非聚焦条件下CS2介质中受激布里渊散射光限幅特性的研究

采用布里渊噪声起源模型,数值研究了非聚焦条件下CS2液体介质中脉冲传输的能量与功率特性.结果表明,在非聚焦条件下,透射光的功率波形仍然表现出光限幅特性,而透射能量随入射能量线性变化,不具有限幅特性,这一点与聚焦条件下的结果不同.以波长1053 nm、脉宽20 ns的Nd:YLF激光器为光源,采用3∶1的缩束系统,通过衰减片调整入射光能量在2 mJ-92 mJ变化,获得了透射光能量和功率波形随入射能量变化的规律,并与聚焦条件下的结果进行了比较.实验结果与理论模拟相符合.由于聚焦条件下当入射能量较高时容易出现介质光学击穿,所以采用缩束结构的功率限幅更适用于高功率大能量的情况.     

透镜焦距对受激布里渊散射光限幅特性的影响

采用受激布里渊散射(SBS)噪声起源理论模型，数值模拟了透镜焦距对SBS光限幅功率特性和能量特性的影响.结果表明，改变透镜焦距就可以方便的控制SBS光限幅的功率波形和输出能量.当采用焦距适中的透镜(f=15cm)时，可以得到较低的限幅输出能量；当采用短焦距的透镜(f=5cm)时，可以得到较理想的限幅功率波形.在实验上将波长1064nm、脉宽8ns、能量16mJ的Nd:YAG激光脉冲聚焦到CCl₄介质中，研究了限幅功率波形和限幅输出能量随透镜焦距的变化规律，实验结果与理论模拟相符合. 关键词： 受激布里渊散射 光限幅 数值模拟     

抽运能量对受激布里渊散射光限幅特性的影响

采用布里渊噪声起源模型,数值模拟了受激布里渊散射介质CCl4对波长1064nm的Nd：YAG纳秒激光脉冲的传输特性及光限幅特性。入射抽运脉冲能量较低时,非线性介质对纳秒激光脉冲呈光学透明。入射抽运脉冲能量高于受激布里渊散射产生阈值后,透射脉冲峰值受限,脉宽压缩,能量趋于饱和,说明该光学系统同时具有光功率限幅和能量限幅的光限幅特性。利用理论模型模拟了如下光限幅参量：透射脉冲峰值功率、透射能量、能量透射率、脉宽压缩率依赖抽运光能量的变化关系。相应的理论模拟计算结果由实验进行验证,实验结果与理论模拟相符合。     

受激布里渊散射介质CCl4中脉冲传输与功率限幅特性

分析了受激布里渊散射介质CCl4对纳秒激光脉冲的传输特性及功率限幅特性. 结果表明：透射脉冲波形随抽运光能量增加而压缩变形，脉冲前沿保持高斯型，后沿被压缩成功率“平台”，功率“平台”随抽运脉冲能量增加而逐渐变宽，但功率水平不变，表明该光学系统具有的功率限幅特性. 详细分析了功率限幅特性随抽运能量的变化规律和时间响应特性规律，采用脉宽为10ns、波长为1.06μm的Nd:YAG激光脉冲进行实验验证，理论与实验结论相符合.  关键词： 受激布里渊散射 功率限幅     

受激布里渊散射光限幅输出波形控制研究

数值模拟了种子场诱导受激布里渊散射光限幅过程的限幅输出波形特性.得到利用种子场控制限幅脉冲波形的规律：选取种子场脉冲宽度为抽运脉冲宽度的五倍，抽运脉冲相对于种子脉冲的延迟时间控制在与抽运脉冲宽度相当时，限幅输出波形最佳.限幅输出波形同时受抽运光功率影响，随抽运能量的增加，限幅脉冲功率不断下降，直至趋于0. 关键词： 受激布里渊散射 限幅输出波形 注入种子     

利用染料片吸收控制受激布里渊散射功率限幅波形

理论研究了受激布里渊散射过程中功率限幅响应特性.采用高斯型入射脉冲，数值模拟了抽运参数(抽运峰值功率密度、抽运脉冲宽度)，介质参数(增益系数、声子寿命)和结构参数(聚焦透镜焦距、介质池长)等物理参数影响限幅输出波形中剩余峰的特性规律.对如何控制剩余峰进行了理论分析.研究表明，声子寿命较小、增益系数大的布里渊介质光限幅响应较好，声子寿命较大、增益系数小的布里渊介质光限幅响应较差；无法单纯通过控制布里渊介质参数来完全消除剩余峰.实验上采用染料片吸收剩余峰，获得接近平顶的限幅输出波形.     

