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Experimentally measure the degree of circular polarization

  1. Jul 16, 2015 #1
    Dear All:

    I have recently encountered a small question regarding the determination of the degree of circular polarization of light. In an optical experiment, we are trying to create a circular polarized light beam by passing a HeNe-laser through a linear polarizer and a quarter wave plate (in which the fast axis is at 45 degree with the LP transmission axis). We need to determine 'the degree of circular polarization' of the output beam -the ratio between the intensity of the desirable circular polarization component in the output beam versus the intensity of the whole output beam.

    My intuition is placing another linear polarizer after the beam, and measure the overall transmission versus the rotation of the 2nd linear polarizer (in a 30 degree's step from 0 to 180 degree). We have the set of measured transmission, and they are very close but not exactly the same. Can I deduce the degree of circular polarization based on this data?

    Or else, could anyone give me a hint on how to measure and calculate it by other method?

    If possible, please provide a link of a published paper or a textbook that describes the calculation process, and I can study it in details.

    Thanks a lot!

    Best regards!
  2. jcsd
  3. Jul 16, 2015 #2
    I don't think your linear polarizer can distinguish between circularly polarized and unpolarized (mixed) light. If that doesn't matter for your application, then maybe it's fine.
  4. Jul 16, 2015 #3
    Thanks for your reply!

    Here, I assume our beam (after the QWP) is fully-polarized-it either contains circular polarization component, or linear polarization.

    However, the problem is, say, I have the spectra set (transmissions in different angles of 2nd linear polarizer), how can I calculate the degree of circular polarization based on these data?

  5. Jul 16, 2015 #4
    Intensity versus polarizer angle should look sinusoidal. Fit a sinusoid to it and get the maximum and minimum.
    degree of circular polarization is just I_min/I_max
  6. Jul 16, 2015 #5

    Andy Resnick

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    The standard ellipsometry reference is Azzam and Bashara's book:


    One issue which you don't discuss is the angular accuracy of your QWP and efficiencies of your polarizers. The reference I mentioned has all of the measurement and data processing details. Here's another reference you may find useful:

    Last edited by a moderator: May 7, 2017
  7. Jul 16, 2015 #6
    Thanks a lot for your hint!

    If I understand you correctly, the output beam can be approximated as a elliptical polarized beam, and the sinusoidal function is used to fit the short and long axis of the ellipse. Here, I suppose the sinusoidal function should describe the field amplitude rather than intensity? In specific, I should first do square-root of the measured transmission, then fit the transmission versus angle to the function such like 'Emin+(Emax-Emin)*sin(theta0+theta)'?

    I suppose this is sensible method. I still feel some uncertain that it seems this method ignore the possible phase difference between the linear and circular polarization component (in addition for the direction of the linear polarization).. I will think more carefully about it.

    Thanks a lot!
  8. Jul 16, 2015 #7
    Thanks a lot! I will try to find a way to read this book and study the details of it.

    Here, I assume a perfect angle reading and linear polarizer, and the only source for imperfection is from the imperfect quarter wave plate. But still I am eager to learn the standard method.

    Last edited by a moderator: May 7, 2017
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