Benyahia,
In the paper they do not describe exactly all the parameters they have chosen to get that exact waveform, it might be difficult to reproduce it exactly with that in mind. The first thing I tried was removing the noise from the two Electric Amplifiers and that seemed to already make a big difference. Therefore the noise from the amplifiers is probably too large and either the initial signal power should be higher or a different amplifier with lower noise should be used. Also changing the switching voltage of the MZMs will have a large impact on the output! Try playing with those to increase the amount of generated harmonics.
Damian