Hi Leebu Babu.
By default, when you use the PRBS as a carrier, it is OOK modulation only.
RZ and NRZ are the formats of the signal you employ while converting the binary PRBS to electrical form before giving as one of the inputs to the MZM. The other input would ofcourse come from the CW LASER source.
So, like most of the people mentioned above, you need to use RZ pulse generator to convert PRBS to RZ electrical format and hence the resultant modulated signal after MZM would be RZ-OOK. Hope I am clear.
Regards.