Research On Modulation Distortion And Its Suppression Methods Of Electro-optical Intensity Modulators

Today the information technology is developing rapidly, especially the maturely and widely used of big data, cloud storage and broadband mobile terminals, all these need ultra-high-speed, wide-bandwidth communication systems. The ROF system which combines the advantages of high-frequency radio wave transmission and fiber optic communications make possible high-speed transmission of large volumes of RF signals and meet the ultra-broadband wireless access(greater than 1Gbps), there is no doubt that it will be the effective solution for the next generation of mobile and network. In the ROF system, the nonlinear distortion of optical transmitter, optical receiver and single-mode fiber reduce the sensitivity of the receiver, which results in a bit error rate(BER) increasing and spurious-free dynamic range(SFDR) decreasing. LiNbO3 M-Z intensity modulators are largely applied in the analog ROF system, in this case, this paper study on its modulation distortion, and mainly focus on suppressing nonlinear distortion by using the dual parallel modulator(DPMZM).First, the theoretical analysis and simulation on the transmission characteristics of M-Z intensity modulator is preformed in this thesis and the effects of bias voltage and input power changing of the harmonics of M-Z intensity modulator is analyzed in detail. Then this paper introduces the evaluation parameters of M-Z electro-optic modulation nonlinear distortion, including spurious free dynamic range(SFDR) and carrier to interference ratio(CIR). The last, the SFDR and CIR in the case of normal M-Z electro-optical intensity modulator intermodulation are tested through Experiments.Research a method which based on single-drive DPMZM to suppress third order intermodulation distortion(IMD3), theoretically analyzed the impact of the three bias voltage have on the IMD3. The simulation results showed that this method can reduce the IMD3 better than conventional M-Z electro-optic modulator. Experiment results showed that using this method, the average CIR was 15.40 dB higher than that of conventional M-Z electro-optic modulator.Proposed a method which based on dual-drive DPMZM to suppress third-order intermodulation distortion, theoretically analyzed that third-order intermodulation distortion can be suppressed when three bias voltages of the dual-drive DPMZM satisfy certain relationship. The simulation and experimental verification show that the average CIR is 18.43 dB higher than an electro-optical intensity MZ modulator. And during process of the input signal power increasing from 5dBm to 13 dBm, third-order intermodulation signals are basically limited in the noise level. Finally, design downconverter based on the double-drive DPMZM, which can produce a stable intermediate frequency signal(IF). And analyzes the impact three bias voltages of DPMZM have on the output power of intermediate frequency signal.