受激布里渊散射介质CCl_4中脉冲传输与功率限幅特性

分析了受激布里渊散射介质CCl4 对纳秒激光脉冲的传输特性及功率限幅特性 .结果表明 :透射脉冲波形随抽运光能量增加而压缩变形 ,脉冲前沿保持高斯型 ,后沿被压缩成功率“平台” ,功率“平台”随抽运脉冲能量增加而逐渐变宽 ,但功率水平不变 ,表明该光学系统具有的功率限幅特性 .详细分析了功率限幅特性随抽运能量的变化规律和时间响应特性规律 ,采用脉宽为 10ns、波长为 1 0 6 μm的Nd :YAG激光脉冲进行实验验证 ,理论与实验结论相符合 .     

多壁碳纳米管光限幅特性的研究

用化学气相沉积法制备了多壁碳纳米管,并将其溶解在甲苯溶液中.用波长为1064nm的皮秒脉冲激光测量该样品的透过率,发现了非常明显的光限幅特性.当入射光强较小时,透射光强度随入射光强度的增大而增大,输出与输入为线性关系;随着入射光强的增大,透射光强增长的速度明显变慢,并逐渐趋于饱和.当入射光强度较小时,样品的透过率接近100%;而当入射光强为8GW/cm²时,非线性透过率达到30%.根据三光子吸收理论计算,理论拟合与实验结果非常符合,说明多壁碳纳米管的三光子吸收产生了光限幅效应.实验测 关键词： 多壁碳纳米管 光限幅 三光子吸收     

碳纳米管悬浮液的光限幅特性实验研究

 通过超声波降解法制备了多壁碳纳米管的水-表面活性剂悬浮液,测量了其对于1 064 nm,脉宽10 ns Nd:YAG调Q脉冲激光的光限幅曲线。实验发现：入射激光能量密度较低时,出射能量密度随入射能量密度的增加而线性增加;当入射能量密度为160 mJ/cm²时,出射能量不再线性增加并且逐渐趋近于光限幅器的嵌位输出值,约16 mJ,同时,对激光的透过率从71%下降到15%。通过Z扫描和探针光实验以及45°散射角下散射能量、散射率随入射激光能量变化曲线的测量,对碳纳米管悬浮液的光限幅机理进行了研究。结果表明：其限幅机理可能源于碳纳米管吸收激光能量后升华产生的膨胀的碳气泡对入射激光产生的非线性散射;另外,非线性折射对光限幅效果也有一定的作用。     

碳纳米管悬浮液的光限幅特性实验研究

通过超声波降解法制备了多壁碳纳米管的水-表面活性剂悬浮液,测量了其对于1 064 nm,脉宽10 ns Nd:YAG调Q脉冲激光的光限幅曲线。实验发现：入射激光能量密度较低时,出射能量密度随入射能量密度的增加而线性增加;当入射能量密度为160 mJ/cm2时,出射能量不再线性增加并且逐渐趋近于光限幅器的嵌位输出值,约16 mJ,同时,对激光的透过率从71%下降到15%。通过Z扫描和探针光实验以及45°散射角下散射能量、散射率随入射激光能量变化曲线的测量,对碳纳米管悬浮液的光限幅机理进行了研究。结果表明：其限幅机理可能源于碳纳米管吸收激光能量后升华产生的膨胀的碳气泡对入射激光产生的非线性散射;另外,非线性折射对光限幅效果也有一定的作用。     

Influence of medium parameters on power limiting characteristic in stimulated Brillouin scattering process

By adopting noise initiation model of stimulated Brillouin scattering (SBS), the influence of phonon lifetime and gain coefficient of medium on power limiting characteristic is numerically investigated. Through using actual parameters of three media, CCl4, acetone, and CS2, the waveforms of transmitted pulses are simulated. The result shows that different media have little effect on the front peak of waveform,while have an obvious effect on the height of power limiting platform. When the medium which has short phonon lifetime and small gain coefficient is used, the height of power limiting platform is comparatively high. In experiment, by focusing 1064-nm, 8-ns, 18-mJ pulses into these three media, the waveforms of transmitted pulses are obtained. The experimental results are in good agreement with conclusions of theoretical simulations.     

