An optical modulating device including a driving circuit generating a
driving voltage corresponding to an input signal, an oscillator
generating a signal having a low frequency signal lower than that of the
input signal, an MZ modulator receiving a DC bias voltage superposed with
the low frequency signal and the driving voltage, to generate a modulated
optical signal according to the input signal, a photoelectric converting
unit for converting the modulated optical signal into an electric signal,
a low frequency signal detecting circuit which extracts the low frequency
signal component contained in the electric signal, multiplies the low
frequency signal component by the low frequency signal outputted by the
low frequency oscillator, and then extracts a DC component from a
multiplied output signal, and a control circuit for extracting a
maximized DC component from the low frequency signal detecting circuit by
controlling the DC bias voltage.