# Experimentally measure the degree of circular polarization

• metatrons
In summary: 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 summary, the degree of circular polarization of a light beam can be determined by measuring the intensity of the desired circular polarization component and comparing it to the whole output beam.
metatrons
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!

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.

Khashishi said:
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.

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?

Thanks!

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

metatrons said:

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?

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

https://www.amazon.com/dp/0444870164/?tag=pfamazon01-20

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:
Khashishi said:
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

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!

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

https://www.amazon.com/dp/0444870164/?tag=pfamazon01-20

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:

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.

Thanks!

Last edited by a moderator:

## 1. What is circular polarization?

Circular polarization is a type of polarization in which the electric field vector of an electromagnetic wave rotates in a circular pattern as it propagates. This is in contrast to linear polarization, where the electric field vector oscillates in a single direction.

## 2. How is the degree of circular polarization measured?

The degree of circular polarization is measured by quantifying the amount of rotation in the electric field vector. This can be done using a polarimeter, which measures the intensity of light in two perpendicular planes. The difference in intensity between the two planes is used to calculate the degree of circular polarization.

## 3. What factors affect the degree of circular polarization?

The degree of circular polarization can be affected by the angle of incidence of the light, the material through which the light is passing, and the wavelength of the light. Additionally, the degree of circular polarization can be influenced by external factors such as temperature and magnetic fields.

## 4. Why is it important to experimentally measure the degree of circular polarization?

Measuring the degree of circular polarization allows us to analyze the properties of light and the materials through which it passes. This information is useful in various fields such as optics, material science, and telecommunications. Additionally, circularly polarized light has unique properties that make it useful in applications such as 3D movie projection and medical imaging.

## 5. What are some techniques for experimentally measuring the degree of circular polarization?

Some techniques for experimentally measuring the degree of circular polarization include using a polarimeter, a quarter-wave plate, or a circular polarizer. These tools can be used individually or in combination to accurately measure and analyze the degree of circular polarization in a given sample of light.

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