Improved output energy characteristic of optical limiting based on double stimulated Brillouin scattering

In order to improve the optical limiting performance based on stimulated Brillouin scattering (SBS), a method based on double SBS is proposed in this paper. The dependence of the output energy of optical limiting based on double SBS on the pump energy is numerically simulated, and experimentally validated with an Nd:YAG seed-injected laser. The results indicate that only the first SBS optical limiting works in the case of low pump energy. However, as the pump energy increases, the second SBS process can be activated if the transmitted power of the first SBS is still above the SBS threshold. Therefore, the output energy characteristic of optical limiting based on double SBS is much better than that based on single SBS. Owing to the sub-nanosecond response time and a high power threshold, the SBS optical limiting can provide protection in high power laser systems.     

High power pulse amplification of ytterbium-doped double-clad fiber amplifier

By solving a set of time-dependent equations, the characteristics of the ytterbium-doped double-clad fiber amplifier are presented. Besides the steady state in the fiber of the upper-state population, pump power and amplified spontaneous emission without the input signal, the dynamic characteristics of the high power Gaussian pulse amplification like the evolution of pulse waveform distortion, upper-state population distribution and stored energy and pulse energy of the amplifier under the forward and backward pump,are simulated. The relations between the output pulse energy of the amplifier and the different input pulse peak power or pump power are also discussed. The models and results can provide important guide for the design and optimization of the high power pulse amplification.     

Investigation on the output energy characteristic of optical limiting based on stimulated Brillouin scattering

Stimulated Brillouin scattering (SBS) featuring sub-nanosecond response time takes place at a high power threshold, which enables its application at a high power density. When the intensity of input light excesses the SBS threshold, strong SBS process takes place through SBS medium, leading to a quick energy transfer from pump to the Stokes and thereby an optical limiting characteristic in the output energy. In this paper, the correlation between SBS output energy and input power density is numerically simulated and validated in the Nd: YAG Q-switch laser system. The results indicate that not only the output energy exhibits an optical limiting characteristic, but also the clamped value of output energy can be controlled by changing the medium or the focal length.     

Investigation on the effect of beam divergence angle upon output waveform based on stimulated Brillouin scattering optical limiting

This paper investigates the effect of beam divergence angle on output waveform based on stimulated Brillouin scattering optical limiting. Output waveforms in the case of different pump divergence angles are numerically simulated, and validated in a Nd:YAG seed-injected laser system. The results indicate that a small pump divergence angle can lead to good interaction between pump and Stokes, and a platform can be easily realized in the transmitted waveform. In contrast, a peak followed by the platform appears when the divergence angle becomes large.     

Measurement of stimulated Brillouin scattering （SBS） threshold based on waveform variation of SBS optical limiting

This paper proposes a method for measuring the stimulated Brillouin scattering (SBS) threshold based on waveform variation of SBS optical limiting. The output waveforms for different pump power densities are numerically simulated, and validated in the Nd:YAG seed-injected laser system. The results indicate that SBS does not take place in the case of a low pump power density and thus the output power scales up linearly with pump power. Once the pump power density exceeds the SBS threshold, SBS takes place and thereby the energies are transferred from pump to Stokes. As a result, a small shoulder appears in the trailing edge of the output waveform, which provides another method to determine the SBS threshold.     

Maximizing TEM00 solar laser power by a light guide assembly-elliptical cavity

For maximizing TEM₀₀ solar laser power, a modified light guide assembly-elliptical cavity pumping approach is proposed. By observing refractive and total internal reflection principles, highly concentrated solar radiations from a primary parabolic mirror are firstly collected by a near-hexagonal input face and then transmitted, at low propagation angles, to a near-rectangular output end of the modified assembly. A sharp elliptical pump cavity with an intervening optics is utilized to further couple and concentrate the radiations from the assembly to a thin Nd:YAG rod. Optimum pumping parameters are found through ZEMAX? non-sequential ray-tracing and LASCAD? laser cavity analysis. Compared with the output performances of the parallel-packed light guide assembly-elliptical-cylindrical cavity, 45% increase in TEM₀₀, 10% in multimode solar laser powers are attained by the proposed approach. Experimental results on both the enhanced transmission efficiency of the assembly and the improved focusing capacity of the elliptical cavity with are finally provided.