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PAX1000
8.2 Polarization Measurement Technique
There are different methods to measure the state of polarization. Among them, the four de-
tector method is a well known technique. A similar technique can be used to measure the polar-
ization in fibers. Pairs of special Fiber Bragg grating (FBG) couple out a portion of light. The po-
larization can be calculated from the power ratios.
Different methods are used to measure the polarization state. Among them, the rotating l/4
waveplate technique is well known. It is described in the following section.
8.2.1 Rotating Waveplate Technique
The optical unit of the PAX1000 polarization measurement sensor consists of a rotating true
zero order quarter waveplate (QWP), a fixed linear polarizer and a photodiode. The waveplate
transforms the input polarization. The polarizer transmits only the portion of light that is orient-
ated in parallel to its transmission axis. The photodiode acts as a power meter.
The incident polarization is transformed (modulated) by the rotating QWP and the polarization
state that enters the linear polarizer depends on the actual rotation angle of the waveplate.
The linear polarizer transforms the polarization modulation into an amplitude modulation of the
light power that enters the photodiode.
The photodiode delivers a current proportional to the optical power and to the square of the
electric field intensity. It is easy to see that unpolarized light will generate a constant optical
power at the input of the photodiode, as it has no defined polarization state.
In general, there are only three spectral components in the photo current:
· a DC component, arising from the unpolarized optical power,
· a component with double the rotation frequency of the QWP and
· a phase-shifted component with quadruple the rotation frequency of the QWP.
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