Hello shanmuga srinivasan,
well i think it has something to do with the logical error you are making. It may be because of the amplitude of the voltages you are applying. The relation between the voltages applied has to be V1 = -V2 . I would suggest you to look at the basic set up of using a mach-zehnder modulator in the system design. Please refer to the link. May be it could help you.
Regards