© 2019 Thorlabs GmbH53
PAX1000
8 Tutorial
8.1 Polarization of Light
8.1.1 The Nature of Polarization
When talking about polarization, light is considered as an electromagnetic wave. It oscillates
perpendicular (transverse) to the direction of the propagation of the light beam. Such a trans-
verse electromagnetic wave can be divided into an unpolarized and polarized part. The plane of
polarization of unpolarized light, also called natural light, fluctuates arbitrarily around the direc-
tion of propagation, so that in average no direction is favored.
All field components of polarized light have a fixed phase difference to each other. Each state
of polarization (SOP) can be split into any two orthogonal states. The following pictures illus-
trate different states of polarization:
The first example is linear polarized light. The phase difference between the two orthogonal
states is 0° or 180°, and superposition results in a total electric field that is always oriented in
the same direction when viewed in the transverse plane.
The second diagram shows an elliptical polarization state. The phase difference between the
orthogonal states is >0° and <=90°. The projection results in an ellipse with a right or left direc-
tion of rotation.
A special case of elliptical polarization is circular polarization (3rd example), where the phase
difference between the orthogonal states is 90°. Then the amplitude of the rotating vector is
constant over the entire 360° rotation.
Common light sources except lasers emit natural light. Helium Neon lasers or DFB laser diodes
normally feature linear polarization. However, if this laser light is launched into a single mode
standard fiber (SMF), the linear polarization is transformed into an arbitrary polarization.
Even if a polarization maintaining fiber is used, the linear polarization has to be coupled into
one of the main axes to prevent the loss of linearity.
A multimode fiber allows a propagation of numerous modes. The state of polarization of each
mode at the end of the fiber is arbitrary. The superposition of the polarization of all modes res-
ults in unpolarized light.
The spectral width of the emitted light needs to be considered as well. If the spectral line width
of the emission is narrow enough, then the light is polarized. An example is a DFB laser diode
with a linewidth of about 3MHz.
Light sources with a broad spectrum like LEDs are generally unpolarized. A polarizer can be
used to polarize light with a large spectral width.
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