Investigation On Flat-top Narrowband Brillouin Gain Spectrum And Low Distortion Filter Amplification

The fundamental and application investigation of stimulated Brillouin scattering(SBS) has been the focus in the field of academy, especially in the high sensitivitydetection of weak laser signals in the field of strong background field. Based onBrillouin amplification technology, narrowband filter amplification of weak opticalsignal can be achieved, and the optimization and measurement of Brillouin gainspectrum (BGS) and the low distortion Brillouin amplification of the signal wave areof great importance. However, Brillouin gain spectrum in the single mode fiber isusually the Lorenz-shaped, which will cause the non-uniform amplification of thesignal, resulting in signal distortion. Aiming at this problem, in this paper, we putforward a method to acquire the narrowband flat-top BGS using unequal-amplitudepump lines, and hence realize the low distortion filter amplification.Based on the theoretical model of multiple-line gain spectrum, we numericallystudy the influence of the ratios of the sideband intensity to the carrier wave intensityand the frequency separations between adjacent spectral lines on the BGS for thecase of intensity modulation. We also calculate the BGS for the equal-amplitude andunequal-amplitude pump under the conditions of the single-frequency andmultifrequency phase modulations. And then we set up the experimental system. TheSBS threshold and BGS without the pump modulation are measured, and thesimulation results are experimentally verified. The results show that, when the pumpspectrum only includes the first sidebands, and the frequency separation is1.2timesthe bandwidth of no pump modulation, the flat-top BGS is achieved. When the pumpspectrum includes the first sidebands and the carrier wave, and the intensity of thefirst sidebands is1.8times that of the carrier wave in the pump spectrum, we alsoachieve a flat-top BGS. By controlling the ratio of the sideband intensity to thecarrier wave intensity and the frequency separation between adjacent spectral lines,we can obtain flat-top BGS for the single-frequency and multifrequency modulations.In phase modulation experiment, we obtain the flat-top gain spectra with the bandwidths of40MHz and125MHz, and with the top fluctuation of less than0.21dB.Using theoretical model of multiple-lines pump Brillouin amplification, wenumerically simulate the spectra of amplified signals of different spectral flat-topBGS bandwidths. Based on the pump phase modulation mentioned above, wepreformed low distortion Brillouin amplification experiment from two aspects oftime domain and frequency domain. The results show that low distortion pulse andspectrum amplification can be simultaneously achieved only when the flat-top widthof the BGS is equal or slightly greater than the probe signal bandwidth.