Alternate-phase return-to-zero transmitter based on integrated dual-parallel Mach-Zehnder modulator

Introduction: In high-speed systems, the major nonlinear transmission penalties generally come from intrachannel interactions such as intrachannel cross-phase modulation (IXPM) and intrachannel four wave mixing (IFWM) [1–3]. IXPM can be effectively suppressed by optimum dispersion management [4], but IFWM remains a limiting factor in long-haul high-bit-rate systems where short pulses may significantly overlap through transmission. It has been reported that p/2 alternate-phase return-to-zero (APRZ) can effectively reduce the IFWM effect owing to the phase shift between its neighbouring bits. To date the most cost-effective scheme to realise APRZ transmitters is reported in [5–8] where a dual-drive Mach-Zehnder modulator (DDMZM) is used to produce phase alternate pulses, followed by a second intensity modulator for data modulation, resulting in an APRZ signal. In this Letter we propose a simple 20 Gbit/s APRZ transmitter by using one 10 Gbit/s dual-parallel Mach-Zehnder modulator (DPMZM). With RZ coding, two 10 Gbit/s data streams are combined in the integrated modulator with stable and adjustable phase relation. This method eliminates an additional alternate-phase pulse generator, thus effectively reducing the system complexity. We experimentally show the transmitter’s feasibility, and perform simulations to prove its enhanced nonlinear performance.