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.     

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.     

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1053nm、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2mJ—92mJ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 关键词： 非线性光学 受激布里渊散射 光限幅 非聚焦     

基于SBS光限幅和金属酞菁光限幅相结合的复合型光限幅器的研究

为了使受激布里渊散射(SBS)光限幅器的输出能量更"平",提出了一种基于SBS光限幅和金属酞菁光限幅相结合的复合型光限幅器的方案,即SBS光限幅后边放置金属酞菁光限幅,在两种光限幅器的共同限幅作用下,使复合型光限幅器的输出光能量更"平". 数值模拟了该复合型光限幅器的输出光能量随入射光强度的变化曲线,并在Countinuum's Nd: YAG种子注入式激光器中进行了实验验证. 理论和实验结果均表明,复合型光限幅器的输出光能量比单独SBS光限幅器更"平". 关键词： 复合型光限幅器 SBS光限幅 金属酞菁限幅     

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。     

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.     

Pulsed solid-state laser system with fiber phase conjugation and 315 W average output power

Solid-state laser sources with high average output power and high beam quality are required for numerous applications in industry and science. To improve the beam quality, stimulated Brillouin scattering (SBS) mirrors based on optical phase conjugation are used to compensate for phase distortions in master oscillator power amplifier (MOPA) systems. SBS phase conjugation in commercially available silica optical fibers facilitates the construction of high-power solid-state laser systems with high beam quality. Here we present a flash lamp-pumped, passively Q-switched Nd:YAP system which delivers an average output power of 315 W with M²=2.6 at a wavelength of 1.08 μm. Received: 11 July 2001 / Revised version: 28 September 2001 / Published online: 23 November 2001     

Optical limiting effect based on stimulated Brillouin scattering in CCl4

The optical limiting effect based on stimulated Brillouin scattering (SBS) in a nonlinear medium was investigated. We numerically treated the nonlinear propagation process with a theoretical model, which includes the spontaneous nature of the initiation of SBS, and obtained optical limiting effect in the process. Energy limiting, pulse reshaping and stabilization have been demonstrated on SBS mechanism with the nonlinear medium CCl_4. The input optical signals were Nd:YAG nanosecond laser pulses with width varying from 16ns to 7ns then to 2ns, the relationship between the transmitted signal and launched pump signal was shown. In the experimental regime, the most stable pulse and a superior energy stabilization of the transmitted pulse were obtained when the laser pulse-width became as short as 2ns. For the energy variation of laser pulses in a wide range of 14－88mJ, the output energy was limited in a quite narrow range 4.5－5.5mJ.     

受激布里渊散射对纳秒激光脉冲光限幅规律

 研究了受激布里渊散射对纳秒激光脉冲的光限幅效应。针对四氯化碳介质研究了受激布里渊散射对2ns激光脉冲的光限幅和稳定输出能量的特性,给出了光限幅特性及输出能量与光限幅介质的结构参数之间的关系。对于输入能量波动范围在 13%的激光脉冲,实验获得输出能量波动范围为 7%,与理论分析的结果基本符合。     

受激布里渊散射对纳秒激光脉冲光限幅规律

研究了受激布里渊散射对纳秒激光脉冲的光限幅效应。针对四氯化碳介质研究了受激布里渊散射对2ns激光脉冲的光限幅和稳定输出能量的特性,给出了光限幅特性及输出能量与光限幅介质的结构参数之间的关系。对于输入能量波动范围在 13%的激光脉冲,实验获得输出能量波动范围为 7%,与理论分析的结果基本符合。     

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     

Stimulated brillouin scattering in SF6 with a free-running XeCl laser as pump

A XeCl laser system without dispersive elements is used to investigate near threshold reflectivity and phase-conjugation (PC) fidelity of stimulated Brillouin scattering (SBS) mirrors with SF₆ as the active medium. Using different focal-length lenses to focus the broadband laser radiation into the Brillouin medium, it was found that at threshold the effective interaction length for SBS is equal to the confocal parameter and that the SBS gain is equal to its steady-state value for monochromatic pumping. High PC-fidelity values ranging from 0.8–1.0 were found under most of the experimental conditions investigated.     

