Patent ID: 9217908
Filing Date: 2015-12-22
Classification: G02F,H04B

Abstract:
1. An optical modulator comprising: a main input port; a main output port; a first Mach-Zehnder modulator that includes a first arm waveguide, a second arm waveguide, a first optical coupler connected to the first arm waveguide and the second arm waveguide, and a second optical coupler connected to the first arm waveguide and the second arm waveguide; a second Mach-Zehnder modulator that includes a first arm waveguide, a second arm waveguide, a first optical coupler connected to the first arm waveguide and the second arm waveguide, and a second optical coupler connected to the first arm waveguide and the second arm waveguide; a first branching waveguide optically coupling the main input port to the first Mach-Zehnder modulator and the second Mach-Zehnder modulator; a second branching waveguide optically coupled to a first output port of the second optical coupler of the first Mach-Zehnder modulator and to a first output port of the second optical coupler of the second Mach-Zehnder modulator, the second branching waveguide including a first input port connected to the first Mach-Zehnder modulator, a second input port connected to the second Mach-Zehnder modulator, and an output port connected to the main output port; a first driver circuit connected to the first Mach-Zehnder modulator, the first driver circuit generating a first drive signal having a first amplitude at a first bias point; a second driver circuit connected to the second Mach-Zehnder modulator, the second driver circuit generating a second drive signal having a second amplitude at a second bias point; a first monitor light output connected to a second output port of the second optical coupler of the first Mach-Zehnder modulator; a second monitor light output connected to a second output port of the second optical coupler of the second Mach-Zehnder modulator; a first photoelectric converter optically coupled to the first monitor light output; a second photoelectric converter optically coupled to the second monitor light output; and a signal processing circuit connected to the first driver circuit, the signal processing circuit receiving a first monitor signal from the first photoelectric converter and a second monitor signal from the second photoelectric converter, the signal processing circuit including a difference circuit that generates a differential signal corresponding to a difference between the first monitor signal and the second monitor signal, wherein the first drive signal and the second drive signal satisfy at least one of a first condition and a second condition, the first condition is that the first amplitude differs from the second amplitude, and the second condition is that the first bias point differs from the second bias point, the first driver circuit generates the first drive signal depending on the differential signal, and the first driver circuit adjusts the first amplitude based on the differential signal such that the first Mach-Zehnder modulator outputs a modulated light having an intensity equal to an intensity of a modulated light output from the second Mach-Zehnder modulator.