Digital Phase Noise Cancellation for a Coherent-Detection Microwave Photonic Link

An intensity-modulation and coherent-detection (IM/CD) microwave photonic (MWP) link with digital signal processing (DSP)-based phase noise cancellation is proposed and experimentally demonstrated. At the transmitter, the optical carrier is intensity-modulated by a radio-frequency (RF) signal and sent to a coherent receiver over a single-mode fiber (SMF). At the receiver, the intensity-modulated optical signal is coherently detected using a local oscillator (LO) laser source. The phase noise introduced by both the transmitter laser source and the LO laser source is converted into an amplitude noise at the output of the coherent receiver, which would be added to the in-phase component and quadrature component from the coherent receiver. The noise can be completely cancelled by summing the squared magnitudes of the in-phase and quadrature components from the coherent receiver, which can be implemented using a DSP unit. Error-free transmission of a 1.25-Gb/s quadrature phase shift keying modulated RF signal with a center frequency of 1.6 GHz over 25-km SMF is experimentally demonstrated. For a signal with a bit rate of 834 Mb/s, the receiver sensitivity can reach
${-}{\rm 24.5}~{\rm dBm}$
which is 20 dB better than that based on an intensity-modulation and direct-detection MWP link.