Novel Design of Laser with SBS Phase Conjugation Mirror

The beam quality of a single-rod Nd: YAG laser with wavelength of 1.06 m was improved almost up to the diffraction limit by using a phase-conjugating mirror (PCM) based on stimulated Brillouin scattering (SBS) with acetone as the SBS medium. The new start cavity, which is the concave-convex cavity operating near the limit of stability in the g diagram, and the phase conjugation cavity of the SBS oscillator were designed. The thermal lens of the material with focal lengths of less than 140 m at pump powers of 60 J was compensated by the PCM. The laser is Q-switched by the nonlinear reflectivity of the PCM and the system works with an average repetition rate of about 5 kHz and single Q-switch pulse showed a width of 10 ns. It is demonstrated that if the start cavity is fit for SBS and the SBS phase conjugation mirror can be optimized, the beam quality of solid laser can be surely enhanced because of this technology of nonlinear optics.     

全氟碳化合物新介质SBS相位共轭镜和光限幅器选取原则的研究

介绍了低吸收高负载特性的全氟碳化合物SBS新介质,研究了SBS相位共轭镜和SBS光限幅器对新介质的选取原则,并在Nd: YAG调Q激光器中进行了实验验证.结果表明,SBS相位共轭镜需要平均分子量小,运动黏度小,增益系数大的介质;而SBS光限幅器需要平均分子量大,运动黏度大,增益系数小的介质.新介质的发现增加了SBS介质的种类,而介质的选取原则对新介质的选用打下了良好的基础. 关键词： SBS新介质 选取原则 平均分子量 运动黏度     

利用受激布里渊散射改善偏振抽取腔光束质量的研究

在理论上分析了受激布里渊散射相位共轭效应和阈值效应对后向受激布里渊散射光束发散角的影响。在实验上，用受激布里渊散射相位共轭镜（SBS PCM）代替普通反射镜构成Nd:YAG激光器的偏振抽取腔，补偿了激活介质和光学元件造成的波前畸变，压缩了发散角，改善了光束质量。研究了抽运能量、透镜焦距等因素对受激布里渊散射偏振抽取腔输出光光束质量的影响。     

A new method to improve the loading capacity of stimulated Brillouin scattering phase-conjugating mirror

A new method based “Split-combined” principle is proposed to improve the load capacity of stimulated Brillouin scattering (SBS) phase-conjugating mirror (PCM). The incident light is split into two beams by polarizer, which will respectively produce SBS in the same experimental conditions, and then the generated two beams of Stokes light are combined together so as to improve the system’s load capacity. The method is verified in Continuum Nd: YAG seed-injected laser system and use FC-72 as the SBS medium. The results show that this method not only has a simple structure and high efficiency characteristics, but also improves the system’s power-load capacity and acquires a better quality linearly-polarized beam, which is expected to provide a new program for the application of SBS phase-conjugating mirror in a high-power system.     

抽运光对双布里渊放大池放大控制脉冲波形的影响

在传统的独立双池结构的受激布里渊相位共轭镜中,引入修整放大池,采用主放大池与修整放大池相结合的双布里渊放大池结构控制脉冲波形,主放大池控制脉冲的整体波形,而修整放大池对脉冲波形进行微调. 研究了抽运光功率密度对双布里渊放大池放大控制脉冲波形的影响,得到了不同抽运光功率密度下,脉冲波形随双池间距的变化规律. 研究表明,抽运光功率密度决定了种子光的放大效率,是控制脉冲波形的重要参数. 关键词： 受激布里渊散射 布里渊放大 脉冲波形控制     

Investigation on high-power load ability of stimulated Brillouin scattering phase conjugating mirror

