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A novel technique for monitoring the timing alignment between the pulse carver and the data modulator in return-to-zero differential phase-shift keying systems is proposed and experimentally demonstrated. A simple off-center optical filter is employed to extract optical power for alignment monitoring. Compared with the previous degree of polarization-based monitoring scheme, in which the monitoring power dynamic range (MPDR) is ∼0.2 dB, a larger MPDR of 3.35 dB is achieved within the same timing alignment range of half-bit period, thus achieving a higher monitoring sensitivity. In addition, this simple alignment monitoring scheme features high-speed operation, polarization independence, and easy integration with transmitter for synchronization feedback control.