Stimulated Brillouin Scattering Theoretical Modeling And Experimental Study

Underwater detection systems based on the stimulated Brillouin scattering of laser not only can discover objects with no reflection, but also can be fixed on low level aerocrafts because of the small size of the system, allowing fast scanning of a large area to be realized. In addition, the system owns such advantages as strong abilities of resisting electromagnetic disturbances and water sound noises, which surpasses sonars and traditional lidars a lot.In order to explore the best parameters of the pump to insure high energy reflectivity as well as high SNR, but without optical breakdown to insure safe performing, it is necessary to study the variation law of the reflectivity versus the pump energy as well as that of the threshold of SBS versus systematic parameters. Hence, it is essential to construct the theoretical model of SBS and find the numerical value of the coupled wave equations. Based on this, the article is divided into two parts: theoretical study and numerical simulation of SBS, and experimental study and comparison.In the part of theoretical study, we will begin with the coupled wave equations which describe the interactions of the pump, the sound and the Stokes wave in the process of SBS. The equations are then simplified according practical conditions, and one-dimension theoretical models are obtained aiming at unfocused pump as well as focused pump. Then the coupled equations are turned discrete using the method of implicit finite difference in terms of time and backward difference in terms of space. After this, we can then make programs to find the numerical values of the equations, and further explore the variation laws of the energy reflectivity and threshold energy, so that we can find the basic laws of SBS underwater.In the condition of unfocused pump, it requires a high pump energy to obtain the threshold intensity of SBS, so what we get is primarily spontaneous Brillouin scattering with a low intensity of backscattering. In order to increase the pump density in the SBS cell and make the occurrence of SBS easier, focusing of the pump wave is quite essential, which is also the stress of the article. Under focused pump conditions, the coupled wave equations need to be improved in order to comply with the transmission laws of light wave. In terms of this problem, most researchers simply divide the amplitude with the square root of cross section of the beam without giving any reasonable explanation. The article deduces the coupled wave equations with focused pump according the energy transmission law during the propagation of light, and then builds up the theoretical model for numerical computing. The results of the model prove to fit well with practical ones.In the experimental study, precision light paths are designed, and a series of experiments are performed using the pulsed laser beams of 532nm frequency doubled from 1.64nm which is generated by a single vertical mode laser. Obvious SBS phenomena of the laser beam getting into water are observed. Changing the pump energy and the focus of the lens, we find the basic laws of SBS in practice. Comparing the experimental results with those of the numerical computing, we discover that they agree with each other very well in terms of their variation trend.Concerning the underwater detection based on the SBS of laser, we have designed a focusing optical system, making the focusing depth variable by changing the distances of the lenses. In the experiment, we also designed a novel scheme to separate the pump wave and Stokes wave, and receive the signals using a PIN tube combined with a Fabry-Perot etalon, and display the pulses on a digital oscillograph. As a result, the SNR is largely improved. In this way we can detect reflected light from different planes; according to the appearance of the scattered wave we can judge the existence of underwater objects. Furthermore, we can compute the distance of the objects from the reference plane by recording the numbers of the oscillograph.In terms of the calculation of the frequency shift according to the interference stripes acquired using the Fabry-Perot etalon, many researchers get the results simply through the proportional relationship of the distances between stripes; others using a complex and troublesome method called edge detection. In this article a new method is introduced, making the calculation of the frequency shift more convenient but more accurate.