TIA's Glossary of Telecommunication Terms
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extinction ratio ( r e )
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. [2196].