hi zukarnain,
Optical transmitters used in high-speed digital communication systems are typically required to maintain a specific set of performance
levels. One parameter, extinction ratio, is used to describe optimal biasing conditions and how efficiently available laser transmitter power is converted to modulation power.Please visit the links for further:
http://cp.literature.agilent.com/litweb/pdf/5966-4316E.pdf
I hope it helps