Patent ID: 9686017
Date: 2017-06-20
CPC Classifications: H04B

Claim:
1. A bias control circuit for multi-level phase modulator that includes an outer modulator and a first power monitor, the outer modulator including a pair of optical waveguides each providing inner modulators, each of the inner modulators including a pair of optical waveguides, the first power monitor receiving an optical signal output from the multi-level phase modulator, the bias control circuit comprising: a bias generator configured to provide first bias signals to the respective optical waveguides of one of the inner modulators, each of the first bias signals including first dither signals superposed thereon having predetermined frequencies equal to each other; a resistor connected in series to the first power monitor, the resistor converting a current signal output from the first power monitor to the first monitor signal; a first differential amplifier configured to receive the first monitor signal and a first reference signal, the first differential amplifier generating a first amplified signal corresponding to a difference between the first monitor signal and the first reference signal; a first analog-to-digital converter (ADC) that converts the first monitor signal to a first digital signal; a first digital-to-analog converter (DAC); and a controller configured to detect first frequency components contained in the first amplified signal and originating to the first dither signals, and provide a first control signal according to intensity of the first frequency components to the bias generator, wherein the controller calculates an average of the first digital signal, wherein the first DAC converts the average of the first digital signal into the first reference signal that is provided to the first differential amplifier, and wherein the first bias signals provided to the first optical waveguides are adjusted according to the first control signal.