A method and device for sampling ultra-fast optical signals by generating a sampling signal comprising a train of short pulses and coupling the sampling signal together with an optical data signal to be sampled into a highly non-linear optical fiber. Four-wave mixing (FWM) occurs between the two signals resulting in a secondary signal output from the fiber consisting of pulsed components having energy proportional to the instantaneous power of points along the data signal. An analysis of this secondary signal can be carried out to create a normal trace or eye-diagram of the data signal.