Low timing-jitter, “σ”-type passively mode-locked fiber laser is used as a high-performance optical sampling pulse source, of which the operation stability is high and the repetition frequency is tunable. Then the generated sequence of ultrashort optical sampling pulses is launched together with a 80 Gb/s optical pulse signal under test into a 100 m-long highly nonlinear fiber (HNLF). The all-optical sampling of an optical signal under test is realized by utilizing the four-wave mixing (FWM) in a HNLF. Consequently, the waveform of a 80 Gb/s optical pulse signal with RZ format is precisely reproduced by a computer and is displayed on its screen by employing our developed software for digital signal processing and computer image processing, respectively. Moreover, the optical sampling oscilloscope prototype is used to measure the pulse waveform and pulse width at the output of a commercially available, actively mode-locked semiconductor laser which produces optical pulses of 1.8 ps width (measured by an autocorrelator) at the 10 GHz repetition frequency. In the experiment, the pulse width measured by this oscilloscope prototype system is 2.0 ps. It is clearly shown that our developed optical sampling oscilloscope prototype has a temporal resolution better than 900 fs.