To obtain eye measurement data as a means for displaying signal degradation by inputting a sampling light pulse to an interferometer, detecting an optical pulse modulated and sampled according to the state of interference of the interferometer, and obtaining an electric data signal.

A sampled light pulse 8 generated by a pulse generator 7 is inputted to the arms 3 and 4 of an interferometer 2 and recombined at an output wave-guide path 9. At this time, an input wave-guide path 12 connected to the arm 3 guides a light signal 13 in a direction opposite to the propagation of the sampling light pulse 8 via a semiconductor light amplifier 5. Here, when there is not any signal pulse of the light signal 13, the output wave-guide path 9 generates null (zero) output. When a signal pulse is present, cross phase modulation occurs in the active medium of the semiconductor optical amplifier 5 to cause a phase shift to occur in the component of a propagating control light pulse and to change the state of interference in the interferometer 2. By this, a sampled light pulse 10 with amplitudes determined by the amplitudes of the light signal 13 is generated. A detector 14 converts it into electric signal data sample 15, and an output device 18 outputs eye measurement data.