According to an aspect of an embodiment, an optical device comprising: a first modulator for independently modulating first light having a first predetermined polarization mode; a second modulator for independently modulating second light having a second predetermined polarization mode; and a polarization beam coupler having a first port, a second port, a third port, and a fourth port; the polarization beam coupler for inputting the first light from the first modulator via the first port, inputting the second light from the second modulator via the second port, outputting the first light via the third port and inputting reflected and polarization converted light on the first light by a wave plate and a mirror, and outputting the first light having the converted polarization mode and the second light having the predetermined polarization mode via the fourth port.