Delay-interferometer based timing alignment monitoring for data and clock signal in optical RZ-DPSK and RZ-OOK systems

A simple scheme for monitoring the alignment status between the pulse carver and the data modulator in both return-to-zero differential phase-shift keying (RZ-DPSK) and return-to-zero ON–OFF keying (RZ-OOK) systems is proposed and experimentally demonstrated. Not relying on any high-speed components, the scheme needs only an athermal delay-interferometer with properly designed delay and one or two optical power meters. For 10-Gb/s RZ-DPSK signals with a duty cycle of 50%, a large monitoring power dynamic range (MPDR) of ∼13 dB was observed using differential monitoring. For 10-Gb/s RZ-OOK signals, the MPDR of ∼2 dB was also observed. The feasibility of the proposed scheme for RZ-DPSK and RZ-OOK signals with a duty cycle of 33% was also investigated. The proposed scheme is also applicable to the single-modulator transmitter configuration, in which the data signal and the clock are mixed in the electrical domain.