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Polarization Extinction Ratio, PER

Cross coupling in regards to a birefringent fiber, quantified by extinction ratio, indicates the amount of light which is able to mix between the two polarization axes. Polarization extinction-ratio, PER,  is important because it is a measure of the polarization-maintaining performance of an optical fiber.

In most applications for PM fiber, only one of the two polarization orientations (states) is used - this is sometimes called the ‘wanted’ polarization-state. The extinction ratio simply compares the optical power held on the wanted axis to that which is on the unwanted axis, the orthogonal polarization state, expressed in decibels (dB).

To test the best possible PER of the fiber, light is launched onto one axis from a polarized source, the PER is a ratio of the light which remains on the wanted axis to that which has managed to couple onto the unwanted axis. For example, if you are using only the x-polarization (the ‘wanted’ polarization) then you would measure the optical power in both ‘x’ and ‘y’ polarizations and perform the following calculation.

 
 
 
 
 
So you can see, an Extinction-Ratio of -30dB is 1000:1 and -20dB is 100:1

The value of extinction ratio is dependant upon the length of fiber and the environmental conditions.  If you subject the fiber to high mechanical stress and small-diameter bends, then it is possible to disrupt the internal stress (which generates the birefringence) and this reduces the extinction-ratio significantly.

Typical PER under various deployment conditions are indicated below.

Length

Conditions

Extinction Ratio

1000m

Loose-tube cabled, wound on a 300mm cable reel

-25 to -30dB

500m

Wound at low tension on a 150mm spool

-30 to -35dB

100m

Wound at low tension (<5g) on a 60mm spool

-25 to -30dB

10m

Laid straight

-35dB*

*limit of measurement resolution; ask about our test set-up

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