The ratio of the power of a plane-polarized beam that is transmitted through a polarizer placed in its path with its polarizing axis parallel to the beam's plane, as compared with the transmitted power when the polarizer's axis is perpendicular to the beam's plane.

The extinction ratio is used to describe the Efficiency with which the transmitted Optical Power is modulated over the fiber-optic transport. It’s simply the relationship of the power used in transmitting a logic level “1” (P1) to the power used in transmitting a logic level “0” (P0). Extinction ratio can be defined as a linear ratio, P1/P0; as a power measurement, 10 * log(P1/P0); or as a percentage, (P0/P1) * 100. The convention adopted for logic levels is that Transmission of Light (laser on) represents a logic “1” and no light transmission (laser off) represents a logic “0.”