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Thread: Knife edging and beam size after correction question

  1. #1
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    Oct 2007
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    Default Knife edging and beam size after correction question

    I'm trying to understand how a cylindrical lens set affects a dual knife edged pair vs a single beam.

    I'll be upgrading my dual combiners 445nm to a quad, planning on using 2 462nm's and 2 445nm's for each quad module. Currently I'm setup with waveplate and pbs, so I will have to knife edge each pair and then waveplate / pbs.

    My question is if for example, each beam is 3.5mm x 0.8mm after the collimator, when knife edged (say for this question a perfect knife edge with no gap) the beam size would be 3.5 x 1.6mm. I am currently passing my dual pbs through a set of Dave's cylindrical lenses (which I'm not sure but believe it's a 4x expander).

    If I were to pass the 3.5mm x 1.6mm knife edged beam through the cylindrical lenses, would the resulting beam simply be 1.6mm x 4 = 6.4mm ? (Too big for my scanner mirrors) Or does it work differently for a knife edged pair ? (such as each beam being expanded separately and overlapping each other ?)

    Is it necessary then to either use prisms and set for a lower magnification, or perhaps purchase a plano-convex cylindrical lens of half the focal length, effectively making a 2x expander ? Or find an appropriate focal length to make a 2.2x for example if I wanted a perfectly square 3.5mm beam ?

    Final question, does anyone know the focal length of lenses of Dave's cylindrical lens pair ? From the distance they need to be and if it is a 4x, I'd guess them to be somewhere around -6.3mm fl and +25mm fl ? (Is my math in the general ballpark ?)

    Thanks,

    Miles

  2. #2
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    Default

    A somewhat related question about cylindrical lenses in general. How does distance from the diode affect beam expansion ratio ? For example at a longer distance, beam size after expansion is larger, is the divergence therefore smaller at far field than if the same cylinder set was mounted closer to the diode ?

    If so, how can it be calculated ?

  3. #3
    Join Date
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    Default

    While you're waiting for someone better qualified to respond, you might read this post from planters, and possibly some of the other posts in that thread:

    http://www.photonlexicon.com/forums/...380#post299380

    Not sure if the same conditions exist with your blues as with the reds planters was talking about, but if so the physics should be the same.

  4. #4
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    Thanks for the link. I always wondered why distance would affect final divergence at all. Couldn't find a reason in physics to explain that Interesting that it's the curvature of the lens itself that is a factor there.

  5. #5
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    You may be better off using shorter FL collimators and "stacking" the beams instead of "knife edging". Another added benefit would be more change in the magnification of the cylinders in relation to distance from the collimators thus allowing more adjustment of your near field size in a shorter distance.

  6. #6
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    Is there a way to predict initial beam size using 2mm fl collimators with blue ? Would it be half that of a 4mm fl lens, or is there more to it ?

    I've thought about that, and definitely would be preferred as my red worked out very well vertically stacking, but had read that Dave's lenses are ar coated specifically for red, so there are definitely some considerable losses with the blue (also not sure if it altogether fits or clips some of the beam).

    Unless anyone knows of a source of shorter fl lenses that work well with the blue diodes ? To stack, I'd have to get under a 2.25mm high stripe to fit a 4.5mm beam profile. Is that even possible with the emitter size of the blue ?

  7. #7
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    Pretty much 1/2 the size you are getting with the 4mm. Dave's 2mm lenses work good for blue. I tested it awhile back. I don't remember the throughput compared to the optima 4mm's. It was real close. I know I posted it somewhere here on PL.

    Edit: fount it. http://www.photonlexicon.com/forums/...785#post221785 go up a few posts.

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