. I wouldnt want to have much more than ~1.1mRad on blue...


    Quote Originally Posted by planters View Post
    Dan,
    What collimator lens are you using?
    Thanks
    From a nice batch of o-likes. :-) The half threaded ones with an o-ring.

    Cheers,
    Dan
    - There is no such word as "can't" -
    - 60% of the time it works every time -

  8. #8
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    ~1mrad from a O-Like? Awesome
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  9. #9
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    Quote Originally Posted by danielbriggs View Post



    From a nice batch of o-likes. :-) The half threaded ones with an o-ring.
    How are you getting 1.1 mRad from an O-like without correction?

  10. #10
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    Regarding O-like divergence. I also am getting in the neighborhood of 1.8 mrad raw divergence (this is typical of other posts) which only reduces to 1.5mrad with a 1:1.5 expanding telescope. And this is at the cost of a near doubling of the slow divergence due to the aberrations I already discussed. I am currently using 24 of these lenses and their variation in performance has been small. Is it possible, as I have done way too many times, that your math is in error?

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