Study On Key Technologies Of Microwave Photonic Analog-to-digital Conversion

High-speed and high-resolution analog-to-digital converters(ADCs)are indispensable components in many modern information systems,especially for ultrawideband applications with high performance requirements,such as military radars,broadband wireless access networks,biomedical imaging and optical communications.However,the performance of traditional electronic ADCs in terms of bandwidth and time jitter is severely limited due to the so-called “electronic bottleneck”.In this context,it is a valuable research topic from an academic and application perspective to use the microwave photonics technology to improve the ADCs' performance due to the advantages of ultra-large bandwidth,ultra-low loss and anti-electromagnetic interference.Starting from the research background of ADC technology,I introduce the basic principles and development status of four types of typical photonic ADC schemes.In this dissertation,some innovative implementation structures and improved schemes are proposed and demonstrated.For the phase-shifting-based optical quantization technology,problems of low bit resolution and threshold decision error are analyzed and two improved schemes are proposed and experimentally verified.The influence of bias phase shift on the performance is further studied through the simulation.A novel serial implementation of photonic flash ADC is proposed and experimentally verified.Performance improvement,realization of non-uniform quantization and improved schemes suitable for large input signal are discussed.In addition,a photonic time stretch system based on phase modulation is proposed and the system properties are analyzed.The correctness of the theoretical model is verified by both simulations and experiments.The major innovations of this dissertation are as follows.(1)In order to solve the problems of low bit resolution and threshold decision error in the phase-shifting-based optical quantization system,an improved scheme based on the principle of linear combination and an adaptive threshold decision scheme based on the balanced detection are proposed and verified by experiments.The improved scheme based on the principle of linear combination combines the output signals of the phaseshifting-based optical quantization system through an electric circuit to increase the number of quantization channels,thereby improving the nominal bit resolution at a low cost.The adaptive threshold decision scheme uses a dual-output modulator and a balanced detector to keep the threshold voltage of the system at zero voltage,so as to avoid the threshold decision error caused by the power fluctuation of the optical sampling pulses.Through the comparative experiments of adaptive and non-adaptive threshold decision schemes,the effectiveness of the adaptive threshold decision in improving the resolution performance is verified.(2)A novel photonic flashADC scheme is proposed and experimentally verified for the first time to our knowledge.With the serial structure,it realizes the analog-todigital conversion in the optical domain,which can greatly reduce the complexity of the system.Based on the multi-wavelength sampling and the group velocity dispersion effect in the fiber,the scheme greatly reduces the number of comparators and enables the realization of non-uniform quantization.A simplified scheme that is suitable for large input signal is proposed and simulated.By properly setting the power ratio of the multi-wavelength pulses,the influence of modulation nonlinearity on the quantization process can be canceled out.To overcome the problems of low bit resolution and large maximum-minimum power ratio of the sampling pulses in the simplified scheme,an improved scheme based on a dual-output modulator is proposed.(3)A photonic time stretch system(PTS)based on phase modulation is proposed.Compared with the system based on amplitude modulation,it has a simplified structure and has higher stability.Due to the sub-octave bandwidth,the system can avoid the influence of second-order nonlinear spurs.All frequency components of the PTS system are gven by strict mathematical analysis,and the correctness of the theoretical analysis is proved by simulations and experiments.For the problem of power fading caused by dispersion,a dual-channel PTS system using a phase modulator and a push-pull intensity modulator is proposed to overcome the bandwidth limitation caused by the power fading.