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The authors demonstrate in-service channel monitoring of a chromatic dispersion (CD) and polarization-mode dispersion (PMD) for a return-to-zero differential phase-shift keying (RZ-DPSK) signals based on the asynchronous amplitude-histogram evaluation (AAHE) technique. The waveform evolution of the RZ-DPSK signal under the effects of the CD and PMD is investigated analytically and numerically. The CD leads to a power level splitting at the valley of the waveform before demodulation. The intensities of these two levels further increase due to the PMD. By tracking the intensities of these levels using the AAHE method, the CD and PMD can be measured. The principle of a simultaneous monitoring of the CD and PMD is demonstrated experimentally for 10-Gb/s RZ-DPSK signals. The effects of the signal's optical signal-to-noise ratio and the number of the sampling points on the accuracy of the monitoring results are also presented.