Research Of Photonic Time-Stretched ADC

As a bridge between the analog and digital world, the traditional electronic analog to digital converter is limited by one or more of the following problems:(1) jitter in the sampling clock; (2) settling time of the sample-and-hold circuit; (3) speed of the comparator or comparator ambiguity;(4) mismatches in the transistor thresholds and passive component values. Due to the limitations of electronic bottleneck, the higher frequency is the more severe limitations are. Recently, researchers do a lot of work aimed at the limitations, but the use of pure electric field digital signal processing to capture the high speed and large broadband signal, many limitations.still exist. With the aid of photonic time-stretch assistive as a new type of electro-optic ADC, the RF signal, through the time-stretch, will be slowed down and captured below the Nyquist sampling rate, then breakthrough the limitation of the electronic bottleneck.PTS-ADC is completed through the following steps:First, the transform-limited optical super-continuum pulse is transformed through the first period of the dispersion medium, and linear chirped; Second, RF signal is modulated on the chirped pulse through the electro-optic modulator to complete time-to-wavelength mapping; Third, realize the time-stretch of RF signal in time domain to slow down, while the chirped pulse with RF signal is time stretched through the second period of dispersion medium to complete wavelength-to-time mapping; Last, the slower RF signal is captured below the Nyquist sampling rate by electronic ADC.The propertie of dispersion medium is one of the important factors that decides the sampling rate and input bandwidth of PTS-ADC. To this end, using photonic crystal fiber with a hybrid structure of dual-concentric core and depressed-clad as PTS-ADC’s dispersive medium, a solution is proposed in this dissertation. By the advantage of the fiber’s high dispersion value and low loss, the PTS-ADC’s performance can be improved, not only has higher sampling rate, larger input bandwidth and lower loss, but also is simpler. For this reason, this paper started from the basic theory and work principle of PTS-ADC, discussed the noise characteristic of the system, completed the mathematical formula derivation, established the mathematical model, and completed simulation and comparative analysis about PTS-ADC based on double sideband modulation and single sideband modulation on dispersion penalty, residual phase distortion, and time-bandwidth product. Moreover, in view of the existing photonic time-stretched RF with small power, detection is a difficult problem, the coherent detection and envelop removing technique to improve the whole fidelity of RF signal methods are discussed.