A highly sensitive optical sampling system has been produced by using sum-frequency generation in a periodically poled lithium niobate crystal. When the relations between crystal length and wavelength bandwidth and SFG conversion efficiency were investigated theoretically and experimentally, a system with a 1.4-mm-long periodically poled lithium niobate crystal was found to have a 22.5-nm wavelength bandwidth and a SFG conversion efficiency ten times that of a similar system with a 3-mm-long KTP crystal. The SNR of the system with the 1.4-mm-long PPLN crystal was about 7 dB higher than that of the system with a 3-mm-long KTP crystal, and a temporal resolution better than 1 ps was obtained by using compressed optical sampling pulses. The eye diagram of a 10-Gbit/s RZ optical signal with a 1-mW peak power could be observed, and the eye diagram of a 160-Gbit/s RZ optical signal could be observed clearly.

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 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% (0.15 dB) in the wavelength range from 1520 to 1620 nm.

The timing jitter of the optical pulse from a gain-switched laser diode is reduced by CW light injection. The reduction ratio of the timing jitter is 5. 5. The pulse width was compressed by a nonlinear optical loop mirror to a pedestal-free optical pulse with a pulse width of 420 fs.