External modulator and method for externally modulating light

An external modulator comprising:

• an external modulating device of an interference type;
• low frequency signal generating means for generating a predetermined low frequency signal;
• bias generating means for generating a bias voltage and capable of changing the level of the bias voltage within a predetermined range centered on a ground level;
• superimposing means for amplitude-modulating a main signal by said low frequency signal, and superimposing said bias signal thereon to obtain a signal serving as a modulation signal to be supplied to said external modulating device;
• low frequency signal extracting means for extracting a component corresponding to said low frequency signal from an output light of said external modulating device;
• bias controlling means for controlling a voltage level of said bias voltage based on the low frequency signal extracted by said low frequency signal extracting means so that the operation point of the external modulating device interference type is set to a predetermined operation stable point;
• initial setting means for initially setting the level of the bias voltage so that the operation point of the external modulating device is set to an operation stable point nearest the operation point attained when a bias voltage of ground level is applied;
• discriminating means for discriminating whether or not said bias voltage is in a predetermined allowable range;
• and
• initial setting controlling means for setting the level of said bias voltage by said initial setting means only when it is discriminated that said bias voltage is not in the predetermined allowable range by said discriminating means for a predetermined period of time after the power supply is started.

A method for externally modulating light using an external modulating device of an interference type, comprising the steps of:

• generating a predetermined low frequency signal;
• generating a bias voltage whose level is within a predetermined range centered on a ground level;
• amplitude-modulating a main signal by said low frequency signal, and superimposing said bias signal thereon to obtain a signal serving as a modulation signal to be supplied to said external modulating device;
• extracting a component corresponding to said low frequency signal from an output light of said external modulating device;
• controlling a voltage level of said bias voltage based on the low frequency signal extracted in said extracting step so that an operation point of the external modulating device is set to a predetermined operation stable point;
• and
• initially setting the level of the bias voltage so that the operation point of the external modulating device is set to an operation stable point nearest the operation point attained when a bias voltage of the ground level is applied.