The Dynamics Of Phase Transitions In The Process Of Stimulated Brillouin Scattering

In recent years, driven by laser medicine, industrial material processing and inertial confinement fusion (ICF) and other applied research, solid-state lasers and related technology research have developed rapidly, in which the development of solid-state laser system of high quality laser beam has been pursued by the practical applicants and researchers. SBS (Stimulated Brillouin Scattering) is a nonlinear process. Beside the pulse compression feature, it also has many other features, for example, a small amount of frequency shift, phase conjugation, scattered light back transmission and high conversion efficiency and so on. For the reason , SBS can not only be used for pulse compression for high power laser output ,but also can be used to compensate the the beam phase distortion caused by the thermal lens effect of material and optical components. In this way, we can improve the beam quality, and get the beam output that spatial intensity distribution was nearly diffraction Limit high-quality. Similarly, SBS can be used to correct the wave front distortion of laser system, and at the same time we can use laser amplifier and the SBS system to get the two-way amplification effect. All in all, for the SBS, the phenomenon of pulse compression and phase conjugation phenomenon is associated with it, and therefore we can use SBS to improve the beam quality, at the same time get the purpose of shaping the compressed output pulse by changing the pump light or the relevant parameters of SBS, then obtain a narrow pulse, high power, light intensity spatial distribution of high-quality laser beam. Thus, SBS is an effective method to improve quality of the laser beam with high power and high repetition rate. In this way, the size and cost of the laser can be greatly reduced .SBS phase transition is the sudden change of the phase of the stokes laser, accompanied by dramatic changes of stokes intensity. The production of phase transition interrupters the process of SBS phase conjugation, resulting in the degree of SBS phase conjugation decreasing. This brings some difficulties to the application of SBS. The study on the mechanism of the phase transition is very important to get a laser pulse with high energy, high stability, high-quality and narrow pulse width.This article analyzes the phenomenon of phase transitions through the one-dimensional complete transient numerical simulation, for the two models of the SBS noise distributed generator and the amplifier with external input seed , we have gave separate discussion. Present, the simulation about SBS generally ignores the time differential coefficient of coupling item, because the coupling item is much more important than the time differential coefficient of coupling item under the steady state. However, this item played an important role, when the oscillator and phase transition of SBS are researched. Because when one of the scattered light, pump light or sound wave is near to zero(usually occurring phase transition at this time ), the value of the the time differential coefficient of coupling item is far greater than the coupling term, in the course it can play a role of the connecting link between the preceding and the following.In the process of SBS, because each of the pump laser, scattering laser and sound waves exist relative motion, we can't observe relation ship of each other at the same time. In the previous study about SBS , they directly observed the connection between the relevant parameters and the scattering laser or sound waves. In the study, we have got the connection among the scatter-laser, the pump-laser and the sound waves, Through simulation and comparison time and again, and we also have analyzed and summarized process and reasons of the phase transition of SBS. The transient relationship between the sound waves and the pump-laser near the focus in the generator can offer base for understanding the breaking mechanism and the oscillation of SBS ,what we have studied in the word.The SBS system, that is used in recent years, is oscillation-amplification two-cell structure. For the difference between generator and amplifier, this paper have respectively studied the phenomenon of phase transition in the generator and the amplifier. Our numerical simulations show that, the phase transition in the generator is mainly caused by random fluctuations of sound waves. When the phonon lifetime of the medium is short, the sound waves is wavy, the instability of sound waves induces that the scattering rate of pump laser is not stable, so when pump energy arrive on the focus, if it is higher than the spontaneous threshold of SBS, it will result in dramatic increase in stokes laser intensity and sound waves, which can cause phase transition in the generator. For medium with a longer phonon lifetime, sound waves generally is stable, so the probability of occurring phase transition is lower.The phase transition in the amplifier is mainly caused by the anti-stokes processes, and the phase transition will occur only when the process of SBS have accumulated to a certain extent, and so the faster or the longer SBS accumulate, the phase transition have more chance to occur. Therefore, the phase transition induced by anti-stokes SBS generally occurs in the amplifier, but if the pump laser is very strong, it can also occur in the generator. Phonon lifetime is the medium parameters that determine the speed of attenuation of sound waves, and the attenuation of sound waves is an important factor that affect the cumulate speed of SBS. The phonon lifetime of medium affect directly the phase transition that caused by anti-stokes. Phonon lifetime is longer, the sound waves in medium attenuate slower, and the occurrence of phase transitions is easier. For medium with a shorter phonon lifetime, because the sound waves attenuate fast, the SBS-intensity is Suppressed, suppresses the anti-stokes processes, reduces the probability of occurrence of phase transitions. In addition, the phase transition is also affected by many other factors, such as energy density, pulse shape.In the end, the phase transition of SBS has been analyzed and summarized, the study can be used as device for pulse compression and phase conjugation.