Full-duplex Radio-over-fiber Transmission And The Down-conversion Technologies

The thesis mainly focuses on full-duplex Radio-over-fiber (ROF) transmission and the down-conversion technologies. As for the down-conversion technology, the key technology in ROF system, we proposed a down-conversion scheme based on a polarization modulator (PolM). The proposed down-converter can avoid the disadvantages of traditional schemes by using the electrical mixers, such as narrow bandwidth, low conversion efficiency and severe signal distortion. Thanks to the PolM applied, no bias-drifting problem exists. Besides, compared with other schemes, no complex loops are required in this scheme, thus the proposed system is more simple, compact and stable. The experimental results show that this scheme is feasible and the proposed microwave photonic down-converter can down-convert the signal in a wide frequency range with high conversion efficiency.If the local oscillation (LO) signal with poor spectral purity and large noise are used in the down-converter, the received signal may be covered by noise, considering that the power of the received signal is small. Therefore, LO signals with high quality are quite desired in the down-converter. RF or microwave signals generated by the optoelectronic oscillators (OEOs) have the advantages of low phase noise, high spectral purity and high frequency stability, which can be used to realize seamless connection with ROF systems. The traditional way to realize OEO is by use of an electrical filter, but the tunability of the generated OEO is limited by the bandwidth of the electrical filter. To solve this problem, we proposed an OEO based on an optical-injected distributed feedback (DFB) semiconductor laser. By utilizing the wavelength-selective optical gain of the DFB laser under optical injection, the phase modulated signals can be converted into intensity modulated ones. As a result, a microwave photonic bandpass filter (MPBF) is generated, which can be utilized as the frequency selector of the proposed OEO. A tunable OEO can be obtained by changing the injection parameters. The experimental results show that this scheme is feasible and the frequency tuning range of the OEO is only about 10 GHz because the bandwidths of the photo-detector and the low noise amplifier used in the experiment are limited. However, the tuning range can be further improved by employing those devices with larger bandwidths.In ROF systems with centralized light source, no specific light source is required in the base station (BS). In fact, the light sources are centralized in the central office (CO). Thus, ROF systems with this configuration are low-cost and flexible to manage, which can satisfy the demand for high bit rate and large bandwidth communication. In this situation, we proposed a full-duplex ROF system with centralized light source and bidirectional fiber transmission based on optical sideband reuse. A single dual-parallel Mach-Zhender modulator (DPMZM) is applied in the CO to simultaneously generate two first order sidebands and modulate the optical carrier by the downstream data. In the BS, the optical carrier and one of the sidebands are used to generate the downstream RF signal, and the other sideband is used as the upstream light source. The proposed ROF system is simple and compact, because it uses only one DPMZM in the CO and avoids the frequency up/down-conversions in the BS.A full-duplex ROF system with bidirectional transmission through 20 km single mode fiber (SMF) is established. The system performance is investigated through bit error rate (BER) and error vector magnitude (EVM) measurements, and the results verify that the proposed architecture is a good candidate for future high-speed and low-cost ROF networks.