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In this letter, we propose and experimentally demonstrate an enhanced radio-frequency (RF) spectrum analysis technique to simultaneously monitor optical signal-to-noise ratio (OSNR) and polarization-mode dispersion (PMD) for return-to-zero ON- OFF keying (RZ-OOK) systems. In this method, by cascading a large differential group delay element at the monitoring module, the PMD and OSNR parameters can be derived from the analysis of the position shift and the minimum power of the RF spectral dip, respectively. Experimental results show that this scheme can monitor PMD from 0 to 90 ps with less than 2-ps error and OSNR from 16 to 35 dB with less than 1-dB error in a 10-Gb/s RZ-OOK transmission system with a pulsewidth of 2.5 ps. The scheme possesses the advantages of simple implementation, large monitoring range, and high PMD monitoring sensitivity by using the RF spectrum analysis only at low-frequency range.