The performance of 1 and 2 Mach--Zehnder modulator (MZM)-based simple carrier suppressed return to zero differential quadrature phase shift keying (CSRZ-QDPSK) transmitters is analyzed in a 20 Gb/s single-mode fiber optic link. The link's performance under the transmitter schemes is evaluated by analyzing the received constellation, eye diagram, quality factor, and eye opening penalty (EOP) in a passive optical networks (PON) scenario. It is observed that the simple transmitter provides performance closer to the case of the standard 3 MZM CSRZ-QDPSK-based transmitter reported in the literature and implemented in experiments.