Patent ID: 7031042

Claim:
A signal converter for converting a digital input signal to an optical modulation signal, comprising: a Mach-Zehnder type optical modulator to be supplied with the digital input signals controlled in amplitude, and a bias signal for providing the optical modulation signal; a pilot signal-superimposing circuit for superimposing a pilot signal of a frequency on a bias control signal; a monitor circuit for providing a monitor signal by receiving a part of the optical modulation signal supplied from the optical modulator; a first feedback system for providing an amplitude control signal to control an amplitude of the digital input signal in accordance with a frequency deviation signal obtained from the monitor signal; and a second feedback system for providing the bias control signal to control the bias signal in accordance with a multiplying frequency deviation signal obtained from the monitor signal, wherein: the first feedback system comprises a first mixer for multiplying the pilot signal and the monitor signal; a first low pass filter for providing the frequency deviation signal based on a low frequency component obtained from an output of the first mixer; and a first differential amplifier for providing the amplitude control signal in accordance with a difference between an output of the first low pass filter and a first reference signal.