A signal light pulse to be transformed into a two-dimensional space signal and a reference extra-short light pulse are directed to a dispersion device (2). One-dimensional frequency light distributions produced by a one-dimensional Fourier transformation lens (5) are made to enter a nonlinear crystal (6) to form a 2nd-harmonic. The light wave distribution produced by subjecting the 2nd-harmonic to time-space transformation through the one-dimensional Fourier transformation lens (9) is filtered by a time-frequency filter (11) provided on the filter surface (102) of a one-dimensional space frequency filtering optical system and transformed into a two-dimensional space signal (13) corresponding to the two-dimensional light distribution which is time-frequency-developed and which represents the relation between the time and frequency of the signal light pulse. Thus, a specific technique of realizing extra-high speed transformation of a signal form from a time signal into a two-dimensional space signal without necessity of active scanning and displaying the 2-dimensional signal in a visible region in a temporally steady state is provided.
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