US Patent 7245828 Extinction ratio determination using duty cycle modulation

An optical transmitter that determines and controls the extinction ratio of an optical signal by modulating the duty cycle of the optical signal. The transmitter obtains a signal from the monitor circuit that represents optical power emitted by the electro-optic transducer during normal conditions in which the duty cycle of the optical signal is between a bias high and bias low modulation condition. The transmitter also obtains a signal from the monitor circuit that represents optical power emitted during the bias high modulation condition in which the time at the optical high power level in the eye diagram is greater. Also, the transmitter obtains a signal that represents optical power emitted by the electro-optic transducer during the bias low modulation condition in which the time at the optical low power level in the eye diagram is greater. Based on these signals, the transmitter then derives the extinction ratio.