An integrated optical linewidth reduction system detects/estimates the phase noise of an incoming optical signal and subtracts the detected phase noise from the phase noise of the incoming signal. A first coupler/splitter of the linewidth reduction system may split the incoming signal into first and second optical signals travelling through first and second optical paths. A second coupler/splitter may split the second optical signal into third and fourth optical signals travelling through third and fourth optical paths. The third optical path has a longer propagation delay than the fourth optical path. Two different coupling ratios of the third and fourth optical signals are used to generate an electrical signal representative of the phase noise of the incoming signal. A phase detector/estimator estimates the phase noise from the electrical signal. A phase modulator subtracts the detected/estimated phase noise from the phase noise of the incoming signal.