Extinction ratio, when used to describe the performance of an optical transmitter used in digital communications, is simply the ratio of the energy (power) used to transmit a logic level '1', to the energy used to transmit a logic level '0'. The extinction ratio may be expressed as a fraction, in dB, or as a percentage. ♦ What is the Extinction Ratio (ER)? Extinction Ratio (ER) is the ratio of the optical power when the. Extinction ratio is an important measurement for characterizing the performance of optical transmitters.