THORLABS ERM100, Operation Manual, Page: 14 / 59

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ERM100
3.3 Measurement Procedure
Depending on the alignment task there exist different measurement setups and measurement
procedures. For example the alignment of a laser diode to a PM fiber pigtail has different re-
quirements than the testing of PM fiber patch cords.
The ERM100 can be used to couple linear polarized light into the slow (or fast) axis of a polari-
zation maintaining (PM) fiber as well as align the connectors key to the slow (or fast) axis of the
PM fiber.
3.4 Measurement Principle
The ERM100 is based on a rotating polarizer. A polarizer transmits only the polarized part of
the light which is parallel to its transmission axis. The transmitted power is not only dependent
on the input polarization but also on the degree of polarization (DOP). A photo diode after the
spinning polarizer detects the power.
If the input light is completely linear polarized and the DOP is 100% the measured photo cur-
rent is sinusoidal with its minimum at zero. If the minimum is not equal to zero it can due to an
elliptical input polarization or due to a DOP less than 100%. There are also more factors which
can influence the measurement data like dark current and noise of the photo diode but let us
assume perfect optical elements for our considerations.
If the input light is circularly polarized with a DOP of 100% a DC photo current would be the res-
ult. A purely unpolarized input light would also generate a DC photo current.
Both properties the ellipticity as well as the DOP will affect the measurement result. This should
be considered when measurements with the ERM100 are performed.
3.5 Coupling of linear polarized light into a main axis of a PM
fiber
If linear polarized light is coupled to one of the main axis (slow or fast) of a PM fiber the linear
polarization will be maintained in the fiber. Light with an elliptical polarization or linear polarized
light which is not parallel to one of the main axis will not preserve its polarization state along the
fiber. The output polarization state is not predictable. It depends on the current conditions like
bends and stress of the fiber.
There are several ways to set up an ER measurement. The light source is one of the key ele-
ments. Using a broadband source like an ASE or SLED will simplify the alignment procedure
compared to a narrow band source like a DFB laser.
Narrow bandwidth light source (DFB laser)
It is not sufficient to measure a single ER value to characterize the quality of an PM fiber align-
ment. The output polarization can be linear with a misaligned linear polarization at the input or
even with a elliptical polarization entered into the input. There are many different output polariz-
ations possible. The worst case has to be found to characterize this fiber.
An easy way to generate the worst case is to stress the fiber by pulling it gently. Stress is in-
duced and the output polarization will be changed. The ERM100 stores the worst case / min-
imum ER value and displays this data in the ER
min
screen.
ASE Source
Instead of a laser diode a SLED (super-luminescent diode) or an ASE is used as light source. A
SLED has a 3dB bandwidth of 10nm to >60nm and an ASE source typically more than 100nm.
As larger the bandwidth as better suitable is a broadband source for this ER measurement on
PMF method.
In general the polarization of a broadband source is not known. It is necessary to generate a
linear polarization for the complete spectrum. A fiber bench with a polarizer is convenient to
perform this task. The polarizer can be mounted with its transmission axis vertical orientated to