A silicon-based optical modulator. An optical modulator of a Mach-Zehnder type includes an optical coupler configured to separate a single optical signal into two optical signals having same output power, two phase shifters having a PN junction through which the two optical signals separated through the optical coupler pass respectively, a plurality of electrodes configured to apply an electrical signal to the two phase shifters, and two PN diodes disposed between the two phase shifters and configured to adjust an operating bandwidth of the optical modulator. The optical signals respectively passing through the two phase shifters have phases that change as a width of a depletion region changes based on a magnitude of a reverse voltage provided to the two phase shifters through the electrodes and a refractive index changes.