The factors that affect the performance of stimulated Brillouin scattering phase conjugating mirror (SBS-PCM) in high-power condition are demonstrated. In high-power condition, the reflectivity is limited by both the SBS medium and the PCM configuration. FC-72 is found to be the best SBS liquid medium for its very high optical breakdown threshold and very low absorption coefficient by comparing several media. As FC-72 is chosen, the impurity of the liquid is the most remarkable factor which affects the reflectivity of PCM. The Millipore membrane filters with aperture of 0.22 μm was used to clean the liquid, and the reflectivity was evidently increased. Among the parameters of the SBS-PCM, the focal length of lens is one of the most important parameters related to the load ability. In the condition of the input laser of 1 J and 10 Hz, the appropriate focal length is proved to be 50 cm. As lens is chosen, the rotating wedge plate (RWP) is used to decrease the effects of optical breakdown and thermal effects in the condition of high power and stable reflectivity is achieved. Synthetically considering all the factors of medium and configuration, an energy reflectivity of 82% is achieved when the input energy is 0.94 J and the repetition rate is at 10 Hz.     

基于“桶中能量法”的相位共轭保真度测量研究

 给出了基于“桶中能量法”的相位共轭保真度测量原理、方法与结果。通过对适当大小的空间滤波针孔能量透过率的直接测量可以得到相位共轭保真度并简单实现系数 标定, 给出了针孔大小的选择原则、系数标定方法和共轭保真度的计算公式以及单池受激 布里渊散射相位共轭镜在几种条件下的实验结果, 指明了这种方法的重要性与不足之处。     

Theoretical investigation on the threshold value of stimulated Brillouin scattering in terms of laser intensity

The threshold value of stimulated Brillouin scattering (SBS) was investigated theoretically basing on a new concept of real gain length (RGL). It shows that for ideal steady-state SBS or ideal transient SBS, the threshold values are both constants and the later is five orders of magnitude larger than the former. Under general conditions, whether steady-state SBS or transient SBS will occur is determined by RGL. The SBS occurred in a long material may not be steady-state SBS.     

振放双池受激布里渊散射相位共轭镜的脉冲波形研究

受激布里渊散射应用到激光驱动器中，就要考虑脉冲的变形。针对上升前沿２ｎｓ的毫 秒泵浦脉冲，在其脉冲前沿根部引入缓慢上升的的预脉冲后，研究振放双池受激布里渊散射相位共轭镜所产生的受激布里渊散射脉冲波形。     

陡前沿脉冲产生的受激布里渊散射稳定性研究

受激布里渊散射转换效率及相位共轭保真度的稳定性与泵浦脉冲的前沿上升时间有关，对前沿上升时间与介质声学声子寿命接近的泵浦脉冲，在泵浦能量超过受激布里渊散射阈值较多时，会造成严重的相位共轭保真度不稳定。本文针对上升前沿时间为２ｎｓ的泵浦脉冲及声学声子寿命的１ｎｓ和ＣＣｌ４介质，在泵浦脉冲前沿根部引入缓慢上升的小信号后，采用振放双池受激布里渊散射相位共轭镜，在泵浦激光能量达到５００倍受激布里渊散射阈值时     

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.     

Lattice Boltzmann simulation on thermal performance of composite phase change material based on Voronoi models

Phase change materials(PCMs) are important for sustaining energy development. For the thermal performance enhancement, the composite PCM with metal foam reconstructed by the Voronoi method is investigated in this work. The lattice Boltzmann method(LBM) is used to analyze the melting process on a pore scale. The melting interface evolution and temperature contour of the composite PCM are explored and compared with those of pure PCM. Moreover, structure parameters including the pore density, porosity and irregularity are investigated comprehensively, indicating that the additive of metal foam strengthens the melting performance of PCM obviously. Compared with pure PCM, the composite PCM has quick rates of the melting front evolution and heat transfer. The heat conduction plays a great role in the whole melting process since the convection is weakened for the composite PCM. To improve the melting efficiency, a larger pore density and smaller irregularity are recommended in general. More significantly, a suitable porosity is determined based on the requirement for the balance between the melting rate and heat storage capacity in practical engineering.     

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.     

