A Phase-Modulated Microwave Photonic Link With an Extended Transmission Distance

A novel technique to transport a microwave signal over an optical fiber based on phase-modulation and coherent in-phase (I) and quadrature-phase (Q) demodulation with an extended transmission distance is proposed and experimentally demonstrated. In the transmitter, a Sagnac loop incorporating a phase modulator (PM) is used to generate two orthogonally polarized optical signals with one being phase modulated and the other with no modulation that acts as a remote optical reference signal. The orthogonally polarized optical signals are transmitted over a single-mode fiber (SMF) to a polarization and phase diversity coherent receiver, and are coherently detected with a free-running optical local oscillator at the receiver. Since the phase-modulated and the reference signals are transmitted over the same SMF, the optical phases are correlated, and the original signal can be recovered based on a digital signal processing algorithm. The proposed technique is experimentally evaluated. Compared with a phase-modulated coherent I/Q demodulated link without using an optical phase correlated reference signal, the transmission distance is extended from 50 m to 10 km, while providing a link gain and a spurious-free dynamic range (SFDR) of -9.5 dB and 115.8 dB · Hz^2/3, respectively.