A quadrature amplitude modulation, QAM, signal generating method using a quadrature amplitude modulation, QAM, signal generator, wherein the quadrature amplitude modulation, QAM, signal generator comprises: a first waveguide; a first quadrature phase-shift-keying, QPSK, signal generator provided on the first waveguide; a second waveguide being connected with the first waveguide at a wave-combining point; and a second quadrature phase-shift-keying, QPSK, signal generator provided on the second waveguide, wherein each of the first QPSK signal generator and the second QPSK signal generator has a main Mach-Zehnder waveguide (7a, 7b) and two sub Mach-Zehnder waveguides (8a, 9a, 8b, 9b), wherein the main Mach-Zehnder waveguide (7a, 7b) has a first arm and a second arm; wherein the two sub Mach-Zehnder waveguides (8a, 9a, 8b, 9b) are provided on the first arm and the second arm of the main Mach-Zehnder waveguide (7a, 7b), wherein the QAM signal generating method comprises steps of: generating a first amplitude-shift-keying, ASK, signal which is a quadruple ASK signal by means of the main Mach-Zehnder waveguide of the first QPSK signal generator; generating a second amplitude-shift-keying, ASK, signal which is a quadruple ASK signal by means of the main Mach-Zehnder waveguide of the second QPSK signal generator, the second ASK signal having a phase difference of 90° with respect to the first ASK signal; and said QAM signal generating method being characterized in that it further comprises the steps of: adjusting the amplitude of the output of the second QPSK signal generator in such a way that the amplitude of said output signal from the second QPSK signal generator is between 30% and 70% of the amplitude of an output signal from the first QPSK signal generator, combining said first ASK signal and said second ASK signal, said second ASK signal having an amplitude adjusted according to said adjusting step. a first waveguide; a first quadrature phase-shift-keying, QPSK, signal generator provided on the first waveguide; a second waveguide being connected with the first waveguide at a wave-combining point; and a second quadrature phase-shift-keying, QPSK, signal generator provided on the second waveguide, wherein each of the first QPSK signal generator and the second QPSK signal generator has a main Mach-Zehnder waveguide (7a, 7b) and two sub Mach-Zehnder waveguides (8a, 9a, 8b, 9b), wherein the main Mach-Zehnder waveguide (7a, 7b) has a first arm and a second arm; wherein the two sub Mach-Zehnder waveguides (8a, 9a, 8b, 9b) are provided on the first arm and the second arm of the main Mach-Zehnder waveguide (7a, 7b), wherein the QAM signal generating method comprises steps of: generating a first amplitude-shift-keying, ASK, signal which is a quadruple ASK signal by means of the main Mach-Zehnder waveguide of the first QPSK signal generator; generating a second amplitude-shift-keying, ASK, signal which is a quadruple ASK signal by means of the main Mach-Zehnder waveguide of the second QPSK signal generator, the second ASK signal having a phase difference of 90° with respect to the first ASK signal; and said QAM signal generating method being characterized in that it further comprises the steps of: adjusting the amplitude of the output of the second QPSK signal generator in such a way that the amplitude of said output signal from the second QPSK signal generator is between 30% and 70% of the amplitude of an output signal from the first QPSK signal generator, combining said first ASK signal and said second ASK signal, said second ASK signal having an amplitude adjusted according to said adjusting step. generating a first amplitude-shift-keying, ASK, signal which is a quadruple ASK signal by means of the main Mach-Zehnder waveguide of the first QPSK signal generator; generating a second amplitude-shift-keying, ASK, signal which is a quadruple ASK signal by means of the main Mach-Zehnder waveguide of the second QPSK signal generator, the second ASK signal having a phase difference of 90° with respect to the first ASK signal; and said QAM signal generating method being characterized in that it further comprises the steps of: adjusting the amplitude of the output of the second QPSK signal generator in such a way that the amplitude of said output signal from the second QPSK signal generator is between 30% and 70% of the amplitude of an output signal from the first QPSK signal generator, combining said first ASK signal and said second ASK signal, said second ASK signal having an amplitude adjusted according to said adjusting step. adjusting the amplitude of the output of the second QPSK signal generator in such a way that the amplitude of said output signal from the second QPSK signal generator is between 30% and 70% of the amplitude of an output signal from the first QPSK signal generator, combining said first ASK signal and said second ASK signal, said second ASK signal having an amplitude adjusted according to said adjusting step.