Operating Principle; Applications - THORLABS ERM200 User Manual

ERM2xx Series Extinction Ratio Meter

5.6 Operating Principle

The ERM2xx models are based on a rotating polarizer which transmits light depending on the transmis-
sion axis of the polarizer and the polarization of the analyzed light. A photodiode positioned behind
the rotating polarizer quantifies the transmitted light. Thus, whenever the analyzed light is linearly po-
larized and the transmission axis of the rotating polarizer is oriented parallel to the polarization axis, a
peak in optical power is detected. The higher the degree of polarization (DOP) of the analyzed light,
the lower the minimum polarization extinction will be.
In theory, if the input light is completely linearly polarized and the DOP is 100%, the measured photo
current is sinusoidal with its minimum at zero. If the minimum is not equal to zero, it might be due to
an elliptical input polarization or due to a DOP less than 100%. This occurs when the light has an unpo-
larized contribution. Aside from the DOP, other factors may influence the measurement data, such as
dark current or noise of the detection system.
Circularly polarized light with a DOP of 100% would result in a DC photo current in the ERM2xx module.
A purely unpolarized input light would also generate a DC photo current.
The following table shows the measured ER value in relation to the DOP assuming an ideal linear polar-
ization.
DOP [%] 90.0
PER [dB] 12.8
If the DOP posts a challenge in a particular PER measurement application, a more complex measure-
ment system like Thorlabs' Polarization Measurement System
PER and PM fibers
As long as the linearly-polarized light of a broadband source travels along the slow (or fast) axis the po-
larization is maintained and the ERM2xx module will show a high PER. If the linearly-polarized light is
not coupled into one of the main axes of the PM fiber the linearly-polarized light is separated into two
parts. One is traveling along the fast axis and the other along the slow axis. Both parts will travel with
different speeds. Depending on the birefringence of the fiber, its length, and the induced stress (for
example by bending or temperature), the output polarization is not predictable and can vary over time.
Furthermore, the birefringence is wavelength dependent. The output of each wavelength of the spec-
trum of the broadband source will be different. This leads to a certain degree of depolarization of the
light coupled into the fiber and leads to a lower extinction ratio measured by the ERM2xx module.
Light with an elliptical polarization or linearly-polarized light which is not parallel to one of the main
axis will not preserve its polarization state along the fiber.

5.7 Applications

Measurement setups and procedures differ depending on the task. For example, the requirements for
testing the coupling of linearly-polarized light into a main axis of a PM fiber and the alignment of a
laser diode to a PM fiber are different, and differ again from the requirements for testing a PM fiber
patch cable.
This chapter suggests application procedures for widely used tasks.
For the cases described below, we assume perfect optical elements.
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Chapter 5 Operating Instructions
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is required.
MTN033660-D02