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We propose and experimentally demonstrate a chromatic-dispersion (CD) monitoring method based on radio-frequency (RF) clock power ratio measurement in 38-Gb/s nonreturn-to-zero differential quadrature phase-shift keying (NRZ-DQPSK) and 57-Gb/s NRZ differential 8-level phase-shift keying (D8PSK) systems. A narrowband fiber Bragg grating (FBG) notch filter is centered at the optical carrier wavelength, such that the RF clock power decreases with CD. On the other hand, RF clock power increases with CD in the absence of the notch filter. By using the RF power ratio of the filtered and nonfiltered signals, polarization-mode-dispersion (PMD) effects on RF power ratio can be eliminated and CD measurement sensitivity can be increased. The CD monitoring range is 0 ~ 250 ps/nm in the systems with a symbol rate of 19-Gsym/s. Moreover, the proposed CD-monitoring method is not affected by received optical power.