Extinction ratio is a measure of the amplitude of the digital modulation on the optical carrier and so affects the power penalty, or distance over which a fiber-optic system can reliably transmit and receive a signal.

The ratio of the low, or OFF optical power level (P) to the high, or ON optical power level (P)

The ratio of the low, or OFF optical power level (PL) to the high, or ON optical power level (PH)

In telecommunications, extinction ratio ( r e ) is the ratio of two optical power levels, of a digital signal generated by an optical source, e.g., a laser diode, where P 1 is the optical power level generated when the light source is "on," and P 2 is the power level generated when the light source is "off."