hi zulkarnain,
good replies by all, but i would like to add up a bit,
extinction ratio is the ratio of power at the on to off mode of the modulator or laser(DMLs). it should be high.BUT>>>>we have to look for the other things also
only high ratio does not imply that system response is good;
e.g Pon=5mW, and Poff=1mW then ext. ratio=Pon/Poff=5;
Pon=1mW, and Poff=0.2mW then ext. ratio=Pon/Poff=5;
which of two is better?????
ofcourse the second one, because poweer at off stage should ideal be zero…because lower power at off stage makes the laser power efficient, brings thermal stability(because laser or modulator cools a bit in this mode, if power is less).
thus high extinction ratio along with lower power value at the off mode of laser or modulator is better than only the system having the better extinction ratio.
with regards