Quantify The Basic Research, In All-optical Analog-digital Conversion

With the development of science and technology, digital technology has been improving tremendously in the past decades, high-speed ADC has been deeply studied which has wide uses in high-definition television, medical imaging, radar, digital instrumentation, communications and other fields. However, the sampling rate and the effective number of bits (ENOB) of electronic ADC are limited because of the inherent electron mobility restriction, which is considered as a bottleneck when applied in the ultrahigh-speed fields. Optical ADC has attracted much attention recently to overcome the disadvantages of electronic ADC, which can achieve both the high speed and resolution.In this thesis, the key technologies of quantization in all-optical analog-to-digital conversion and the transmission characteristics of ultrashort optical pulse in fiber guided by the generalized nonlinear Schrodinger equation (GNSE) were discussed firstly. Then, based on the studies of soliton self-frequency shift (SSFS) and spectral compression of femtosecond pulse in highly nonlinear optical fiber (HNLF), the physical model of quantization on the basis of the transform between peak power and wavelength of pulse was established and experimental verification system was designed to complete the quantization of high-speed sampled signals. Moreover, the spectra of wavelength shifted pulses have been effectively compressed to improve the resolution of quantization.In the experiment of quantization, a passively mode-locked fiber laser was made based on nonlinear polarization rotation effect as a pulse source of quantization, which can export femtosecond pulses with the magnitude of peak power up to kilowatts near 1550nm. Because of the nonlinear effects (especially SSFS) when pulse was propagated in a HNLF with anomalous dispersion, the center-wavelength of pulse will be shifted to longer side following an energy transfer, the shift range can be up to 150nm. As the amount of wavelength shift depends on the peak power of incident pulse, pulses with different wavelength represent various values of input pulse intensity, the direct relationship between them can be used to complete quantization process. Then, the wavelength shifted nonchirped femtosecond pulses were propagated in another section of HNLF with anomalous dispersion, spectra have been greatly compressed by controlling the injected energies, a maximal compression ratio of 4 and a total resolution of 4-bits can be obtained.Pulses propagate in HNLFs with different dispersion curve have been investigated in the experiment to analyze how can the parameters of optical pulse and fiber affect the results of quantization. The evolution characteristics of pulse in time and frequency domain were simulated and compared with experimental results by numerically solving the GNSE using split-step Fourier method, which can be used to optimize the experimental system.