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8 Tutorial
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A fiber polarization controller features two l/4-paddles and one l/2-paddle. These three
paddles can be twisted relative to one another. The diameters of the paddles are chosen in
such a way that all states of polarization can be set. Therefore, 1/2/1 or 2/4/2 turns are neces-
sary, depending on the wavelength range. A rotation of a paddle causes a rotation of the fast
and slow axes in the fiber spool with respect to the incoming electric field.
Another way to cause birefringence in a SMF is to "squeeze" the fiber. Applying pressure to the
fiber results in stress birefringence. By setting up three independent fiber squeezers in series
with a 45° shift each to the other, any desired transformation of polarization can be achieved.
The disadvantage of a fiber squeezer is the strong mechanical stress imposed upon the fiber
that may lead to destruction.
8.3.3 Fiber Polarization Scrambler
Polarization scramblers convert highly polarized light into light with a random polarization.
Piezoelectric components, also known as fiber squeezers, are used to quickly modulate the
state of polarization by inducing a time-dependent birefringence in the optical fiber. Thus it is
possible to reduce polarization dependent effects in optical transmission systems, for example,
polarization dependent losses or gain. Polarization scramblers should have a low insertion loss
and operate in a wide wavelength range.
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