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Thread: waveplate or no waveplate?

  1. #1
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    Default waveplate or no waveplate?

    I'm still trying to finish up my dual p73's for my projector but I feel I may have overlooked the use of a waveplate. Searching around I see some designs that use them and some designs that don't and I'm not sure what the difference is. My plan was to simply combine two beams with a pbs cube and then correct the output but as I got thinking about it wouldn't half of the combined beam be the wrong polarity by the time it gets to the cylindrical lenses? Do I need a waveplate after the pbs in order for the cylinders to correct the whole beam?

    If it turns out I do in fact need a waveplate then my question is what is the difference between a half waveplate and a full waveplate? How do the two affect the output beams?

    Thanks,
    Luke
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  2. #2
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    Default

    In order to combine the 2 beams with the cube and be able to correct them effectively after the cube you need to have both the diodes in the same orientation, with the fast axis in the horizontal plane. However, by doing this both will be polarized the same way. So you need a half wave plate to switch the polarity of one diode to ensure it comes out the cube correctly.
    The wave plate goes before the cube.
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  3. #3
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    Default

    What Norty says. One solution is to use a waveplate and pbs.

    The other solution is to use no waveplate and no pbs, but to stack the beams verticaly, like " = "
    The shape of the p73 beam allows for this configuration, too. The two diode mounts will need to have a slight height offset (like 2mm or so).
    so as the beams are on the same vertical axis, they can be corrected with a single cylinder lens pair

    a more complex approach is to stack two like " = " and then via waveplate and pbs to add another two " = " for a quad


    in terms of terminology, think of it like a "half-wave" plate and not a half "wave-plate." The name is derived from lightwave retardation, so you have half-wave, and quarter-wave. A full wave plate would make no sense as such http://en.wikipedia.org/wiki/Wave_plate
    Last edited by LaNeK779; 08-27-2014 at 02:37.
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  4. #4
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    Default

    I think I'm still missing something. Why would the wave plate go before the pbs cube. If I had both beams oriented horizontally and used a wave plate on one to change the polarity before the cube wouldn't the resulting beam still look like a (+)? I thought this was what I was trying to avoid since the correction optics would only be able to correct half of that beam. Maybe I'm not understanding what the wave plate is actually doing. I'm under the impression that the wave plate would take a beam oriented horizontally and change it to a vertical orientation. Is that correct? If that's the case, couldn't I just rotate on diode 90 degrees to achieve the same (+) shape? I assumed the wave plate would go after the pbs cube to rotate the vertical beam back to horizontal after it had been combined. Is that assumption wrong?
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  5. #5
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    if the two beams are oriented like = before the waveplate, they will still be like = after one beam passes through the waveplate.
    a waveplate will not turn a "-" beam to a "|" beam. it does its magic inside the beam, not changing its shape or orientation

    Waveplate is considered a dark art to most, myself included. It is just that it is not so easy to grasp the whole polarity of the light thingy
    "its called character briggs..."

  6. #6
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    Ok, that makes more sense then. So the polarity of the beam can be changed without changing the orientation of the fast axis. So if I start with both beams oriented horizontally they will still be horizontal after the wave plate and pbs cube.
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