Vector sum phase shifter, optical transceiver, and control circuit

An optical transceiver comprising:

• a laser which outputs continuous light;
• a serializer which outputs serial data to be output and a clock;
• a first Mach-Zehnder modulator which phase- or amplitude-modulates the continuous light input from said laser and outputs NRZ signal light;
• a first modulator driver which drives said first Mach-Zehnder modulator based on the serial data;
• a second Mach-Zehnder modulator which amplitude-modulates the NRZ signal light input from said first Mach-Zehnder modulator and outputs RZ signal light;
• a vector sum phase shifter which receives the clock;
• a second modulator driver which drives said second Mach-Zehnder modulator based on the clock phase-adjusted by said vector sum phase shifter;
• and
• a phase control circuit which outputs a control voltage corresponding to a phase shift amount of said vector sum phase shifter,
• wherein said vector sum phase shifter comprises:
• a 90° phase shifter which generates, from the clock, an in-phase signal and a quadrature signal 90° degrees out of phase with the in-phase signal;
• a first four-quadrant multiplier which changes an amplitude of the in-phase signal based on a first control signal on an in-phase signal side and outputs the in-phase signal;
• a second four-quadrant multiplier which changes an amplitude of the quadrature signal based on a second control signal on a quadrature signal side and outputs the quadrature signal;
• a combiner which combines the in-phase signal output from said first four-quadrant multiplier and the quadrature signal output from said second four-quadrant multiplier and outputs a combined signal as the phase-adjusted clock;
• and
• a control circuit which outputs the first control signal and the second control signal,
• said control circuit comprises:
• a voltage generator which generates a reference voltage;
• and
• a differential amplifier which outputs a difference signal between the control voltage and the reference voltage as one of the first control signal and the second control signal, and
• when the control voltage is near the reference voltage, said differential amplifier performs an analog operation of converting the control voltage into one of the first control signal and the second control signal which is similar to one of a sine wave and a cosine wave.

A control circuit for outputting a control signal to means for adjusting a signal amplitude, comprising:

• a voltage generator which generates a reference voltage;
• and
• a differential amplifier which outputs a difference signal between the reference voltage and an externally input control voltage as the control signal,
• wherein when the control voltage is near the reference voltage, said differential amplifier performs an analog operation of converting the control voltage into the control signal which is similar to one of a sine wave and a cosine wave.