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Thread: Degree of Polarisation Measurement

  1. #1
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    Default Degree of Polarisation Measurement

    How do I measure this very accurately?

    So basically I have collimated laser beam incident on to a polariser and measure the minimum and maximum transmission. The ratio of which is 20:1. I am concerned that my absolute measurement of the minima has a lot of error...

    Is there a better way of determining this?
    Thanks for your time.
    Alex

  2. #2
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    Are you sure the laser is polarized? (Not all lasers are.) And even among those that do produce a polarized output, some are highly polarized while others are not.

    Also, are you sure your polarizing filter is up to the task? If it is a high-grade optical component, that's fine. But if it's cheap optical film (like they use for 3D glasses at the movie theater), then it's not good enough for any serious measurements.

    What exactly are you trying to measure? ("Degree of polarization" can be interpreted two different ways.) Are you trying to determine the precise angle of the polarization plane relative to the baseplate, or are you merely wondering about the power difference between the polarized and non-polarized planes?

    Adam

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    Get a pbs cube and measure the power from each beam. Won’t tell you s or p but will give you a ratio.

  4. #4
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    I am using a thorlabs glen taylor polariser GL10, which says it has an extinction ratio of 100,000:1 in the transmitted ray. See here: https://www.thorlabs.com/newgrouppag...ctgroup_id=815

    I am trying to measure the ratio of the maximum and minimum transmitted power by rotating the polarizer and thus determine the extinction ratio of the laser.
    ER = 10 log (I max/I min)

    The laser has several elemnts which are polarization sensitive and so it should be well polarized.. Tha's why i thnk my measurement procedure is poor...

    Thanks
    Alex

  5. #5
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    Make sure the prism is used in its wavelength region. Find a polarized hene with an internal Brewster window to check the polarizer against (If Visible or near IR)

    There is the ANCIENT stack of transparent slides at Brewster's angle method to ensure you have a highly polarized beam, essentially a stack of Brewster Windows in series. Then use that known polarized beam coming through the stack to check your beam. Keeping track of all the Fresnel reflections requires a really linear optical sensor.

    Speaking of optical sensors, what are you using for your measurement? I'd really look at the linearity and bias of a photodiode before assuming I have a problem.

    I'd also look at the quality and alignment of my rotator. 90' incidence with no tilt while rotating is important.

    Or call up some one like Medowlark Optics or and have a chat about what to do.

    The Poincare sphere is an interesting thing. Really incredible polarization is hard to achieve because the last optic always has some minor thickness difference or phase shift or internal inhomogeneity across the beam.

    There are other techniques like "Law of Malus" experiments or methods involving reflected beams and waveplates etc...

    Steve
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  6. #6
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    Make sure all room lights are off, and there isn't a computer screen the detector can "see". Those are very expensive polarizers. Did you get yours used? Is it damaged in the middle? Try a different polarizer for a sanity check. Could be that you're reading it right though, and the polarization of your laser is elliptical.

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