Extinction ratio as a power measurement in polarization maintaining fiber
Light traveling down a standard singlemode fiber can randomly rotate on a three dimensional axis. The output of such signals has been an increasingly undesirable by-product when a system or application is dependent on minimal polarization losses as in sensors, interferometers, or high speed switching networks.
The introduction of polarizing and polarization maintaining fiber and components has effectively addressed this concern. Extinction ratio is a performance standard measurement of polarization maintaining fiber.
When a fiber core is stressed, it causes the light to be linearly polarized, on a slow and fast axis, as opposed to "tumbling" and rotating down the fiber randomly. This linearly polarized light can be observed by measuring the output light of a polarized patchcord as it passes through a polarizor.
A polarizor acts like a slotted plate, only light that aligns with the slots is able to pass through and be measured. Since polarized light will only pass through a polarizor plate when their axis are aligned, each 180 degree rotation of the plate would result in the light passing through, then becoming extinguished, then passing through and so on. The formula for this measurement is ER = 10 log (Pmax/Pmin) where Pmax and Pmin are the maximum and minimum signal intensities as the light is passing through the polarizor and being measured at the output.
This extinction ratio measurement of light entering and exiting a fiber indicates how well the fiber maintains polarization.
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