Hi Ibn Aziz,
As it was correctly noticed by Jafar, your modulated power spectral signal has DC (constant component) in it. Because of this reason, your modulated signal never comes to zero. Not sure what is the final goal of your project. If you are designing transmitter and want to send this signal through the channel, and your signal-to-noise ratio is within the requirements (i.e. your DC component is at the channel noise level), you can correct it at the receiver which usually has photodetector, amplifier and the correction circuit.