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This very useful optical sampling system, uses a passively mode-locked fiber laser as an optical sampling pulse source and is based on sum-frequency generation. The optical pulse had a sufficiently short pulse width, and its peak power was very high. In addition, it had a very low timing jitter. We could observe optical signals that were jitter-free in terms of single scanning. The sum-frequency generation conversion efficiency was 1.0 X 10^<-4>W^<-1>, and the temporal resolution was 700 fs, when we used a 5-mm-thick KTP crystal. A 320-Gbit/s optical signal could be clearly observed. We have also developed a polarization-insensitive optical sampling system with a two-path configuration based on sum-frequency generation using the type-II phase matching condition in a KTP crystal. The polarization dependency was less than 3.5% (0.15dB) in the wavelength range from 1520 to 1620 nm.