| In the ROF (Radio over Fiber) system, the effect of fiber dispersion on the performance of the system is critical. To improve the anti dispersion performance of the ROF system, the common method is the transmission of SSB or OCS optical millimeter-wave signals. Thus in this thesis the ways to generate these two kinds of signals based on the dual parallel MZM and the cascade MZMs are investigated. With the modulation based on these two structures, we not only can realize SSB modulation or OCS modulation, but also can get multi-frequency millimeter-wave signal, to improve both the spectral efficiency and the transmission performance of ROF system.Based on DPMZM, the frequency doubled millimeter-wave signals and frequency quadrupling millimeter-wave signals are generated by SSB modulation and OCS modulation, respectively. In these two forms of modulation, the30GHz and the15GHz RF signal is used respectively, the generated millimeter-wave signals with the frequency of60GHz will transmite through the optical fiber. And in the OCS modulation, one optical sideband signal is used to carry data, while the other optical sideband is used to carry the uplink data to achieve full-duplex systems. The eye diagram and BER curves at different fiber transmission lengths are simulated is shown that:the optical signal generated by SSB modulation own a good transmission performance; while the optical signal generated by OCS modulation, even though can overcome the power periodical attenuation effect caused by dispersion, but the bit walk-off effect induced by dispersion has a greater impact on the transmission performance. Analysis showed that the use of the single optical sideband carrying data signals can reduce the influence of dispersion and improve the transmission performance of the system. In addition, the use of multi-frequency technology and full-duplex structure can reduce the cost of the system.Based on the cascaded MZMs, the optical signal generated by the OCS modulation has a pure spectrum (only±4order optical sidebands are contained), theoretically there is no power periodic attenuation effects due to dispersion. Meanwhile. the baseband signal is only modulated on one optical sideband, and the other optical sideband is used repeated to carry an uplink data signal. In the simulation, with the use of lOGHz RF signal, the80GHz millimeter-wave signal is generated, and the eye diagram and BER curves in different fiber transmission distant of the uplink and downlink signal are analyzed. The results show that: the generated optical signal can overcome the power periodic attenuation effects and bit walk-off effect which both caused by dispersion, so they have good transmission performance, and suitable for long distance transmission. |
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