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Thread: How to rotate polarisation of a laser beam

  1. #1
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    Default How to rotate polarisation of a laser beam

    We need to rotate the polarisation of our argon beam by 90 degrees to simplify our beam table layout...

    I hear that, with a vertically polarised laser, if you reflect the beam UP, and then use another mirror to reflect it back horizontally again, this will rotate the polarisation by 90 degrees....

    is this the case ?

    Any other easier ways to do this for a multiline argon beam ?
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    Quote Originally Posted by aijii View Post
    We need to rotate the polarisation of our argon beam by 90 degrees to simplify our beam table layout...

    I hear that, with a vertically polarised laser, if you reflect the beam UP, and then use another mirror to reflect it back horizontally again, this will rotate the polarisation by 90 degrees....

    is this the case ?

    Any other easier ways to do this for a multiline argon beam ?
    I'm not sure that would work... but I was looking at these a while ago: http://www.sp3plus.co.uk/quartz-rotation-plates.php

    Pit.
    A little bit werrrr, a little bit weyyyyyy, a little bit arrrrgggghhh

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    you can also simply use 3 mirrors, buy nice 1/20 wave or better to keep
    divergence low.

    Bounce it up, then to the right then bounce in the original direction of the
    beam...

    2 mirrors if you don't mind sending the beam out 90 degrees to the orignal path.
    Or you can use waveplates...

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    Broadband 1/2 wave plates are $USD750 from thorlabs.

    Pit - that link you posted looks cool - but are they broadband plates ?

    Also - with a multiline beam - will each wavelength be rotated by the same amount ?
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    Might try the 3 mirror trick first. We need to raise the beam up off the base plate anyway so this might just do the trick.

    How critical are the angles of reflection here ?
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    If you eyeball everything for parallelness then it's good enough.
    In a literal sense, if your mirror sends your beam up at 5 degrees off,
    then your polarization will be 5 degrees skewed.

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    The 3-mirror trick is the least expensive. But (not so obvious for those who are not in the know...) you should use mirrors that have a metalic reflective surface (i.e. aluminum, enhanced aluminum, or silver -- or gold if you are using red or IR). I have seen many times, that dielectric mirrors disturb the polarization.

    Using a broad-band half-wave plate is also a good way to go, but as mentioned earlier, it is more expensive. However, it does allow you to "dial in" the polarization as you see fit.

    One other alternative has not been discussed here. Simply rotate the laser by 90 degrees. Unless it is a medium frame or large frame laser, it won't really care what the axial orientation is...

    Best regards,

    William Benner

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    Quote Originally Posted by Pangolin View Post
    Using a broad-band half-wave plate is also a good way to go, but as mentioned earlier, it is more expensive. However, it does allow you to "dial in" the polarization as you see fit.

    One other alternative has not been discussed here. Simply rotate the laser by 90 degrees. Unless it is a medium frame or large frame laser, it won't really care what the axial orientation is...
    Bill, thanks for the reply.

    Will the half-wave plate method work with a multiline laser though ? Will the polarisation of each wavelength be rotated by the same amount ?

    Rotating the actual laser was our first thought on how to correct the issue - but it will mean our low profile projector will become not so low profile any more, so it is not our preferred option at this point.

    The power loss from using the 3 mirrors is going to be significant, but what about the waveplate ? What do these things do to the beam in terms of throughput efficiency and divergence etc ?
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  9. #9
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    Quote Originally Posted by aijii View Post
    Will the half-wave plate method work with a multiline laser though ? Will the polarisation of each wavelength be rotated by the same amount ?
    Yes, and Yes That is, IF you use a broad-band waveplate. We have used ones from the company Meadowlark in the past. Personally, I think they are the best...


    Quote Originally Posted by aijii View Post
    The power loss from using the 3 mirrors is going to be significant, but what about the waveplate ? What do these things do to the beam in terms of throughput efficiency and divergence etc ?
    They're pretty transparent. We have used them with relatively high power lasers in the past...

    Best regards,

    William Benner

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