Patent ID: 6924916

Claim:
A method for stabilizing operation point and optical output of an external optical modulator including a light source, an external optical modulator modulating light from the light source, an optical detector detecting output-light from said external optical modulator, and means for regulating direct current bias applied to said external optical modulator, which regulates the direct current bias determining the operation point of a modulation curve of said external optical modulator, according to output of said optical detector, comprising the steps of: superimposing onto said direct current bias a low-frequency signal, which is frequency below a lower limit of a signal frequency band of an input signal inputted to said external optical modulator; extracting a low-frequency component included in the output of said optical detector; normalizing on the basis of said low-frequency signal the output of said low-frequency component; and controlling the output-light of said light source in accordance with said normalized low-frequency component, wherein: a means of controlling optical output of light source is equipped, which, in control of the output-light of said light source, detects the output-light from said light source, compares a value of the detected output-light from said light source to a standard value of primary optical output determining primary optical output, and adjusts the output-light of said light source; said standard value of primary optical output is modified according to a ratio of a primary value of said normalized low-frequency component to a subsequent value; and said means of controlling optical output of light source is operated on the basis of said modified standard value.