High-Spectral-Efficiency Photonic Frequency Down-Conversion Using Optical Frequency Comb and SSB Modulation

A high-spectral-efficiency photonic frequency down-conversion approach for wavelength-division-multiplexing radio-over-fiber (WDM-ROF) uplinks is proposed and experimentally investigated. In the approach, an optical frequency comb serves as the carriers of multiple channels, and each comb line is modulated by the upstream radio-frequency (RF) signal via single-sideband modulation. At the central station, the beating between the sideband with upstream signal and its adjacent optical carrier leads to photonic frequency down-conversion without additional optical frequency shifts. Due to the band overlapping or the reduction of guard band between adjacent channels, a high spectral efficiency is achieved for WDM-ROF uplinks. A two-channel experimental uplink is established, where a 15-GHz RF signal carrying quadrature phase shift keying (QPSK) data is successfully down-converted to a 2-GHz intermediate frequency (IF) signal. For the 10-Mb/s QPSK data, the measured eye diagrams and the constellation diagrams clearly show the effectiveness of the frequency down-conversion and the influence of the transmission distance. In fact, the proposed approach is applied to the down-conversion at a high data rate up to several gigabits per second, such as a simulation demonstration at 2.5 Gb/s.