Patent ID: 6510255
Filing Date: 2003-01-21
Classification: G02F

Abstract:
An optical transmission apparatus comprising:a light source operable to output an optical carrier; an external optical modulator operable to modulate the optical carrier outputted from said light source with an electrical signal; a bias voltage applier operable to apply a bias voltage to said external optical modulator; and a bias voltage controller operable to control the bias voltage applied by said bias voltage applier to said external optical modulator based on an amount of second order distortion included in an optical signal from said external optical modulator and caused by non-linearity of said external optical modulator, said bias voltage controller comprising: an optical branching unit operable to branch the optical signal from said external optical modulator into two optical signals; an optical-electrical converter operable to convert one of the two optical signals from said optical branching unit into an electrical signal; a distortion detector operable to detect the amount of second order distortion included in the optical signal from said external optical modulator and caused by the non-linearity of said external optical modulator by extracting a component in a specific band from the electrical signal provided by said optical-electrical converter and measuring a level of the component; and a bias voltage control circuit operable to control the bias voltage applied by said bias voltage applier to said external optical modulator so as to minimize the amount of second order distortion detected by said distortion detector, said bias voltage control circuit increasing or decreasing the bias voltage applied by said bias voltage applier to said external optical modulator, determining whether the second order distortion detected by said distortion detector is increased or decreased before and after increasing or decreasing said bias voltage, and determining, based on a determination result, whether to increase or decrease the bias voltage at a next time.