Patent ID: 8676060

Claim:
A quadrature amplitude modulation (QAM) signal generating method the method comprising: combining an output signal from a first QPSK signal generator and an output signal from a second QPSK signal generator, wherein the output signal from the second QPSK signal generator is a half of an amplitude of the output signal from the first QPSK signal generator, wherein each of a main Mach-Zehnder waveguide of the first QPSK signal generator and a main Mach-Zehnder waveguide of the second QPSK signal generator has a first bias electrode, a second bias electrode, and a third bias electrode, wherein the first bias electrode adjusts a bias voltage applied to a first arm of the main Mach-Zehnder waveguide, wherein the second bias electrode adjusts a bias voltage applied to a second arm of the main Mach-Zehnder waveguide, wherein the third bias electrode applies a bias voltage to signals that go though the first arm and the second arm of the main Mach-Zehnder waveguide and are combined at an wave-combining part of the main Mach-Zehnder waveguide; and controlling bias signals applied to the first bias electrode, the second bias electrode, and the third bias electrode, so that four sub Mach-Zehnder waveguides allow a phase difference of signals propagating through the first arm and the second arm before being combined with each other to be 0° or 180°, so that the main Mach-Zehnder waveguide allows a phase difference between signals propagating through the first arm and the second arm before being combined with each other to be 90° or 270°, and so that the first QPSK signal generator and the second QPSK signal generator allow a phase difference between signals propagating through the first waveguide and the second waveguide before being connected with each other at the wave-combining point to be 0° or 180°.