An optical signal generator includes a splitter, a first modulator, a second modulator, a signal-switching unit and a multiplexer. The splitter splits an optical pulse train into a first pulse train and a second pulse train. The first modulator receives the first pulse train and first data signal, and generates a first modulation signal by performing on-off-keying or phase modulation on the first pulse train based on the strength of the first data signal. The second modulator receives the second pulse train and second data signal, and generates a second modulation signal by performing the on-off-keying or phase modulation on the second pulse train based on the strength of the second data signal. The signal-switching unit delays pulses of the second modulation signal or adjusts a phase of a carrier included in the second modulation signal according to a switching signal. The multiplexer generates an optical modulation signal by multiplexing the first modulation signal and the second modulation signal.