© 2019 Thorlabs GmbH
8 Tutorial
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Linear Polarization
This section considers input light with a linear state of polarization (SOP).
Assume that for a rotation angle of 0° the fast axis of the quarter waveplate is aligned with the
polarization direction of the input light. The linear polarized light travels along the fast axis and
is not changed.
When the waveplate is rotated 45°, the SOP is transformed to right circular.
Another 45° rotation, which gives in total 90°, aligns the slow axis of the waveplate with polari-
zation direction of the incoming light. The SOP is not changed by the waveplate.
A rotation of 135° yields left circular polarization.
A half turn, corresponding to 180°, again aligns the fast axis of the waveplate with the polariza-
tion of the light. The waveplate does not change the SOP.
The following picture demonstrates the change of the polarization on the Poincaré Sphere. The
dark-green trace represents points laying in the foreground of the sphere, the light-green - in
the background ("dark side"). As can be seen, after a half revolution of the QWP, a trace with
the shape of an "8" is drawn onto the sphere:
In the case of linearly polarized incident light, the photo diode current I(t) is proportional to the
quadruple rotation frequency, w
QWP
, while the phase shift, j
4
, reflects the azimuth angle:
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