The device is structured to have a first electric modulation signal generator, a second electric modulation signal generator, a two-mode beat light source and an optical intensity modulator. The first electric modulation signal generator generates and outputs a first electric modulation signal. The second electric modulation signal generator generates and outputs a second electric modulation signal of a same frequency as the first electric modulation signal and to which a phase difference of δ radians is provided (δ is a real number satisfying 0≦δ≦π). The two-mode beat light source is driven by the first electric modulation signal, and generates and outputs two-mode beat light. The two-mode beat light is inputted to the optical intensity modulator, and the optical intensity modulator generates and outputs a CS optical pulse train. Light transmittance of the optical intensity modulator is modulated by the second electric modulation signal.