Polarization Extinction Ratio (PER) is a measure of the quality of polarization in optical systems and is defined as the ratio of the power in the strongest polarized component of the light to the weakest polarized component. In other words, it represents the difference between the highest and lowest intensity of light in two orthogonal polarization states.

A laser beam is a transverse beam with the electric and magnetic fields oscillating perpendicular to the direction of propagation. Most of the laser beams are polarized, i.e. the direction of the electric field vibration is well defined.

So, in lasers, the polarization extinction ratio is the ratio of the optical powers of the transverse electric and transverse magnetic fields in the beam. It is given by,

Here P₂ and P₁ are the optical powers corresponding to the transverse electric and transverse magnetic fields in a laser beam.

The PER is expressed either in decibels (dB), ratios, or percentages. The value of PER of a laser beam is usually 100:1.

PER is important in many optical systems, including fiber-optic communication systems, where it is critical to maintain the polarization of the light to avoid signal loss and degradation. A high PER ensures that the light remains polarized and that the signal remains strong, while a low PER may result in signal loss or crosstalk between the different polarization states. According to device, typical values range from 18 to 20 dB in many passive components to 50 to 60 dB or even more in some polarizers or polarising waveguides.

PER can be measured using various techniques, including Jones matrix analysis, Mueller matrix analysis, and Stokes vector analysis. The Jones matrix method is the simplest and most common method for measuring PER, and involves analyzing the intensity of light in two orthogonal polarization states.

There are several factors that can affect the PER of an optical system, including manufacturing and design tolerances, environmental factors, and the nature of the optical components themselves. To maintain a high PER, optical systems are often designed with polarization-maintaining components, such as polarization-maintaining fibers and polarizers.