A High Spectral Efficiency Coherent RoF System Based on OSSB Modulation With Low-Cost Free-Running Laser Sources for UDWDM-PONs

A coherent radio over fiber (RoF) system based on optical single-sideband modulation with no optical carrier (OSSB) for ultradense wavelength division multiplexing passive optical networks (UDWDM-PONs) is proposed and experimentally demonstrated. For one channel of the UDWDM-PONs, at the transmitter, the OSSB RoF signal is generated using a dual parallel Mach-Zehnder modulator (DP-MZM) with a real-valued precoded microwave vector signal which is applied to the DP-MZM via the two electrodes with one having a 90° phase shift. Then, the OSSB RoF signal is sent to a coherent receiver over a single-mode fiber (SMF). The coherent receiver with a free-running laser source as a local oscillator (LO) is used to perform the coherent detection. To recover the standard microwave vector signals from the real-valued precoded microwave vector signal that is embedded in a strong phase noise introduced by both the transmitter laser source and the LO laser source, a digital signal processing algorithm is developed to perform phase noise cancellation. An experiment is performed. The transmission of a 1.25-Gbps quadrature phase shift keying, a 1.875-Gbps eight-phase shift keying (8-PSK), and a 2.5-Gbps 16 quadrature amplitude modulation (16-QAM) microwave vector signals with a channel spacing as narrow as 3 GHz over a 25-km SMF is experimentally demonstrated. For the transmission of the 2.5-Gbps 16-QAM microwave vector signal, the received optical sensitivity at forward error correction level over a 25-km SMF link is -24.8 dBm, while for the 1.875-Gbps 8-PSK microwave vector signal, it can reach -29.8 dBm which leaves an enough margin to accommodate the splitting losses in the optical distribution networks.