透射衍射光栅内全反射级次

当前高功率激光装置系统中, 国内使用透射式衍射光栅进行光束采样, 国外甚至使用光栅聚焦并同时结合谐波分离和采样功能. 高功率激光辐照下, 除了0级透射打靶光以及目标取样光外, 其他级次衍射光的影响也不可忽视, 可能造成诸如鬼像聚焦破坏、时间脉宽测量不稳定、近远场测量噪声等问题. 尤其是透射衍射光栅内部全反射级次会导致光栅出射一系列规律的衍射花样, 造成上述干扰和危害. 首先理论上针对透射式光栅(包括取样光栅和聚焦光栅)进行了相应的计算和分析, 然后从实验上观察并测量了取样光栅内部全反射级次的现象, 并且结合镀减反膜前后的衍射现象进行了讨论, 最后分析并给出了如何消除或规避全反射级次和其他冗余级次影响的方法.     

The damage of the optical components induced by the stimulated Brillouin scattering

A theory of excitation of ultrasonic waves in the stimulated Brillouin scattering (SBS) process is presented in this paper. By using several reasonable approximations, a numerical calculation of the transient longitudinal SBS shows that large amplitude of acoustic waves can be built up by the nanosecond pulse of high-power laser, which may result in the damage of optical glasses. The maximal density change and the maximal acoustic wave intensity in optical glasses of 5\,cm in thickness are calculated by using different parameters of the high-energy laser, such as the intensity, the pulse width, and the wave length. The damage threshold of the optical glasses is about 80 GW/cm^{2} when using a 1064 nm laser. The dynamic mechanism of SBS is the electrostriction effect of the components coupling with the high-power laser.     

基于飞秒瞬态反射/透射技术的纳米Au半透膜热效应研究

以飞秒激光放大器作为光源联合使用瞬态反射/透射实验技术研究了纳米Au半透明纳米薄膜中非平衡载能粒子的热传导过程. 在相同实验条件下, 发现该薄膜的瞬态透射和反射信号明显不同并且延迟时间在5.0–7.5 ps时瞬态透射信号的符号发生改变. 对纳米薄膜的透射和反射信号进行了对比分析, 分别使用双温模型和Crude近似进行数据模拟并拟合, 分析认为沿膜厚方向的温度梯度变化和界面热阻效应引起介电函数的变化不同, 从而引起了瞬态透射信号和反射信号的不同. 对于半透明金属纳米薄膜需要同时考虑其瞬态透射和反射影响才能得到准确的瞬态吸收结果. 随着抽运脉冲能量的增加, 可以看到上升时间约为1.0 ps, 电子-晶格弛豫时间增加.     

光学窗口材料激光辐照热-力效应的解析计算研究

建立了高功率连续激光辐照透明光学材料的热力学模型,通过积分变换方法求解三维热传导方程,得出了激光辐照引起的瞬态温度场分布的精确解析解,并在此基础上进一步求得热应力场的瞬态分布。以1.315μm的高能氧碘激光辐照熔石英玻璃为例,计算了熔石英在激光辐照下的温度场与热应力场分布,分析了其激光损伤机理。研究结果表明由于熔石英具有优良的热稳定性,温度不均匀分布所产生的热应力相对较小,激光损伤主要是受辐照区域温度值超过材料熔点发生熔融破坏。理论分析结果与相关的实验结论一致,说明所建立激光辐照效应模型的合理性。     

Total internal reflection apodizers

A novel type of apodizer using total internal reflection is suggested for use in high-power laser systems.     

大功率激光单模光纤远距离传输的注入光纤光功率限值分析

对于大功率激光单模光纤远距离传输, 采用传统的受激拉曼散射(SRS)阈值作为注入光纤激光功率值取值过大。本文对单模光纤远距离传输过程中泵浦光和拉曼斯托克斯光(Stokes)光功率随注入光纤激光功率的变化进行仿真与理论分析。根据光纤中SRS产生的机理, 提出以拉曼Stokes光在单模光纤中传输的光功率变化曲线曲率极大值点对应的注入激光功率为限值, 对注入单模光纤光功率限值随着光纤长度变化进行仿真, 通过曲线拟合得出注入单模光纤激光功率限值公式, 并搭建实验系统进行验证。结果表明, 提出的注入单模光纤激光功率限值适用于大功率激光远距离单模光纤传输。     

强紫外激光在空气中长程传输受激旋转拉曼散射效应

 采用瞬态受激旋转拉曼散射（SRRS）模型，详细考虑了激光脉冲波形、介质的非线性极化、泵浦光的衰减、斯托克斯光的非线性放大以及拉曼线宽等诸多因素，对强紫外激光束在空气中长程传输过程产生的SRRS效应进行了研究。讨论了产生SRRS效应的阈值条件，详细分析了SRRS效应对激光光束质量的影响。     

