Enhanced feed forward optical frequency/phase demodulator

An optical demodulation system for demodulating an optical signal modulated by a transmitted RF signal, said demodulation system receiving the modulated optical signal at a receiver end of an analog optical link, said demodulation system comprising:

• an optical splitter responsive to the optical signal, said splitter splitting the optical signal into a first split modulated optical signal and a second split modulated optical signal;
• a coarse optical demodulator responsive to the first split signal, said coarse demodulator including a first Mach-Zehnder interferometer, first and second photodetectors and a first differential amplifier, said first interferometer being responsive to the first split signal and generating a first set of complementary optical signals from the first split signal, said first and second photodetectors converting the first set of complementary optical signals to a first set of complementary electrical signals, and said first differential amplifier converting the first set of complementary electrical signals into a coarse frequency demodulated RF signal, said coarse RF signal including the transmitted RF signal and the additive inverse of an error RF signal;
• a phase modulator responsive to the second split signal and an additive inverse of the coarse RF signal, said phase modulator modulating the second split signal by the inverted coarse RF signal so that the transmitted RF signals in the second split signal and the coarse RF signal cancel each other, said phase modulator generating a modulated error optical signal modulated with the error RF signal;
• a narrow band optical filter responsive to the modulated error optical signal, said filter narrowing the frequency band of the error optical signal to a narrower band than the second split modulated optical signal;
• a fine optical demodulator responsive to the filtered error optical signal, said fine demodulator including a second Mach-Zehnder interferometer, third and fourth photodetectors and a second differential amplifier, said second interferometer being responsive to the error optical signal and generating a second set of complementary optical signals from the error optical signal, said third and fourth photodetectors converting the second set of complementary signals to a second set of complementary electrical signals, and said second differential amplifier converting the second set of complementary electrical signals into the error RF signal;
• and
• a combiner responsive to the coarse RF signal from the coarse demodulator and the error RF signal from the fine demodulator, said combiner generating the transmitted RF signal.

A method of demodulating an optical carrier signal modulated by a transmitted RF signal, said method comprising the steps of:

• splitting the optical carrier signal into a first split modulated optical signal and a second split modulated optical signal;
• demodulating the first split optical signal to generate a coarse RF signal that includes the transmitted RF signal and the additive inverse of an error RF signal;
• inverting the coarse RF signal;
• modulating the second split optical signal with the additive inverse of the coarse RF signal to cancel the transmitted RF signal in the second split signal and the coarse RF signal and to generate a modulated error optical signal modulated with the error RF signal;
• filtering the modulated error optical signal to a narrower bandwidth than the second split modulated optical signal;
• demodulating the error optical signal to generate the error RF signal;
• and
• combining the coarse RF signal and the error RF signal.