激光与材料参数对瞬态SBS 诱导的激光超声破坏效果的影响

 通过数值求解一维瞬态受激布里渊散射（SBS）声光耦合波方程，从理论上分析了泵浦激光参数及光学材料参数对SBS过程发生阈值的影响。以SBS过程中建立起来的应力场抗拉（压）强度和散射场的反射率为判据，分析了激光脉宽及作用区域长度对激光超声破坏材料效果的影响，探讨了短脉冲激光（约ns）引起的激光超声对材料的破坏机理；讨论了通过参数配置有效遏制SBS过程激光超声对光学材料的破坏问题。     

Investigation of stimulated Brillouin scattering for broadband KrF laser

Stimulated Brillouin scattering (SBS) numerical mode for broadband multimode KrF laser pump with optical breakdown has been built up in this paper. The basic premises of the model are that KrF spectrum is composed of an umber of lines and SBS arises from these lines with coupling to some extent. The broadband SBS threshold strongly depends on pump bandwidth. SBS and optical breakdown occur nearly simultaneously when broadband SBS and breakdown have the same threshold. The decrease of saturation reflectivity for broadband pump radiation is explained with this model. Experimental results agree well with the model.     

Phase conjugation of gap solitons: A numerical study

We study the effect of a nearby phase-conjugate mirror (PCM) on the gap soliton of a Kerr non-linear periodic structure. We show that phase conjugation of the gap soliton (in the sense of replication of the amplitude profile in the reverse direction) is possible under the condition of PCM reflectivity approaching unity. This is in contrast with the results for linear structures, where the wave profiles can be conjugated for arbitrary values of the PCM reflectivity. The sensitivity of the conjugation of the gap solitons to PCM reflectivity is ascribed to the fine balance of non-linearity with dispersion, necessary for their existence.     

受激布里渊散射介质——全氟聚醚的温度特性研究

受激布里渊散射(SBS)介质——全氟聚醚(PFPE)不仅具有低吸收高负载的SBS特性,还具有低的流点或高的沸点,可适合于较宽温度范围之内工作.通过分析PFPE的化学结构,对PFPE具有良好的SBS特性及独特物理化学特性进行了解释.在连续Nd:YAG种子注入激光系统中选用HT230,DET和HS260三种PFPE进行了不同温度下SBS特性研究.结果表明,PFPE不仅具有良好的SBS特性,还可以在低温或高温条件下工作,这为不同温度下研究SBS相位共轭镜打下了良好的基础.     

Effect of water temperature on pulse duration and energy of stimulated Brillouin scattering

The water temperature has a strong effect on the kinematic viscosity, which is inversely proportional to the phonon lifetime and the gain coefficient. The higher the temperature is, the smaller the kinematic viscosity is, and the larger the phonon lifetime is. At a low pump power and a short focal length, we can derive a single-peak stimulated Brillouin scattering (SBS) pulse. The duration of this single-peak SBS pulse depends mainly on the phonon lifetime of water. With the increase of the water temperature, the duration of such a single-peak SBS pulse will become longer, and the SBS energy will become higher for the gain coefficient, which is related to the phonon lifetime. Therefore, varying the medium temperature can lead to the changes of SBS pulse duration and SBS energy.     

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.     

相位调制信号对窄线宽光纤放大器线宽特性和受激布里渊散射阈值的影响

高功率窄线宽光纤放大器的输出功率主要受限于受激布里渊散射(SBS)效应,通过相位调制进行线宽展宽可以有效抑制SBS效应.基于窄线宽光纤放大器中的SBS动力学模型,研究了正弦信号、白噪声信号和伪随机编码信号(PRBS)对窄线宽光纤放大器光谱特性与SBS阈值的影响.研究发现,采用不同信号进行相位调制时,调制频率和调制深度等参数对调制后激光光谱的谱线间隔、谱线数目与光谱平整度的影响存在较大差异,进而影响放大器的线宽特性和SBS阈值.通过对比分析,给出了调制信号的类型选择和参数优化原则,能够为窄线宽光纤放大器的相位调制系统设计提供参考.