EOMs Operating Manual
Ref. Ed. 0 – 08/02/2021 24
The PER is the optical power ratio between the two main polarization axis (slow/fast) measured at
the PM output of the modulator. We measure the PER of the modulator by injecting a linearly
polarized light on one axis only and then measuring the optical power on each axis P
Fast
and P
Slow.
Fig. 37
For this set up, a broadband source like an ASE or SLED coupled with a polarizer is used. The use of
such a light source ensures a stable PER measurement:
Fig. 38
→ PER is read directly on the PER meter (rotating polarizer).
A narrow linewidth laser such as a DBF can also be used to measure PER. However, with such a
highly coherent source, if a fraction of the polarization travels along the wrong axis, any stress on
the fiber (ex: temperature change) will directly affect the phase relation between the slow and fast
axes in the PM fiber and therefore the PER, in a time dependent manner. In order to evaluate the
worst situation, you need to apply some stress on the output fiber and record the lowest PER:
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