The alternate-phase return-to-zero modulation format in high-speed optical-fibre communications

Licentiate thesis, 2006

Increasing the channel bit rate is an efficient way to upgrade traffic capacity in installed transmission links. At bit rates of the order of 40 Gb/s, on-off keying transmission becomes severely impaired by non-linear effects,particularly Intra-channel Cross-Phase Modulation (IXPM) and Intra-channel Four-Wave Mixing (IFWM), which cause pulses in different bit slots to interact with each-other. IFWM is a phase sensitive process and can therefore be suppressed by proper phase modulation of the optical signal. This thesis introduces a new modulation format, called Alternate-Phase Return-to-Zero (APRZ), in which the phase of the optical signal alternates between neighbouring pulses. A variation of APRZ-Pairwise APRZ (PAPRZ) - is also studied. Theoretical analysis shows that a proper choice of the phase-modulation amplitude can make IFWM contributions coming from different pulse combinations to interfere destructively, therefore reducing the impact of IFWM. The theoretical predictions are verified through numerical simulations and laboratory experiments. It is shown that APRZ and PAPRZ can achieve a significant improvement in non-linear tolerance, compared to the standard return-to-zero (RZ) transmission format.