Study Of Generation And Receiving Of Multiple Frequency Optical Millimeter-waves In Radio-over-fiber System

The technology of radio-over-fiber(ROF)is originally to improve efficiency of spectrum and reduce the size of remote base station. As the further studies of ROF, the technology has many outstanding capabilities, such as low penalty, broad-band, high-capacities and seamless transmission and so on. So it is forecasted as one key technology for the ultra wideband wireless communication network. Based on the considerations, we have investigated the optical generation and demultiplexing of multiple frequency millimeter-wave signals in ROF systems, specifically including:Firstly, a full-duplex WDM-ROF system with passive base station based on optical carrier reuse has been proposed and experimently demonstrated. This system has simple and cost-efficient configuration, the optical power is effectively utilized. At central station, a optical intensity modulator is used to generate DWDM optical millimeter(mm)-wave signals for carrying downstream data. At base station, the DWDM optical mm-wave signals are separated by a optical filter before optical carriers are reused in uplink. We recieved the eye diagrams of the mm-wave signals after transmitting different distance by numerical simulation. The results show that the limitation of chromatic dispersion is greatly reduced due to avoiding the generation of higher order sidebands via driving the intensity modulator with optimized RF signal. After transmission over 60km, the eye open penalty of mm-wave signals and baseband data signals for downlink and uplink are both small.Secondly, we have theoretically investigated the transmission performance of the optical mm-wave generation by using an external modulator based on optical carrier suppression. Though the OCS millimeter (mm)-wave can immune the fading effect, the baseband signals with a high bit rate are degraded greatly after transmission along fiber. The main reason is that the fiber dispersion cause the time shift of the code edges; therefor the maximum transmission distance of the baseband signals is limited. We have proposed a scheme to overcome the dispersion affection and extend the transmission distance, in which an external intensity modulator is employed to generate the DSB signal with suppression central carrier, while the baseband data are modulated only onto one of the two optical carriers. Our theoretical analysis results demonstrate the feasibility of the proposed scheme at last.Thirdly, based on wavelength reuse technique, we proposed and simulation demonstrated a novel full-duplex multiple frequency optical mm-waves system in which the bidirectional 2.5Gbit/s data signals have been successfully transmitted over 60 km single-mode fiber. Multiple different RF signals are combined by an coupler and then to drive a phase modulator and to generate different optical mm-waves with double RF frequencies. Each two first-order sidebands are used to carry different downlink data, respectively, while the separated optical carrier is used in uplink with wavelength reuse. In this novel scheme, not only successfully generated doubled frequency mm-wave, but also make the best use of the whole optical power by realizing the wavelength reuse technique, simplified the configuration.Finally, a novel radio-over-fiber system has been experimentally demonstrated to generate multiple different frequency optical mm-wave signals simultaneously by using single optical carrier and one single-arm LiNbO3 modulator. At central station, multiple different baseband signals are electrically up-converted with corresponding RF signals in mixers, the converted RF signals are combined by an electrical power coupler to drive a single-arm LiNbO3 modulator based on optical carrier suppression. The differernt optical mm-wave signals, after transmitted to base station, are directly separated by a optical filter, then detected by different high-speed photo-diodes, respectively. The experiment results show that the repetitive frequency of the generated mm-wave signals is 20GHz and 40GHz, which is the double of the LO frequency based on OCS modulation, and 2.5Gbit/s data on 20GHz mm-wave can be transmitted at least 40 km over fiber. Therefore the scheme is potent and feasibility.