| Real-time wideband digitizers are the key components of many systems and applications,such as advanced laboratory instruments,biomedical imaging,industrial applications and military systems.The study of high-resolution,high-bandwidth ADCs is very important.With the rapid development of optoelectronic technology,the advantages of photonics including large bandwidth,low loss and immunity to the electromagnetic interference are widely used.Photonic analog-to-digital conversion has become the most promising technology to break through the electronic bottleneck.Photonic time-stretch(PTS)can adjust the frequency of the modulated microwave signal by changing the time-domain scale of modulated light pulse.It has arisen worldwide concerns as the most typical preprocessing technology of photonic assisted ADC.Similar to the traditional microwave photonic link,the nonlinear problems also exist in the photonic time-stretch system,which are not conducive to the i2mprovement of the system performance.Unlike the traditional model with small signal approximation,this thesis presents an accurate theoretical model of photonic time-stretch system.On the basis of this model,the nonlinear effects in the process of electronic-to-optical conversion,optical-to-electronic conversion and optical fiber transmission are systematically analyzed,and the corresponding optimization solutions are proposed.The research contents include theoretical modeling and experimental verification.The main innovations in this thesis are as follows:1.The theoretical model of PTS system is optimized,the exact expressions of harmonics and intermodulation products caused by modulation nonlinearity are presented.The design of PTS system is simplified and comprehensive based on this theoretical model,and the performance of the system is easy to estimate including including frequency response(power fading due to dispersion),3dB bandwidth,1dB compression point,system gain,noise figure,third intercept point and spurious free dynamic range,etc.2.In order to eliminate nonlinear modulation distortion,a novel broadband linearized modulation scheme based on modulators cascaded is proposed for enhancing the linearization of modulation under the condition of large signal modulation.The effectiveness of the scheme is verified by simulation experiment and the spurious dynamic range of PTS system is improved.The accurate model of PTS system using modulator with finite extinction ratio is discussed for the first time,and the distortion can be mitigated by the method of bias point predistortion.3.In order to eliminate the linear dispersion induced power fading,a signal modulation design based on parallel phase and intensity modulators is presented.Broadband flat frequency response can be achieved by using this schme and the feasibility is verified by theoretical analysis and simulation experiment results.4.The performance of system with nonlinear fiber group velocity dispersion is studied.The time-dependent amplitude attenuation model is obtained and the theoretical analysis is based on strict derivation rather than small signal modulation.To normalize the impact of 3rd-order dispersion,a ratio factor to measure the extent of time-dependent attenuation is introduced.A time-dependennt frequency transfer function is derived out for the first time.Based on the theoretical analysis,a new correction system for signal frequency chirp in digital domain is designed and implemented.Experimental results prove the proposed correction method applicable to both single tone signal and broadband signal transmissions.5.The nonlinear distortions of the system due to fiber nonlinear optics and the nonlinear response of photodetector at high optical power is dicussed.While improving the signal-to-noise ratio(SNR)of the PTS system,the optical amplifier and the method of photodetection are selected reasonably to reduce distortion. |
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