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Summary form only given. We have successfully observed the eye diagram of a 320-Gb/s optical signal with a time resolution of 700 fs and timing jitter of 80 fs. The sampling pulse was generated stably from a passively mode-locked fiber laser (MLFL). Because the MLFL could directly generate a 480-fs sampling pulse, we did not have to use any pulse-compression techniques. Furthermore, the MLFL had a two-stage delay line for changing the repetition rate of the sampling pulse. This system can be used to observe and evaluate the quality of high-bit-rate optical signals.