Polarisation Extinction Ratio (PER) is an important parameter that quantifies the ability of an optical fibre component to maintain an incident polarisation state. The PER is expressed in decibels (dB) and represents the ratio of the optical power of the desired polarization state to the optical power of the undesired polarization state. It is an essential parameter in optical communication systems, particularly when dealing with polarization-sensitive components and signals. It is mathematically calculated as:

Here P_desired is the optical power in the desired polarization state and P_undesired is the optical power in the undesired polarization state.

In optical communication systems, light can have different polarization states, which describe the orientation of the electric field vector of the light wave. Maintaining the correct polarization state is crucial in polarization-sensitive components and systems.

If plane polarised light is launched into one axis of a birefringent fibre there will inevitably be a small amount of cross-coupling of power between the two polarisation modes and some light will emerge in the other axis. The polarization extinction ratio quantifies the polarisation cross-talk. It is, however, a ratio of two power measurements and ignores the relative phase of the polarisation components and the degree of polarization.

The measured value is highly dependent on:

• the alignment of the launched polarisation state with the polarisation axis of the device under test
• the length of the fibre