| The slow light technology based on stimulated Brillouin scattering in optical fiber exhibits several advantages, such as low power threshold, continuously adjustable delay, room-temperature operation, harmonize with optical fiber communication systems. It contributes to the potential applications in the field of tunable optical delay line, the "true delay" of microwave signal etc. However, the intrinsic bandwidth of the Brillouin gain spectrum is narrow and the Lorentz shape make the delayed signal be often accompanied with the distortion. Thus, it is of important practical significance to study the mechanism of delayed signal distortion in different shape of the Brillouin gain spectrum, and to explore effective solutions.There are already several ways to broaden the Brillouin gain spectrum. For example, one can directly modulate pump source by pseudo-random code or Gaussian noise, or broaden the pump source by the superposition of multiple Brillouin gain spectrum. But both of them are limited to the current experimental conditions, therefore, it is difficult to achieve broadband pump light with wider and more lines. Another method is by using the broadband light as pump lights, but the ideal broadband rectangular spectrum does not exist in practice, and gain hole burning will be occurred when the pump intensity is relative large. Considering the problems for the methods mentioned above, this thesis focuses on the application scopes and limiting factors of several Brillouin gain spectrum broadening technologies, and the delay performance of output Stokes signal in different shapes of broadband Brillouin gain spectrum. The main contents including:1. Continuous light source is modulated in M-Z(Mach-Zehnder) intensity modulator to produce multi-line pump light. If the modulation signal is multi-line signal and the spectrum of the continuous optical wave light source is Gaussian spectrum and rectangular spectrum, respectively. The results show that the modulation signal broadened the spectrum of the pump lights, and the output pump light obtains a flat-top spectrum through modulating the continuous light source with rectangular spectrum.2. To solve numerically the light waves-sound waves coupled equations, the finite difference method is used. Firstly, the distortion of output Stokes signal after the interaction of the pump light with rectangular spectrum and Stokes signal are analyzed. Furthermore, under the conditions of pump light with Gaussian wide spectrum and rectangular wide spectrum, we have analyzed specifically the effect of the Brillouin gain spectrum bandwidth and top flatness on the distortion of output Stokes signal. The results showed that:due to the frequency detuning between optical frequency of the Stokes signal and its center frequency, frequency shift and pulse compression are occurred for frequency domain Stokes in the process of Brillouin scattering. Besides, a flatter and wider gain spectrum can be obtained for rectangular wide spectrum of pump light, and the distortion of the Stokes signal can be reduced at the flat gain spectrum3dB bandwidth range. When the Stokes signal pulse width is even wider, the distortion of output Stokes signal is lower when the rectangular spectrum of the pump power is lower.3. Under the effects of double Gaussian spectrum broadband pump lights and single Gaussian spectrum broadband pump light, the distortion and delay of output Stokes signal are compared. The results show that double broadband Gaussian pump lights can construct a flat top gain spectrum. Moreover, when the intensity ratio of the double broadband Gaussian pump is2. both phase distortion and amplitude distortion reach minimum in the3dB bandwidth range of the flat gain. Besides, for the case of double Gaussian spectrum pump lights, when the bandwidth and intensity of stokes signal are fixed, adjustable delay can be realized through changing the light intensities ratio of two pump pulses, and larger delay is preferred for greater intensity ratio. |
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