Investigation of Phase Noise in a Radio-Over-Fiber Link Based on an InAs/InP Quantum-Dash Mode-Locked Laser

The phase noise of a radio frequency (RF) signal generated by beating two optical comb lines from an InAs/InP quantum-dash mode-locked laser (QD-MLL) in a radio over fiber (RoF) link is studied theoretically and experimentally. At the central office (CO), a QD-MLL with equidistant comb lines is employed. By selecting two comb lines and sending them to a remote radio unit (RRU) via an optical fiber, a RF signal is generated by beating the two comb lines at a photodetector (PD) in the RRU. The phase noise from each of the comb lines and the differential phase noise between the two comb lines are directly translated to the generated RF signal, which will deteriorate the transmission performance of a RoF link. In this study, we perform a thorough theoretical examination of the phase noise in the generated RF signal. Our findings reveal that the time delay mismatch between the two comb lines will create a phase noise to be added to the RF signal, and the differential phase noise between the comb lines will also create a phase noise to the RF signal. We show that the RF phase noise can be significantly reduced by a hybrid approach using analog time delay matching and digital phase noise compensation. As a result, utilizing a QD-MLL and the hybrid phase noise compensation approach, we successfully transmit a 16QAM orthogonal frequency division multiplexing (OFDM) signal at 10 Gb/s at 25 GHz over a 10 km optical fiber and a 2 m wireless span with improved performance. Error free transmission is achieved within the feedforward error correction (FEC) limits. This finding opens up new possibilities for phase noise management in broadband RoF transmission systems.