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8.3 Polarization Control and Transformation
The polarization of light can be manipulated by retarders, polarizers and other polarization op-
tics. Even single mode fibers operate on the polarization state of the guided mode, so that the
input and output states of polarization differ. Polarization control is important since many optical
components require a well defined polarization state. In many applications, a polarization con-
troller is a key component. In the following sections a short summary of polarization controllers
is given.
8.3.1 Bulk Optics
Polarizers transmit only those light waves with a specific polarization, which makes them use-
ful as optical filters when analyzing the light's SOP. Linear polarizers pass one of the two ortho-
gonal components of light. They generate linearly polarized light aligned with the transmission
axis of the polarizer. Circular polarizers can have a clockwise or counterclockwise orientation,
and they pass clockwise or counterclockwise generated light, respectively. If two polarizers are
used in an optical setup, the second is called an analyzer.
Retardation Plates (Retarders) can be used to control the polarization of a light wave. In gen-
eral, a retarder is a birefringent optical element, in which orthogonally polarized components of
light have different indices of refraction. Light incident on a waveplate is divided into one com-
ponent polarized along the fast axis, which has a lower index of refraction, and one polarized
along the slow axis, which has a higher index of refraction. Traveling through the waveplate
delays the light polarized along the slow axis relative to the light polarized along the fast axis,
so that a there is phase difference between them. After passing through a QWP, the slow axis
lags the fast axis by l/4 and the phase difference is 90°. Quarter waveplates convert elliptically
polarized light to linearly polarized light, and vice versa. After passing through a half waveplate,
the slow axis lags the fast axis by l/2 and the phase difference is 180°. A half waveplate flips
the polarization direction of linear light with respect to the fast axis and inverts the handedness
of elliptically polarized light.
Polarization Controller consisting of one half waveplate placed between a pair of quarter
waveplates can be used to produce light with any SOP from arbitrarily polarized input light.
The first QWP is adjusted so that linearly polarized light is incident on the half waveplate. The
half waveplate is rotated to adjust the azimuth angle of the light incident on the second QWP.
The second QWP transforms the incident linearly polarized light into elliptically polarized light.
For QWPs and half waveplates, if the incident light is linearly polarized and its direction of polar-
ization is aligned to the fast or slow axis, the light passes unmodified through the optical ele-
ment. If a QWP is aligned so that the azimuth of the input light is at 45° to the fast and slow
axes, a circular state of polarization is generated.
8.3.2 Fiber Polarization Controllers
A fiber optic polarization controller (aka Lefèvre loops) is used to accurately set the state of po-
larization of light in a single mode fiber. These polarization controllers apply bend-induced
mechanical stress to the fiber to induce birefringence, so that the orthogonally polarized com-
ponents of the guided fundamental mode travel at different velocities. By controlling the length
of fiber over which the stress-induced birefringence is applied, the function of bulk QWPs and
half waveplates, which are described in the Bulk Optics section, can be duplicated in optical
fiber. As the stress-induced birefringence in the fiber is low compared with the birefringence of
the bulk optic crystals, relatively long lengths of fiber (typically on the order of 1 m) are needed
to induce the desired phase shift. The mechanical stress is induced by wrapping the fiber
around a spool, and the number of turns determines the total phase shift induced between the
two polarization components.
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