All-optical Quantization Based On The Raman Self-frequency Shift And Spectral Compression

Analog-to-digital converters (ADCs) which convert analogue signals into digital ones, play an important role in the signal processing system. With recent tremendous growth of the high-speed digital technique, the effect of the ADCs on the high-speed communications, real-time measurements, radar systems, image processing, and space communications enhances. And the ultrawide-bandwidth applications also encourage the demands of high-speed and high-resolution ADCs. Recently, electrical ADCs are most widely used in practical applications. But to electrical ADC, the enhancement of performance is limited by its electron mobility. Superconducting material ADCs need to work under cold condition, which limits their applications. All-optical ADCs can overcome the disadvantage of the electrical and superconducting material ADCs. All-optical analog-to-digital conversion, which is characterized by high-speed and high-resolution will be extremely beneficial to signal processing systems in future. And all-optical quantization is a key technology to the realization of the all-optical ADCs. The researches on the all-optical quantization are highly significant. The works presented in the dissertation focus on the investigation of all-optical quantization based on the Raman self-frequency shift and spectral compression. The main contents of the dissertation are shown as follows:(1) Research backgrounds of the all-optical ADCs and all-optical quantization are simply introduced. The development history and status of the all-optical quantization at home and broad are reviewed.(2) An all-optical quantization configuration is constructed with highly nonlinear fiber (HNLF), dispersion-increasing fiber (DIF) and arrayed waveguide grating (AWG). Firstly, the Raman self-frequency shift (RSFS) occurs as pulses with narrow pulse widths and high powers propagation in the HNLF, which can realize the conversion from power to wavelength. Where, the RSFS is the mainly part of the all-optical quantization. Then, the spectra of the shifted pulses are compressed as propagating along the DIF, which can effectively improve the resolution of the all-optical quantization. Finally, the AWG separates the compressed pulses, which prepares for the following coding process.. Generalized nonlinear Schr?dinger equation (GNLS equation) is used to describe the propagation of the pulse in fiber. In this dissertation, the inference of the GNLS equation is shown. And the split-step Fourier method, the mostly used numerical method to analyze the GNLS equation, is depicted.(3) Pulse propagation in an optical fiber is analyzed through using the moment method to solve the evolution equation of the pulse parameters, such as pulse width, chirping, energy, time delay, and frequency shift. More accurate expressions are obtained by modified the mistakes in the reference by J. Santhanam and G. P. Agrawal. And the results before and after modification are contrasted for the case of subpicosecond pulses injected.(4) The physical origin of the RSFS in fibers is related to the delayed nature of the Raman response. So the RSFS depends on not only the pulse characteristics, but also the propagating fiber.The RSFS of subpicosecond pulses in three different fibers, viz. photonic crystal fiber (PCF), polarization maintaining fiber (PMF) and commonly HNLF, are detailedly investigated. When the shape, width and peak power of input pulses are different, the output pulses after the RSFS are compared. And several rational suggestions are given for choosing suitable fibers. Experimental results about the RSFS in two different HNLFs are presented. By changing the input peak power, the frequency shift of 140nm can be obtained.(5) Based on the characteristics of the Raman solitions origined from the Raman self-frequency shifted pulse, the spectral compression processes of subpicosecond solitons are firstly described. The fundamental solitons are injected into the DIF as input pulses. Five different DIFs, viz. linear-type, exponential-type, Gaussian-type, hyperbolic-type, and logarithm-type DIFs, are used to compress the spectra of fundamental solitons. The evolutions and spectral compression in five DIFs are investigated. The results show that the linear-type, exponential-type, and logarithm-type DIFs are more suitable for compressing the spectra of the shifted pulses.(6) The performance of the all-optical quantization system composed of PCF, DIF, and AWG is analyzed as subpicosecond pulses input. The feasibility of the suggested all-optical quantization scheme is testified. Under ideal condition, the time delay of soliton is derived first time by analyzing the GNLS equation using the moment method. And the influence on the sampling velocity from the time delay which is accompanied with the RSFS and spectral compression is discussed.