激光等离子体相互作用的受激拉曼散射饱和效应

在激光等离子体相互作用过程中,受激拉曼散射（SRS）会通过Langmuir波衰减不稳定性（LDI）和电子俘获两种机理饱和.文章给出均匀一维等离子体和低强度非相对论激光作用中,LDI和电子俘获两种机理下的SRS饱和时间的解析表达式.SRS饱和时间与入射激光强度,电子密度,电子温度,初始电子密度微扰等参数有关.解析理论计算得到了与模拟和实验相符的结果. 关键词： 受激拉曼散射 饱和 Langmuir波衰变不稳定性（LDI） 电子俘获     

水中受激拉曼散射的能量增强及受激布里渊散射的光学抑制

为提高液体介质中受激拉曼散射的输出能量,提出了通过温度调控来抑制受激布里渊散射的方法,设计了532 nm多纵模宽带脉冲激光泵浦的受激拉曼散射发生系统,测量了不同温度下水中前向受激拉曼散射及后向受激布里渊散射的输出能量,分析了水温、泵浦激光线宽及热散焦效应对受激拉曼散射输出能量影响的物理机制.实验结果表明:通过降低水温可实现对受激布里渊散射过程的有效抑制,同时减小热散焦效应带来的光束畸变,从而有效提高受激拉曼散射的输出能量.研究结果对液体介质中的受激拉曼散射多波长转换具有重要意义.     

Nonlinear spectral properties of stimulated Brillouin scattering in plasma

New phenomena in which the stimulated Brillouin scattering (SBS) changes the number and magnitude of frequency shifts in a plasma upon a nonlinear increase in the pumping field intensity are discovered. It is found that the SBS becomes forbidden and then allowed again (upon a subsequent increase in the pumping intensity).     

Theoretical investigation on the pumping effect of stimulated Brillouin scattering on stimulated Raman scattering in water

The pumping effect of stimulated Brillouin scattering on stimulated Raman scattering is investigated theoretically through the coupled wave equations of stimulated Brillouin scattering and stimulated Raman scattering. The numerical simulations are in agreement with the experimental results. They indicate that the backward stimulated Raman scattering is excited and amplified collectively by both pump laser and stimulated Brillouin scattering.     

Experimental investigation on the competition between wideband stimulated Brillouin scattering and forward stimulated Raman scattering in water

The utilization of a simple focused optical cell to bring to light the competition between wideband stimulated Brillouin scattering (WSBS) and forward stimulated Raman scattering (FSRS) is investigated experimentally. A pulsed, wide bandwidth second-harmonic Nd:YAG laser is used as the pump source. We found that, the competition between WSBS and FSRS is an alternate process, which one dominated depends on the linewidth and energy of the pump laser, focal length, and optical breakdown.     

Nonlinear high-order effects in stimulated Raman scattering spectrum

A dependence of the stimulated Raman scattering (SRS) of water solutions of HNO₃ on the pumping level has been studied. The lines of the fourth and higher order effects were found. The results are treated in the basis of a spectrally uniform model of radiation centres and dependence of the coefficients of nonlinear susceptibility on the pumping.     

Nonlinear optical properties of stimulated Brillouin scattering process in submerged object detection

Nonlinear optical properties of stimulated Brillouin scattering(SBS)to signal detection in water are analyzed.With the threshold characteristics,SBS only occnrs when the high power laser is focused in the SBS cell.When there is an object present in front of the focus,it leads to lower incident intensity and then SBS does not occur.The backward SBS signal depends on the focusing location.The nonlinear optical properties of SBS process in the focusing regime axe analyzed theoretically.With the object coming near to the focusing center,the backward Stokes signal rises up from zero to a maximum,and then grows to saturation.The delay time of the echo signal to pump signal can give the object location.In experiment,the peak position of varying rate of energy can give object location.     

Investigation on the suppression of stimulated Brillouin scattering in single-frequency Raman fiber amplifiers

High power operation of single-frequency Raman fiber amplifiers (SF-RFAs) is usually limited by the onset of stimulated Brillouin scattering (SBS). In this paper, we present our theoretical investigation on the suppression of SBS in SF-RFAs, based on the intensity equations combining SBS and stimulated Raman scattering (SRS). The effects of increasing the mode area of fiber and utilizing temperature and strain gradients along the fiber on the suppression of SBS are discussed. The simulation results suggest that it is not an advisable method to suppress SBS in SF-RFAs by increasing the mode area. Besides, although tensile strain and temperature gradients can suppress SBS in RFA singly, the conditions are too rigorous to realize. Based on the numerical simulation results, a feasible scheme for the suppression of SBS in RFA is proposed using the temperature gradients together with tensile strain gradients along the fiber length, resulting in an increase of ～3 times of amplifiers output power.     

