| The electro-optic intensity waveguide modulator based on Mach-Zehnder interferometer has the advantages of high modulation rate, low insertion loss, low driving voltage, small chirp and weak wavelength dependence, which gradually becomes a kind of important high-speed modulation devices and plays an irreplaceable role in the DWDM system, optical pulse modulation system and Brillouin distributed fiber sensing system. However, the modulator has the problem of working points drifting which causes serious impact on its performance and greatly restricts its applications.To solve the problem and promote the practical engineering of MZ-type electro-optic modulator, the thesis proposes and studies several working point control schemes. The improvement of performance in practical applications are verified by experiments. The main research of this paper includes:(1)The working principle and modulation characteristics of the MZ electro-optic modulator and dual parallel Mach-Zehnder modulator(DPMZM) are introduced in detail. The frequency shift characteristics of both are analyzed and the expressions of each optical sidebands intensity are deduced. The reasons that cause working points drifting are summarized and the effect of drifting on applications is analyzed.(2)Working points control schemes based on phase-locked on first harmonic of the dither and contrast between harmonic and average power are studied. The schemes based on harmonic operation and phase-locked on second harmonics of the modulation are proposed. The feasibility and reliability of the these control schemes are verified in experiments. Finally, we realize the automatic control of Peak point, Null point, Quad ± point. Under microwave modulation, any working point control is also realized. The control accuracy k ? 10 mV.(3)The time-division-circulation working points control method for DPMZM is proposed and the control program is written based on Labview. By introducing the dither and scaning these bias at different time, the bias of the three MZ modulators are controlled automatically. Finaly, the single sideband signal after frequency shift with a SMSR of 25 dB is obtained.(4)The control systems are applied in equal-amplitude optical frequency combs(OFCs) generating system, optical pulse modulation system and BOTDA system, and each system performance is effectively improved. In the equal-amplitude OFCs generating system with control, five equal-amplitude sidebands can be obtained when C=1.84 by MZ electro-optic modulator. The standard deviations of the fundamental and first-order harmonic are 0.08 dB and 0.11 dB, respectively. The flatness of the OFCs is 0.35 dB. In pulse modulation systems, stable and high extinction ratio(ER) optical pulse is obtained using the working point control system. The pulse average ER is 23.22 dB and standard deviation is 0.39 dB within 2 hours. In the BOTDA system which MZ electro-optic modulator modulates the bump light and DPMZM shifts the frequency of probe light, the influenceof working points drifting on the system signal and Brillouin gain spectrum is analyzed. Experiment results show that the application of working point control can effectively improve the SNR, sensing accuracy and sensing distance of system. |
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