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Summary form only given. Mode-locked lasers exhibit timing and amplitude jitter that can limit the performance of photonic analog-to-digital converters and ultrafast optical communication systems. In addition to random jitter, harmonically modelocked lasers also generate correlated pattern noise that is periodic at the fundamental cavity frequency. Methods used to measure modelocked laser noise include spectral analysis of the detected laser output and optical cross-correlation. Special techniques are required to separate amplitude and phase noise. We report on the use of phase-encoded optical sampling to exclusively measure the timing jitter of a harmonically mode-locked fiber laser. Our technique provides an upper bound of the laser's white-noise timing jitter (55 fs) and allows pattern phase-noise to be observed. The all-polarization-maintaining modelocked Er-fiber ring laser being characterized serves as the timing source in the phase-encoded optical sampling system.