Each link consists of PRBS generator, transmitter, optical filter, attenuator, receiver, and BER tester. Transmitters represented as compound components blocks, i.e. combination of blocks. For example, NRZ transmitter combines CW Laser source, Electrical signal generator (NRZ driver), external Mach-Zehnder modulator, and attenuator. The following parameters of transmitter can be set: power, wavelength, extinction ration, rise/fall time, RIN, etc. In case of RZ transmitter the electrical signal generator generate RZ signal with raised cosine shape and given duty cycle. In CRZ transmitter we add a chirp to RZ optical by applying a phase modulation. And finally in case of CSRZ transmitter the RZ optical signal after Mach-Zehnder modulator goes through phase modulator driven by analog sine wave generator at frequency equal to half of the bit rate. That will introduce a pi phase shift between any two adjacent bits and the spectrum will be modified such that the central peak at the carrier frequency is suppressed.
Figure below shows transmitter optical spectrum for different modulation formats. One can observe the central peak suppression in case of CSRZ.