In telecommunication, 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."
Note: The extinction ratio may be expressed as a fraction or in dB.
Source: from Federal Standard 1037C and from MIL-STD-2196