Frequency upconversion of multiple RF signals using optical carrier suppression for radio over fiber downlinks

Abstract

We propose and analyze a technique of an optical carrier transmitting two RF signals using optical carrier suppression. A single optical Mach-Zehnder modulator is used for both optical carrier suppression and signal modulation, and optical carrier suppression modulation is also used for frequency conversion of RF signals. This work shows that in contrary to the case of an optical carrier transmitting a single RF signal with optical carrier suppression where stronger optical carrier suppression improves the upconverted RF signal, weaker optical carrier suppression is preferred for an optical carrier transmitting two RF signals due to nonlinear distortion because the nonlinear distortion is reduced by using weaker optical carrier suppression. We find that the usable range of optical carrier suppression ratio is from 10 to 18 dB for RF signal upconverted to 20 GHz and beyond, and the best optical carrier suppression ratio is around 10 dB. We verify the concept and analysis with experiment. In experiment, we used two RFs at 6 and 18 GHz transmitting two 750 Mb/s signals. The experiment for the first time demonstrated that an optical carrier can transmit two RF signals using optical carrier suppression and showed that upconverted RF signals are degraded by nonlinear distortion, particularly for upconverted RF signal at 12 GHz, i.e. the RF signal at the lower frequency.