Editing Extinction ratio (section)

{{Short description|Concept of this parameter}}

[[File:On-off keying eye diagram.svg|thumb|300px|right|[[Eye diagram]] showing an example of two power levels in an [[On-off keying|OOK]] modulation scheme, which can be used to calculate extinction ratio. ''P''<sub>1</sub> and ''P''<sub>0</sub> are represented by (binary 1) and (binary 0) respectively.]]

In [[telecommunications]], '''extinction ratio''' (''r''<sub>e</sub>) is the [[ratio]] of two optical [[Power (physics)|power]] levels of a [[Digital signal (electronics)|digital signal]] generated by an optical source, e.g., a [[laser diode]]. The extinction ratio may be expressed as a fraction, in [[decibel|dB]], or as a [[percentage]].<ref>{{cite web|url=http://www.timbercon.com/Extinction-Ratio.html|title=Extinction Ratio|publisher=Timbercon|access-date=May 14, 2009|archive-date=November 21, 2019|archive-url=https://web.archive.org/web/20191121000919/http://www.timbercon.com/Extinction-Ratio.html|url-status=dead}}</ref> It may be given by
:<math>r_e = \frac{P_1}{P_0},</math>
where ''P''<sub>1</sub> is the optical power level generated when the light source is on, and ''P''<sub>0</sub> is the power level generated when the light source is off.

The '''polarization extinction ratio''' ('''PER''') is the ratio of optical powers of perpendicular polarizations, usually called TE (transverse electric) and TM (transverse magnetic). In telecommunications, the PER is used to characterize the degree of polarization in a polarization-maintaining device or fiber. For coherent transmitter and receiver, the PER is a key parameter, since X&nbsp;polarization and Y&nbsp;polarization are coded with different signals.