Self-referenced method and apparatus for in-operation measurement of a coherent optical transmitter's modulation loss

A transmitter, comprising:

• a laser operable to supply an optical signal;
• a digital signal processor operable to supply first electrical signals based on data input to the digital signal processor;
• a digital-to-analog conversion circuitry operable to output second electrical signals based on the first electrical signals;
• a modulator driver circuitry operable to output third electrical signals based on the second electrical signals;
• an optical modulator having a first arm with a first high-speed phase modulator and a first phase adjuster operable to modulate the optical signal into a first modulated optical signal based on the third electrical signals, the first modulated optical signal having a first power, and a second arm with a second high-speed phase modulator and a second phase adjuster operable to modulated the optical signal into a second modulated optical signal based on the third electrical signals, the second modulated optical signal having a second power;
• an optical combiner operable to combine the first modulated optical signal and the second modulated optical signal into a combined optical signal having a combined power and directing a first portion of the combined optical signal to a monitor photodetector;
• the monitor photodetector operable to measure the combined power of the combined optical signal;
• and
• control and monitoring circuitry coupled to the monitor photodetector and in communication with the optical modulator, and operable to cause the first phase adjuster to apply a first phase to the first modulated optical signal and the second phase adjuster to apply a second phase to the second modulated optical signal at a first point in time, to cause the first phase adjuster to apply a third phase to the first modulated optical signal and the second phase adjuster to apply a fourth phase to the second modulated optical signal at a second point in time, to compute a contrast ratio based on the combined power at the first point in time and the combined power at the second point in time, and to determine a modulation loss based at least in part on the contrast ratio.

A transmitter, comprising:

• a laser operable to supply an optical signal having a first portion and a second portion;
• a digital signal processor operable to supply first electrical signals based on data input to the digital signal processor;
• a digital-to-analog conversion circuitry operable to output second electrical signals based on the first electrical signals;
• a modulator driver circuitry operable to output third electrical signals based on the second electrical signals;
• an optical modulator having a first arm with a first high-speed phase modulator and a first phase adjuster operable to modulate the first portion of the optical signal into a first modulated optical signal based on the third electrical signals, the first modulated optical signal having a first power, and a second arm with a second high-speed phase modulator and a second phase adjuster operable to modulate the first portion of the optical signal into a second modulated optical signal based on the third electrical signals, the second modulated optical signal having a second power;
• a first monitor photodetector;
• a second optical combiner;
• a complementary photodetector;
• a first optical combiner operable to combine the first modulated optical signal and the second modulated optical signal into a first combined optical signal having a first combined power and directing a first portion of the first combined optical signal to the first monitor photodetector and a second portion of the first combined optical signal to the second optical combiner;
• the first monitor photodetector operable to measure the first combined power of the first combined optical signal;
• the second optical combiner operable to combine the second portion of the first combined optical signal and the second portion of the optical signal into a second combined optical signal having a second combined power and directing a first portion of the second combined output optical to the complementary photodiode;
• the complementary photodetector operable to measure the second combined power;
• and
• a control and monitoring circuitry coupled to the first monitor photodetector and the complementary photodetector, and operable to determine a contrast ratio based at least in part on the first combined optical power and the second combined power and to determine a modulation loss based at least in part on the contrast ratio.