HELLO SANJEEV
What i understand from ur query is that you want to develop a code for a modulator and use it in optisystem..is it??
Well if so for that you can refer to the following link where an QPSK modulator is developed in MATLAB…Although we are not allowed here for posting links from other sites but your convenience i am posting one…I hope u will get some idea about writing a code for a modulator..
http://in.mathworks.com/help/comm/examples/qpsk-transmitter-and-receiver-1.html
Thanks and regards