高峰值功率大模场全光纤脉冲放大器

报道了一种由纤芯直径分别为15 μm和25 μm的大模场光纤组成的全光纤脉冲放大器模块。当注入脉冲宽度为10 ns,重复频率为1 Hz,光谱宽度为0.3 nm,脉冲能量为80 nJ的激光脉冲时,经过两级放大输出的激光脉冲峰值功率为30 kW,单脉冲能量为300 μJ。此外,实验研究了该放大器模块输出的时间特性。研究发现：由于受激布里渊散射（SBS）效应,限制了单纵模光纤激光在光纤中的高能量放大,采用宽带激光脉冲可以有效地抑制SBS效应,提高光纤放大器的SBS阈值。     

Experimental study of a broad-band XeCl double-pass amplifier with SBS mirror

An oscillator-amplifier XeCl laser system has been used to experimentally investigate the effectiveness of a liquid stimulated Brillouin scattering (SBS) mirror for correcting the spatial aberrations of broad-band laser radiation in a double-pass amplifier. It has been found that the SBS amplifier performance is strongly dependent on the intensity I _p exciting the Brillouin medium. A good beam reconstruction has been attained at I _p1GW/cm², whereas highly aberrated output beams have been delivered by the SBS amplifier for I _p>1 GW/cm². By comparing the broad-band SBS amplifier performance to that of the same amplifier with a dielectrically coated flat mirror at one end, it has been found that the use of a SBS mirror is advantageous to obtain lower divergence output beams only for low energy pump beams (<1 mJ).     

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.     

High efficiency and high energy parametric wavelength conversion using a large aperture periodically poled MgO:LiNbO3

We have demonstrated an efficient high energy 2 μm laser generation with a 36 mm long large aperture 5 mol% MgO-doped periodically poled LiNbO₃ (PPMgLN) nonlinear optical crystal. A high power Q-switched Nd:YAG laser (1.064 μm) was used to pump the quasi-phase matched (QPM) optical parametric oscillator (OPO). A total output energy of 186 mJ with 58% slope efficiency was obtained in two separate beams at 2 μm.     

Nonlinear responses and optical limiting behavior of Basic Violet 16 dye under CW laser illumination

Nonlinear optical properties of Basic Violet 16 dye solution in water are studied employing different optical techniques. Experiments are performed using the second harmonic of a continuous Nd-Yag laser beam at 532 nm wavelength and 100 mW power. The effect of nonlinearity of Basic Violet 16 dye in broadening the laser beam is observed. The optical limiting behavior is investigated by measuring the transmission of the samples. The third-order nonlinearity, χ³ of Basic Violet 16 dye, is measured using Z-scan data. The nonlinear absorption coefficient is calculated using the open aperture Z-scan data, while its nonlinear refractive index is measured using the closed aperture Z-scan data. All experiments are done for different concentrations and thicknesses of Basic Violet 16 dye solution. The effect of intensity of input laser beam on the nonlinear susceptibility is studied experimentally. Results indicate that Basic Violet 16 dye is a potential candidate for low-power optical limiting applications.     

All-optical switch and limiter based on nonlinear polarization in Mach-Zehnder interferometer coupled with a polarization-maintaining fiber-ring resonator

A novel all-optical switch based on nonlinear polarization mechanism using polarization-maintaining fiber ring with a polarization rotator is proposed. Optical switching with low threshold of mW order and optical limiting with broader limiting range, less fluctuation, higher damage threshold and response speed are demonstrated numerically. The deterioration of switching and the improvement of limiting originating from losses are also studied. Considering the tradeoff between switching power and bandwidth, the way to increase bandwidth is discussed.     

水的衰减系数及有效增益长度对受激布里渊散射输出能量的影响

本文从理论和实验上分析了水的衰减系数及有效增益长度对受激布里渊散射(SBS)输出能量的影响。实验结果表明,衰减系数越小,SBS输出能量越高。由于脉宽压缩效应,当入射光能量足够高并且有效增益长度相对较长时,SBS易获得高能量而形成极高的峰值功率。一旦这种峰值功率超过受激拉曼散射(SRS)或者二阶SBS阈值,SBS就会作为一个新的激发源去激发SRS或者二阶SBS,从而消耗掉部分SBS的能量,所以会出现后向有效增益长度越长,SBS的输出能量越低的现象。 关键词： 受激布里渊散射 衰减系数 有效增益长度 脉宽压缩     

KrF激光受激布里渊散射反射率稳定性的研究

实验研究发现抽运功率密度（或抽运能量）的波动引起的受激布里渊散射(SBS)反射率的不稳定程度受实验参数（抽运能量、介质气压和透镜焦距）的影响.当抽运能量、介质气压和透镜焦距这三个参数同时满足下列条件时可以获得稳定的输出：抽运能量超过5倍的SBS阈值，介质气压在16×105Pa和透镜焦距在15—50cm的范围内；同时还发现当改变实验参数时SBS反射率与稳定性的变化规律完全一致，反射率越高稳定性越好.通过理论分析得到 SBS反射率的相对稳定度实际上是受GIL因子的影响，理论计算与实验符合得很好. 关键词： KrF激光 受激布里渊散射 反射率 稳定度