The polarization state of a light source is represented by four numerical quantities called the Stokes parameters. These four parameters correspond to intensities of the light beam after it has passed through certain devices such as polarized prisms, films and wave plates. These parameters are described in the Figure below.
The first component, I, represents the total intensity of light.
The second component, Q, represents the difference in intensities between horizontal and vertical linearly polarized components.
The third component, U, represents the difference in intensities between the linearly polarized components at 45 and -45 degrees.
The fourth component, V, represents the difference in intensities between right and left circularly polarized light.
The four components of the Stokes vector allow a single vector to describe polarized, partially polarized, and unpolarized light.
As shown in Figure 2, the Dual PEM Stokes polarimeter contains two PEMs, one PEM oriented at 0°, and a second PEM oriented at 45°, a Glan-Thompson polarizer at 22.5° and a photo detector.
The electronic signals generated at the detector have both AC and DC components. The AC signals contain modulations at different harmonics from both PEMs and they are processed using lock-in amplifiers in this instrument.
To measure Q and U, signals at the second harmonics of both PEMs should be detected. To measure V, the first harmonic signal from the first PEM is detected. Therefore, a waveform capture card is used to detect Q, U and V simultaneously.
The DC signal is recorded after the detector signals pass through an electronic low-pass filter and an analog-to-digital converter. The DC signal represents the average light intensity reaching the detector. A computer is used to record both the AC and DC raw data and to calculate normalized Stokes parameters.