| High-speed and high-resolution analog-to-digital converters(ADCs)with a large input bandwidth are essential devices for digital signal processing,which are widely used in both civil and military applications such as ultra-broadband wireless communication,biomedical imaging,high-speed real-time oscilloscope,wideband signal acquisition,ultra-wideband radar and electronic reconnaissance.Photonic time-stretch ADCs,which utilize photonic time-stretch technique to reduce the frequency and to compress the bandwidth of input analog signals before their digitization,can greatly enhance the sampling rate and the input bandwidth of the subsequent electronic ADCs,and can meanwhile reduce the influence of the time jitter on the conversion accuracy.Hence,it is recognized as one of the most promising techniques to realize high-speed and high-resolution digitization of a wideband signal.However,the signal distortions introduced by the nonuniformity of the pulse envelope and the nonlinearity of the electro-optic modulation inevitablely degrade the conversion accuracy.Besides,the dispersion power penalty puts a stringent limit to the time-bandwidth product,which contraints the input bandwidth enhancement.Therefore,it has great importance from both scientific and practical viewpoints to conduct intensive research in high-speed and high-resolution photonic time-stretch ADCs with a large input bandwidth.In this thesis,special focus is put on the theoretical and experimental research of how to circumvent the aforementioned key technical problems in a photonic time-stretch ADC.The main contents of the thesis are summarized as follows:(1)A broadband linearization scheme for photonic time-stretch ADCs is proposed based on an asymmetrical dual-parallel Mach-Zehnder modulator,which is aimed at reducing the nonlinear distortion produced by the electro-optic modulation.Through establishing physical model,the proposed photonic time-stretch ADC scheme is theoretically analyzed.Most importantly,the modulator parameters for achieving linearization are optimized.Numerical simulation is implemented to verify the linearization effectiveness of the proposed scheme under both single-tone and two-tone modulation,and the results are compared with those employing the existing linearization schemes.Besides,the influence of parameter deviation from optimal values on linearization performance is also discussed.The results indicate that both the even-order harmonics and the third-order intermodulation distortion spurs are effectively suppressed by employing the proposed scheme with optimal modulator parameters,where the spurious free dynamic range is increased by 19 dB compared with the scheme based on the complementary dual-output Mach-Zehnder modulator with assistance of digital linearization processing.In addition,the proposed broadband linearization scheme has a strong tolerance of parameter deviation.(2)A novel photonic time-stretch ADC scheme based on complementary single-sideband modulation architecture is proposed,which can simultaneously realize a large time-bandwidth product and suppress the signal distortion induced by the pulse envelope and the nonlinear response of the electro-optic modulation.Physical model is established to theoretically analyze the proposed scheme,and a phase compensation algorithm is introduced to correct the dispersion-induced phase distortion for a wideband signal.Numerical simulation is carried out in the case of single-tone and broadband modulation to demonstrate the validity of the proposed scheme in eliminating dispersion power penalty,suppressing modulation-induced signal distortion,removing pulse envelop and correcting phase distortion.The simulation results are compared with those of two existing schemes,i.e.,the single-output single-sideband modulation architecture and the complementary dual-output double-sideband modulation technique with assistance of digital linearization processing.Besides,the performance degradation due to parameter deviations in real application is also discussed.The results reveal that the proposed scheme is capable of obtaining a large time-bandwidth product and suppressing the signal distortion,which is favorable for realizing high-speed and high-resolution digitization of a broadband microwave signal.(3)Photonic time-stretch ADCs based on dissipastive soliton are proposed and researched.Through establishing theoretical model and numerical simulation,a dissipastive soliton-based passively mode-locked erbium-doped fiber laser with a wide and flat spectrum is obtained.By employing the designed dissipastive soliton-based optical source in a photonic time-stretch ADC and comparing with the performance of the conventional soliton-based scheme,the feasibility of utilizing its flat spectrum to reduce pulse-envelop-induced conversion accuracy degration is numerically demonstrated.Furthermore,a dissipastive soliton-based passively mode-locked erbium-doped fiber laser is set up,and the photonic time-stretch ADC based on the dissipastive soliton is experimentally studied.The results manifest that the flat spectrum is benefit for increasing the conversion accuracy,and the broadband spectrum helps to increase the time-bandwidth product.In addition,the high power spectral density enhances the signal-to-noise ratio,and the steep edge of the spectrum minimizes the overlap between the adjacent pulses after time stretch.(4)Continuous-time photonic time-stretch ADCs are researched,where a five-channel continuous-time photonic time-stretch ADC is designed based on wavelength multiplexing/demultiplexing technique.In the designed scheme,continuous-time signals are successfully stretched by employing a continuous-time linearly chirped optical carrier which is obtained by using two five-channel wavelength division multiplexers and corresponding fiber delay lines.The stretched signals,after wavelength demultiplexing by a five-channel wavelength division demultiplexer,are digitized by five electronic ADCs with a sampling rate of 3 GS/s and a nominal quantization level of 8 bits.Finally,the datas are reconstructed in the digital domain.The system parameters of the scheme are carefully designed,and the system performance is numerically simulated.Based on the numerical simulation,a prototype is developed,which can achieve photonic time-stretch analog-to-digital conversion of a 5 GHz microwave signal with a 3.84-fold stretch factor,and has an effective sampling rate of 11.52 GS/s. |
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