Investigation On The Technique And Spectral Characteristics Of High-Amplification Low-Noise Brillouin Amplification

Aiming at the problems in the detection systems operating under the conditions of strong background illumination such as the submarine laser communication, lidar system working by day and so on, a new approach to raise detection sensitivity and signal noise ratio(SNR) by using spectrum-selectivity Brillouin amplification is presented. Based on this background, the main problems including the noise in the Brillouin amplifier, the threshold value of stimulated Brillouin scattering(SBS), performance parameter optimization, Brillouin gain spectrum and amplified signal spectrum are investigated theoretically and experimentally. A new method of double-stage non-collinear Brillouin amplification with high amplification and low noise is put forward. Combined with practical applications, Brillouin amplification of weak Stokes signals from water is studied.Firstly, a set of coupled-wave equations of Brillouin amplification with frequency detuning are derived. Considering the competition between the distributed noise and input signal in a Brillouin amplifier, a theoretical model of the transient non-collinear Brillouin amplification including distributed fluctuating noise is constructed. Moreover, by taking the Fourier transform on the equations of Brillouin amplification with frequency detuning, a theoretical model of Brillouin amplification in the frequency domain is obtained.Based on the transient SBS coupled-wave equations including the distributed fluctuating source, the noise and SBS threshold characteristics of a Brillouin amplifier are investigated. Researches show that the threshold gain for quasi-transient SBS increases with increasing the interaction length and the pump wavelength. A new method for measuring SBS threshold by an optical limiting in the transmitted pump energy is presented. These results can provide the basis for designing the optimum working point of the amplifier.We numerically study the dependence of the amplifier performance including the SNR, the sensitivity, the signal amplification factor(SAF) on physical parameters(signal intensity, delay time, exponential gain, and medium parameters) by the theoretical model of transient Brillouin amplification including distributed noise, obtaining the conditions for optimizing the amplifier performances. Experimental verifications are performed. To remove the ABS noise and further raise the SAF, we present the scheme of double-stage non-collinear Brillouin amplification and prove its feasibility. The design principle to achieve high-performance amplification is obtained. Combined with the conditions for achieving the optimum performances,the amplifier performances can be effectively improved by properly choosing the intersection angle between the pump and signal beams, the medium length, the pump energy and the beam diameters. In experiment, CS₂ was used as the amplifier and the generator medium. For an input signal of 5.5×10^-14J, an overall SAF of 6×10¹¹ with 10³ SNR was obtained.Based on the coupled-wave equations of Brillouin amplification in the frequency domain, the theoretical describtions of the dependences of the lineshape and linewidth of the Brillouin gain spectrum(BGS) on the exponential gain are investigated and perfected. The mathametical expression for calculating the BGS linewidth is obtained. The formula holds true for two cases involving low gain and pump depletion. By studing amplified signal spectrum, we find that when the frequency of a weak wide-linewidth signal is shifted in the amplification bandwidth, the frequency shift of the amplified beam is independent of the frequency shift of the input signal, and is close to the Brillouin shift of the amplifier. These results can instruct the detection and processing of the amplified signal.Finally, based on the application background of detecting submerged objects by Brillouin lidar, we propose an approach of selective optical amplification of weak scattering signals from water based on Brillouin amplification. Researches show that weak Stokes signals arising from water can be efficiently amplified by using CS₂ or CS₂/CCl₄ mixture as the amplifier medium. In experiment, the mixture of 90% CS₂ and 10% CCl4 was chosen as the amplifier medium. A Stokes signal of 8.6×10^-14J from water was amplified with an overall SAF of 10¹¹ and 10³ SNR by using the double-stage non-collinear Brillouin amplification system.