Patent ID: 7936996

Claim:
An optical modulator comprising: a first sub Mach-Zehnder waveguide (MZ A ) ( 2 ); a second sub Mach-Zehnder waveguide (MZ B ) ( 3 ); a main Mach-Zehnder waveguide (MZ C ) ( 8 ) including: an input part ( 4 ) of an optical signal, a diverging part ( 5 ), the diverging part ( 5 ) being configured to be connected to the input part, wherein the optical light is diverged at the diverging part into the first sub Mach-Zehnder waveguide (MZ A ) and the second sub Mach-Zehnder waveguide (MZ B ), the first sub Mach-Zehnder waveguide (MZ A ), the second sub Mach-Zehnder waveguide (MZ B ), a combining part ( 6 ) combining the optical signal outputted from the first sub Mach-Zehnder waveguide (MZ A ) and the second sub Mach-Zehnder waveguide (MZ B ), the combining part being configured to being connected to the first sub Mach-Zehnder waveguide (MZ A ) and the second sub Mach-Zehnder waveguide (MZ B ), an output part ( 7 ) outputting the optical signal combined in the combining part, the output part being configured to connected to the combining part; a first electrode (electrode A) ( 9 ) of the first sub Mach-Zehnder waveguide (MZ A ); a second electrode (electrode B) ( 10 ) of the second sub Mach-Zehnder waveguide (MZ B ); a main Mach-Zehnder electrode (electrode C) ( 11 ) of the main Mach-Zehnder waveguide (MZ C ); a signal source ( 12 ) for applying either one or both of a modulation signal and a bias voltage to the first electrode ( 9 ), the second electrode ( 10 ) and the main Mach-Zehnder electrode ( 11 ); a first bias adjusting means for adjusting bias voltage, the bias voltage being applied to the electrode A and the electrode B, the bias voltage being adjusted so as to maximize an intensity of the optical signal outputted from the main Mach-Zehnder waveguide; and a second bias adjusting means for adjusting bias voltage, the bias voltage being applied to the electrode C, the bias voltage being adjusted so that, when the intensity of the optical signal is represented by Max, an intensity of the optical signal is in between 40% to 60%, both inclusive, of the Max, while the bias voltage applied to the electrode A and the electrode B is kept to be the same level as the one obtained in the first bias adjusting means.