In the recent optical communication systems, QPSK (Quadrature Phase Shift Keying) in which two-bit information is carried for each of four phase differences is used as a multilevel modulation method. Two parallel Mach-Zehnder (MZ) modulators (hereinafter referred to as “IQ modulators”), which are composed of two MZ modulators, are used in the QPSK. In the IQ modulators, the MZ modulators each serving as an MZ interferometer are connected in parallel, and a modulating signal of an in-phase channel (referred to as “I-channel” or “I-arm”), which is an in-phase component, and a modulating signal of a quadrature-phase channel (referred to as “Q-channel” or “Q-arm”), which is a quadrature component, are multiplexed by applying a carrier phase difference of 90 degrees.
An optical modulator of this type performs an intensity modulation on continuous light with a uniform intensity. Power for providing the intensity modulation includes signal power, which is proportional to an input digital signal (digital input signal), and DC bias power. To ensure that the digital signal operates properly, it is necessary to apply a bias (bias voltage or current) properly and to apply a signal (signal voltage or current) based on the bias. There are appropriate values for the signal power and bias power to obtain an optical modulating signal that is linear with respect to a digital input signal and has no distortion. However, since this MZ modulator is affected by the environmental temperature or temporal variation, an optimum bias voltage varies. Accordingly, in order to stabilize the signal quality, it is important to control the bias of the optical modulator with a high precision.