Optical modulator and optical transmitter

An optical transmitter comprising:

• a light emitting element that emits light;
• a data generating circuit that generates modulation data;
• a driver that generates an electrical signal corresponding to the modulation data output from the data generating circuit;
• and
• an optical modulator that based on the electrical signal output from the driver, modulates the light emitted from the light emitting element;
• wherein
• the optical modulator includes:
• a first substrate that includes:
• a Mach-Zehnder optical waveguide that is formed on the first substrate having an electro-optical effect;
• a signal electrode and a ground electrode that are disposed along a pair of branching waveguides positioned between an optical branching unit and an optical coupling unit of the Mach-Zehnder optical waveguide;
• plural optical modulating units that are disposed in parallel, and respectively modulate a light beam propagating in the Mach-Zehnder optical waveguide by applying to the signal electrode that is to function as a travelling wave electrode, an electrical signal that corresponds to modulation data;
• and
• an output optical coupling unit that couples modulated light beams output from the optical modulating units;
• a second substrate that is disposed separately from the first substrate and has plural first signal line paths corresponding to the optical modulating units;
• a package that accommodates therein the first substrate and the second substrate;
• and
• a flexible circuit board that is disposed outside the package and has plural second signal line paths corresponding to the optical modulating units, wherein
• input ends of input electrodes of the optical modulating units on the first substrate are each supplied the electrical signal and are disposed side by side on one side face of the first substrate,
• when base points are independently set such that each optical path length from the output optical coupling unit in the branching waveguide disposed along the signal electrode of the branching waveguides of the optical modulating units to the base points is equivalent, each electrical length from an input end of each signal electrode to each respective base point differs,
• input ends of the first signal line paths on the second substrate are respectively supplied the electrical signals respectively corresponding to the optical modulating units and are disposed side by side on one side face of the second substrate, and output ends electrically connected to input ends of the signal electrodes of the corresponding optical modulating units on the first substrate are disposed side by side on another side face of the second substrate,
• input ends of the second signal line paths on the flexible circuit board are respectively supplied the electrical signals respectively corresponding to the optical modulating units and are disposed side by side on one side face of the flexible circuit board, and output ends electrically connected to input ends of the corresponding first signal line paths on the second substrate are disposed side by side on another side face of the flexible circuit board,
• electrical lengths from the input ends of the second signal line paths to output ends thereof respectively differ, and
• electrical lengths of signal paths respectively corresponding to the optical modulating units are equivalent, the signal paths include signal paths that are from the input ends of the second signal line paths and pass through input ends of the first signal line paths connected to the output ends of the second signal line paths, to the base points on the signal electrodes, and signal paths that are from the input ends of the first signal line paths and pass through the input ends of the signal electrodes connected to output ends of the first signal line paths, to the base points on the signal electrodes.