A signal-transmitting system includes a digital-to-analog converter, an optical modulator, first and second electrodes, an optical phase shifter, and an optical coupler. The digital-to-analog converter converts digital data into an electrical analog signal. The optical modulator includes a first optical waveguide configured to transmit a first optical carrier, a second optical waveguide configured to transmit a second optical carrier, a first electrode positioned on the first optical waveguide, and a second electrode positioned on the second optical waveguide. The first and second electrical couplers are configured to couple respective electrical analog signals and electrical carriers to electrodes to generate modulation waves. The modulation waves are different in phase. The optical phase shifter is configured to shift the second modulation wave by a predetermined phase, and the optical coupler is configured to couple the first and second modulation waves to generate an optical output signal.