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8 Tutorial
70
General case
Linear and circular polarizations are special cases of polarization.
Elliptical polarization generates a photodiode current with a DC term, a term that varies with
twice the rotation frequency of the waveplate, and a term that varies with four times the rotation
frequency of the QWP.
The Fourier transformation of the resulting photodiode current obtains the amplitudes and
phases of the DC, the 2w and the 4w QWP components.
The DC component has an amplitudes of A
0
, the 2w component has an amplitude of A
2
and a
phase of j
2
, and the 4w QWP component has an amplitude of A
4
and a phase of j
4
. Other fre-
quency components are assumed to have zero amplitude, in accordance with theory.
The polarization parameters azimuth, ellipticity, degree of polarization (DOP) and the Stokes
parameters can be calculated from the amplitudes A
0
, A
2
and A
4
and the phases j
2
and j
4
, ac-
cordingly.
A typical photo current for elliptical polarization for one waveplate revolution is displayed in the
following figure:
Note
In the PAX1000 a true zero order l/4 waveplate is used. For this type of waveplate, the exact
l/4 retardation is given only at its reference wavelength. In order to enable a broad operating
wavelength range of the polarimeter, the wavelength-dependent retardation must be com-
pensated in order to achieve accurate measurements. For this reason, the operating
wavelength must be entered into the device settings.
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