Patent ID: 7672033

Claim:
A light modulating apparatus of an external modulating type which is used in an optical communication system, comprising: an LN modulator for modulating a light which is incident from a light source and outputting an output light signal; a light branching circuit for branching the output light signal; a photoelectric converter for converting an output signal of the light branching circuit into an electric signal; a bias control circuit for generating a signal based on the electric signal of the photoelectric converter; a driver for an LN modulator which serves to drive the LN modulator in response to an input electric signal; a capacitor connected to an input terminal or output terminal of the driver for an LN modulator and serving to cut off DC and low frequency components of the input electric signal; a low frequency component compensating circuit for compensating the DC and low frequency components of the input electric signal which are lost by the capacitor, wherein said low frequency component compensating circuit outputs DC and low frequency components; and an adding circuit for adding an output of the bias control circuit and an output of the low frequency component compensating circuit and generating a bias control signal for adjusting a bias point of the LN modulator.