Research On The Stimulated Brillouin Scattering Amplification Of Weak Echo Signal In Optical Fibers

Weak optical signal detection embedded in strong background noise remains a big challenge in optical detection,such as real-time monitoring of laser welding,daytime operating lidar,intelligent driving and so on.A high-gain and frequency-selective amplification method for weak optical signal based on stimulated Brillouin scattering(SBS)in a single mode fiber is proposed.The proposed method combines the nonlinear amplification from SBS,the narrow Brillouin gain linewidth and lower pump power,which extends the amplification ability of weak signal in a great amount.In this dissertation,theoretical and experimental studies are carried out to solve the two key problems existing in this technology,i.e.: how to get high gain and low noise amplification in optical fiber and how to achieve low distortion amplification.We theoretically study the SBS process in single mode fiber based on the SBS coupled wave equations.A time-domain Brillouin amplification model including distributed noise is established and relevant research of SBS amplification is carried out by using this model.It provides theoretical support for the experimental study of closed-loop SBS amplification in chapter 3.Moreover,by taking the Fourier transform on the equations of SBS,a theoretical model of Brillouin gain spectrum in the frequency domain is obtained.The theoretical model of wideband Brillouin gain spectrum is obtained which supports the experimental study in chapter 5.The narrow-band SBS amplification process under single-line pumping and the pulse-domain waveform distortion correction process under discrete multi-line pumping are numerically studied in single-mode fiber,respectively.Using the time domain model,we numerically study the dependence of the Brillouin amplifier performance including the signal gain,the signal-to-noise ratio(SNR)on physical parameters(pump power,Stokes peak power,Stokes pulse width,and fiber parameters),obtaining the conditions for optimizing the fiber Brillouin amplifier performances.Researches show that the high gain and spectrum-selectivity Brillouin amplification can be realized with reasonable choice of physical parameters;Using the frequency domain model,we numerically study the influence of the number of discrete pumping lines and the spacing of adjacent spectral lines on the Brillouin gain spectral bandwidth.On this basis,the frequency spacing between the discrete spectral lines is fixed,and the effects of different gain spectral bandwidths on the distortion characteristics of the pulse waveform during SBS amplification are studied.Studies have shown that the signal gain increases as the injection pump power and the pulse width of the signal light increase,the signal gain decreases as the peak power of the injected signal light increases,the signal-to-noise ratio(SNR)decreases as the injected pump power and signal pulse width increase,the SNR increases as the peak power of the injected signal light increases;With discrete multi-line pumping,the waveform distortion during SBS amplification can be effectively suppressed.The higher the correlation between the Stokes optical spectrum and the Brillouin gain spectrum of fiber is,the higher the high gain amplification of weak signal will obtain.In order to achieve high gain imaging of weak signal under strong light background noise.The experimental study of SBS amplification in closed-loop fiber is carried out,and the influence of parameters such as pump light,signal light and medium on the amplification characteristics of SBS is studied to obtain the experimental parameters of weak signal high gain amplification.In closed-loop experiment,a 430 n W(peak power)pulsed signal is amplified by 107 with a SNR of 14 d B.By contrast,the experimental results are agree well with the theoretical simulations.In open-loop experiment,a 430 n W(peak power)pulsed signal is amplified by 106 with a SNR of 11 d B.Results show that the proposed amplifier under the condition of closed-loop or open-loop can realize weak signal amplification with high gain and low noise.Therefore,this paper takes the target image imaging under the background of strong light background as the application background.By directly detecting the scattered light signal of the target,a narrow-band SBS imaging experiment is carried out,and a clear image of the target under the strong light noise is successfully obtained.In order to realize small pulse distortion,The SBS amplification with multi-frequency intensity modulation to generate broadband pumping is studied.Using this method,the effects of Brillouin amplification and waveform distortion for 50-ns pulse are studied under the Brillouin linewidth of 200 MHz.The results prove that the method of generating multi-line spectrum is valid for achieving low distortion with high gain.The SBS amplification and waveform distortion characteristics of 5-ns pulse in the Brillouin gain spectrum bandwidth of 419.5MHz and 600 MHz are studied,respectively.For the 5-ns Gaussian Stokes pulse,the corresponding frequency domain width(419.5MHz)at 2% of the peak of the frequency domain is selected as the bandwidth of the broadband pump light,which is a reasonable value that can ensure the lowest waveform distortion and ensure the highest signal-to-noise ratio.A wideband SBS imaging experiment is carried out,and a clear image of the target under the strong light noise is successfully obtained.