Study on forward stimulated Brillouin scattering in a backward pumped fiber Raman amplifier

Strong multi-order forward stimulated Brillouin scattering (SBS) has been observed in the backward pumped S-band distributed fiber Raman amplifier (FRA) with tunable narrow signal source (less than 100 MHz) when the pump power of FRA reached the SBS threshold. This does not obey the theory that only weak backward SBS lines exist according to the conservation of energy and momentum and the wave vector selected rule. This is because the sound waveguide characteristic weakens the wave vector rule, and the forward transmitted sound waveguide Brillouin scattering lines are generated and amplified in FRA.When the pump power is further increased, 11 orders of SBS lines and comb-like profile are observed. For the excited line, the frequency is 197.2296 THz and the power is 0 dBm. The even order SBS lines are stronger than odd order SBS lines, the power of the 2nd and 4th order SBS lines is 1.75 dBm, which is 16 dB higher than that of the 1st and 3rd order SBS lines. The odd order SBS lines are named BrillouinRayleigh scattering lines.     

横向受激布里渊散射的抑制

从非线性光学的耦合波理论出发,结合流体力学的连续性方程,建立了光学元件中窄带泵浦的横向受激布里渊散射的二维理论模型,并在此基础上,利用数值模拟考察泵浦光带宽增加对受激散射过程的影响。研究表明:在一定的条件下,当泵浦光带宽略大于布里渊线宽时,受激散射过程的Stokes增益将显著降低;带宽越大,Stokes增益降低得越多;当带宽大于20GHz时,可以抑制横向受激布里渊散射带来的不良影响。另外,当带宽一定时,调制频率的改变也会影响受激散射过程,可以通过数值模拟确定最佳的调制频率。     

横向受激布里渊散射的抑制

 从非线性光学的耦合波理论出发，结合流体力学的连续性方程，建立了光学元件中窄带泵浦的横向受激布里渊散射的二维理论模型，并在此基础上，利用数值模拟考察泵浦光带宽增加对受激散射过程的影响。研究表明:在一定的条件下，当泵浦光带宽略大于布里渊线宽时，受激散射过程的Stokes增益将显著降低；带宽越大，Stokes增益降低得越多；当带宽大于20GHz时，可以抑制横向受激布里渊散射带来的不良影响。另外，当带宽一定时，调制频率的改变也会影响受激散射过程，可以通过数值模拟确定最佳的调制频率。     

部分离化激光等离子体的受激喇曼散射不稳定性

由于部分离化激光等离子体中存在束缚电子，它可显著改变其参量不稳定性。本文分析了部分离化激光等离子体的受激喇曼散射的非线性色散关系，计算了钕玻璃倍频和三倍频激光受激喇曼散射的增长率，结果表明，前向受激喇曼散射显著增强，后向受激喇曼散射影响不大。     

Nd:YAP laser pulse compression by three-stage transient stimulated Brillouin and Raman scattering

There is a continuous effort to generate stable, powerful picosecond laser pulses for application in spectroscopy, nonlinear optics and parametric light generation, as well. One of the possible methods is the compression of longer nanosecond laser pulses by stimulated Brillouin and stimulated Raman scattering. The advantages of such a technique, in comparison to the used mode locked picosecond lasers, are as follows: the absence of the active and/or passive mode lockers used to generate a train of picosecond pulses, and the absence of a fast electrooptical shutter used to select a single pulse from a train of pulses. The application of stimulated Brillouin and stimulated Raman scattering permits to generate picosecond pulses in the wavelength regions not covered by mode locked lasers. Of special interest is the wavelength region of 0·8 m, which may be amplified by the attractive Titanium Sapphire lasers. In this paper we are summarizing our results in theoretical modelling and in real generation of picosecond pulses by means of cascaded stimulated Brillouin and Raman scattering. The models of scattering processes have been investigated. The stable generation of 5, 7, 3 picosecond pulses have been optimized for the wavelengths of 0·8, 0·64 and 0·54 m, respectively. In all these cases, the pulses exhibited the far field pattern close to Gaussian, with the pulse energy ranging from 0·2 to 1 mJ.     

Bursting behaviours in cascaded stimulated Brillouin scattering

Stimulated Brillouin scattering is studied by numerically solving the Vlasov-Maxwell system. A cascade of stimulated Brillouin scattering can occur when a linearly polarized laser pulse propagates in a plasma. It is found that a stimulated Brillouin scattering cascade can reduce the scattering and increase the transmission of light, as well as introduce a bursting behaviour in the evolution of the laser-plasma interaction. The bursting time in the reflectivity is found to be less than half the ion acoustic period. The ion temperature can affect the stimulated Brillouin scattering cascade, which can repeat several ×at low ion temperatures and can be completely eliminated at high ion temperatures. For stimulated Brillouin scattering saturation, higher-harmonic generation and wave-wave interaction of the excited ion acoustic waves can restrict the amplitude of the latter. In addition, stimulated Brillouin scattering cascade can restrict the amplitude of the scattered light.