| The control of the speed of a light signal in an optical fiber, especially the reduction of light group velocity (namely slow light) is extremely useful due to its broad applications in all-optical information processing, optic sensor, microwave photonics and so on. Thus an extensive investigation on slow light in an optical fiber has important theoretical significance and application value. Recently, slow light based on stimulated Brillouin scattering (SBS) in an optical fiber has become a hot topic, which has many prominent advantages over other slow-light technologies, such as seamless integration with fiber-optic systems, flexibility of wavelength, controllable delay time and room-temperature operation.However, due to the narrow Brillouin gain bandwidth, the influence of high-order dispersion and gain saturation, it is difficult to obtain large time delay, and the delayed signals suffer large distortion, which are regarded obstacles to its practical applications. To solve these problems, this dissertation addresses these issues by concentrating on two aspects, i.e. the suppression of distortion and the enhancement of time delay. On one hand, the distortion mechanisms of a single pulse, the data stream with different modulation format, as well as periodic signals in the intrinsic Brillouin gain spectrum with Lorentzian profile are thoroughly investigated by employing the small-signal analysis method and the finite difference method to solve the SBS slow light coupled wave equations, and several methods for reducing distortions in SBS based slow light systems are proposed. On the other hand, the Brillouin comb spectrum and a cascaded Brillouin slow light system with the nonlinear optical loop mirror (NOLM) have been applied to improve the delay performance. The main contributions of this dissertation are listed as follows:(1) In SBS-based slow light, the pulse broadening characteristics of a single pulse were studied, while the amount of pulse broadening was measured by the root-mean-square (RMS) width of the pulse. With the help of the frequency domain expression of RMS, the pulse broadening associated with the variations in spectral amplitude and spectral phase of the pulse was calculated firstly when the signal carrier is aligned with the gain peak; the contribution of the two broadening factors to pulse broadening was clarified under various conditions. Then, the effects of the signal carrier deviates from the center of the gain peak and the optimized Brillouin gain spectrum on two broadening factors were further analyzed. In the case of a monochromatic pump field, the spectral-amplitude-distortion-induced pulse broadening plays a dominant role in most cases when the signal was located in the center of the Brillouin gain spectrum. When the spectrum width of a pulse is much larger than that of the Brillouin gain spectrum and the Brillouin gain is in small-signal regime, the spectral-phase-variation induced pulse broadening becomes the dominant one. In the small-signal regime, the pulse broadening can be suppressed effectively by detuning the Brillouin gain spectrum away from the signal carrier, which results from minor variations in spectral amplitude of the signal. Taking Lorentzian, Gaussian and super-Gaussian profiles of the Brillouin gain spectrum as example, the amount of distortions on the spectral amplitude and the spectral phase of the signal is the least simultaneously using the second-order super-Gaussian-shaped Brillouin gain spectrum for the same spectrum bandwidth, leading to the smallest pulse broadening is achieved.(2) The distortion mechanisms of the RZ-OOK with a50%duty cycle in SBS-based slow light system were studied when a monochromatic pump field is used. To reduce the related distortions, then different distortion-suppression methods were proposed and numerically studied in terms of the eye-opening penalty (EOP), the peak-delay variances of different "1" bits and time delay. The results show that the distortion of data stream mainly attributes to two aspects, the pulse-broadening-induced intersymbol interference and the gain fluctuation caused by pump depletion. An advanced modulation format with a narrower spectral width can effectively suppress the intersymbol interference for a given bit rate, which is beneficial to reduce the peak-delay variances of different "1" pulses and to increase time delay. At the same bit error rate, the modulation formats with lower peak power are suggested to mitigate the EOP at higher pump level due to the gain saturation is effectively suppressed; while for a given pump power, a pump field with multi-line spectrum can improve the EOP and the peak-delay variances of different "1" bits at the cost of certain time delay.(3) In accordance with the spectrum characteristics of the periodic signal, an optimized Brillouin comb spectrum with the ability to delay or advance periodic signal without distortion was designed, and the corresponding mathematical expressions were derived. For any periodic signals, the intensity and the position of each frequency component in a comb pump can be obtained conveniently by following the derived equations when the time delay is specified. Compared to the equally-spaced Brillouin comb spectrum with the same number of spectral lines, the optimized Brillouin comb spectrum removes the fluctuation of the extinction ratio during the delay process.(4) A novel scheme to generate wideband-tunable comb spectrum signal was presented and experimentally demonstrated, where a comb spectrum signal with-50GHz bandwidth and a constant line spacing as large as the Brillouin frequency shift was obtained. The experimental setup of the comb spectrum signal generator is characterized by simple configuration and flexible wavelength. By combining multiple comb spectrum signals, the spectrum width and line spacing of the synthesized signal can be flexibly adjusted. The delay performance of the quasi-optical storage technology with the experimental obtained comb spectrum signal was numerically studied. The results indicate that data stream with high bite rate can be effectively delayed by employing the obtained comb spectrum signal, that the obtained delay-bandwidth product is much larger than1, and the quality of the delayed signal can be effectively improved by reducing the intensity differences among the comb lines.(5) Aiming at enhancing time delay, the application of a nonlinear optical loop mirror (NOLM) in the SBS-based slow light cascaded system was proposed, and the delay and distortion characteristics of both a single pulse and data stream were investigated. For the single pulse, the NOLM can inhibit pulse broadening effectively in cascaded SBS-based slow light system, such that the number of stage is increased and a larger time delay is obtained. As a fixed relative delay is specified, the amount of pulse broadening of the output pulse can be modified by adjusting the pump power of each stage, and zero-broadening delay according to FWHM width can be realized at some specified parameters. Chirps induced by NOLM are imposed to a compressed pulse, and the chirp in the leading edge of the pulse is a larger than that in the trailing edge, which could be declined by adopting high power pump field. The optimization of the profile of the Brillouin gain spectrum in the cascaded Brillouin slow light system can not only increase time delay, but also improve the output profile of the compressed pulse via the NOLM. For the data stream, although the NOLM can effectively compensate pulse broadening, it increases "1"-level splitting, so the number of stage is limited."1"-level splitting can be greatly reduced by detuning Brillouin gain spectrum away from the signal carrier frequency, which is conducive to the multi-stage cascade to increase time delay. |
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