An optical modulator includes a light input/output unit receiving an incident optical signal which has not been modulated, splitting the incident optical signal into a first optical signal and a second optical signal, and transmitting the first and second optical signals to a first path and a second path, respectively, of an optical waveguide. A phase shifter is positioned in at least one of the first and second paths and modulates a phase of at least one of the first and second optical signals, which have been received through the first and second paths, respectively, in response to an electrical signal. A phase-modulated signal is output. A reflective grating coupler reflects signals respectively received through the first and second paths back along the first and second paths respectively.