Hi Urvi,
Go through the links which may help you to get more idea about dqpsk-wdm system and which help you in your research works.
http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5588472
http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=5758591&newsearch=true&queryText=dqpsk-wdm
http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=1283616&newsearch=true&queryText=dqpsk-wdm
http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=4800951&newsearch=true&queryText=dqpsk-wdm
Thanks and regards
Ranjeet