A first optical splitter splits a light source to obtain a first optical signal, a second optical signal, and a third optical signal. A first MZ modulator modulates the first optical signal to output a fourth optical signal. A second MZ modulator modulates the second optical signal to output a fifth optical signal. A first optical coupler couples the fourth optical signal and the fifth optical signal to output a sixth optical signal and a seventh optical signal. A power regulator and a phase shifter respectively perform power adjustment and phase shifting on the third optical signal to output an eighth optical signal. A second optical splitter splits the eighth optical signal into a ninth optical signal and a tenth optical signal. A second optical coupler combines the sixth optical signal and the ninth optical signal. A third optical coupler combines the seventh optical signal and the tenth optical signal.