A phase modulator (22) modulates, based on an electrical signal (Sa), the phase of one of optical signals of an input optical signal branched by a light branching circuit (21). Then, a phase modulator (23) performs phase modulation in a smaller amount and opposite polarity as compared to the phase modulator (22) based on an electrical signal (Sc) obtained by delaying the inverse logic signal (Sb) of the electrical signal (Sa) by a predetermined delay time τ shorter than the transient response time of the phase modulator (22) or the rise and fall times of the electrical signal (Sa). A light multiplexing circuit (24) multiplexes the optical signal obtained by the phase modulator (23) with the other optical signal of the input optical signal, thereby outputting a pulse-like output optical signal (36).
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