© 2019 Thorlabs GmbH
8 Tutorial
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An ER meter based on the rotating analyzer considers the Degree of Polarization (DOP) as well
as the absolute ellipticity.
Polarimeter Technique
The polarimeter records the polarization at the exit of the fiber during stress application. If there
is a non-optimal launch condition (misalignment) or a poor fiber, the output polarization will res-
ult in a circle being traced on the Poincaré sphere, in which the diameter of the circle is a meas-
ure for the extinction ratio. The smaller the circle the higher is the ER:
ER Meter Technique
Thorlabs' ERM100 is based on a rotating polarizer.
The polarizer transmits only the polarized part of the light parallel to its transmission axis. The
transmitted power depends not only on the input polarization but also on the degree of polariza-
tion (DOP). A photodiode after the spinning polarizer detects the power.
If the input light is linearly polarized and the DOP is 100%, the measured photo current is sinus-
oidal with its minimum at zero.
An elliptical input polarization or a DOP less than 100% causes a minimum > 0 of the photodi-
ode current. However, there are more factors that can influence the measurement data like
dark current and noise of the photodiode, but these are neglected for our considerations.
If the input light is circularly polarized with a DOP of 100% a constant (DC) photocurrent would
be the result. A purely unpolarized input light would also generate a DC photo current.
Both the ellipticity as well as the DOP will affect the measurement result. This should be con-
sidered when measurements with the ERM100 are performed.
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