位相畸变对强紫外激光在空气中长程传输的影响

 采用瞬态受激旋转拉曼散射（SRRS）模型，详细考虑了激光脉冲的位相变化、时间波形、泵浦光的衰减、斯托克斯光的非线性放大以及初始的随机斯托克斯散射场等诸多因素，研究了空间位相畸变对空气中长程传输过程中产生的SRRS效应的影响。研究结果表明：位相畸变对阈值条件和时间特性影响较小，而对光束质量影响较为明显；随着位相畸变的增大，光束的能量分布变得极不均匀，光束的光强对比度也明显变大，从而使激光光束质量变差，而且当传输距离达到阈值条件后，随着传输距离的增加，光束质量将进一步变差。     

受激旋转喇曼散射效应对强紫外激光聚焦特性的影响

采用Maxwell-Bloch-Langevin 方程,建立了强紫外激光在空气传输中的瞬态受激旋转喇曼散射（SRRS）效应模型；利用相位均方根（RMS）梯度描述的随机相位屏表示低频相位畸变,用扰动幅度表征的随机相位屏描述中、高频相位畸变,建立了相位畸变模型.讨论了强紫外激光在空气长程传输中所产生的受激旋转喇曼散射（SRRS）效应对其聚焦光斑形态的变化,定量分析了焦斑半径随入射强紫外激光的空间相位畸变及传输距离的变化.研究结果表明：当传输距离超过阈值条件时,焦斑半径随传输距离的增大而明显增大,且相位畸变越大,焦斑半径也越大.     

Influence of thermal self-action on the diffraction of high-power X-ray pulses

The space and time temperature distributions in a crystal exposed to the high-power pulses of an X-ray free-electron laser are analyzed by solving the heat-conduction equation. Using synthetic diamond crystals as an example, we examine how the temperature and strain of a crystal lattice affect the space-time and integrated intensities of diffraction reflection and the transmission of X-ray laser pulses. The conditions making it possible to minimize the thermal loads and spectral broadening of the diffracted pulses are determined.     

Femtosecond high-power spontaneous mode-locked operation in vertical-external cavity surface-emitting laser with gigahertz oscillation

We realize a femtosecond high-power spontaneous mode-locked operation with gigahertz oscillation in a vertical-external cavity surface-emitting laser under the condition of eliminating the internal and external unwanted reflection. We find that the reflectivity of the output coupler has a significant influence not only on the output power but also on the output pulse duration. With an incident pump power of 20 W, we have achieved 2.35 W of average output power with 778 fs pulse duration at a repetition rate of 2.17 GHz. The shortest pulse duration was 654 fs at an average output power of 0.45 W.     

The role of electron-phonon coupling in femtosecond laser damage of metals

Femtosecond laser pulses were applied to study the energy deposition depth and transfer to the lattice for Au, Ni, and Mo films of varying thickness. The onset of melting, defined here as damage threshold, was detected by measuring changes in the scattering, reflection and transmission of the incident light. Experiments were done in multi-shot mode and single-shot threshold fluences were extracted by taking incubation into account. Since melting requires a well-defined energy density, we found the threshold depends on the film thickness whenever this is smaller than the range of electronic energy transport. The dependence of the threshold fluence on the pulse length and film thickness can be well described by the two-temperature model, proving that laser damage in metals is a purely thermal process even for femtosecond pulses. The importance of electron-phonon coupling is reflected by the great difference in electron diffusion depths of noble and transition metals.     

Frustrated total internal reflection evanescent switching

The mechanism of frustrated total internal reflection was analyzed according to the theory of optical film. The transmission and reflection functions were developed and the curves of transmission and reflection changing from the relative gap width d/λ, incident angle θ₀, and refraction index n were shown in graphs. We proposed a scheme of experimental device to test the theoretical conclusion. A self-made electronic actuator drove the piezoelectric ceramics of FTIR. Experiments showed that the evanescent of transmission is 80% for He–Ne laser and smaller for YAG laser and Er:glass laser.     

机载激光测深海洋传输通道的吸收和散射特性分析

光在海洋中的传输特性对于机载激光测深有着重要的意义.本文从分析海水对光的吸收和散射的特性出发,对机载激光测深的海洋传输特性进行了细致的讨论,并阐述了其对机载激光测深的影响.     

近海机载激光海洋测深技术

本文详细论述机载激光海洋测深的基本原理;讨论机载蓝绿激光的最大探测深度及测深精度等关键指标;对高频率短冲脉激光器及大动态范围的微弱信号检测等关键技术也进行了论述;并简要介绍该技术的国内外发展动态,最后分析机载激光海洋测深技术在我国的现实需要及可行性。     

General design basis for a final optics assembly to decrease filamentary damage

The high-power laser beam in the final optics assembly of high-power laser facilities is often modulated by contamination particles, which may cause local high light intensity, thereby increasing the filamentary damage probability for optical components. To study the general design basis for a final optics assembly to decrease the risk of filamentary damage,different-sized contamination particles deposited on a component surface are simulated to modulate a 351-nm laser beam based on the optical transmission theory, and the corresponding simulation results are analyzed statistically in terms of the propagation characteristic and the light field intensity distribution of the modulated laser beam. The statistical results show that component thickness and distance between components can to some extent be optimized to reduce the appearance of local high light intensity, and the general design basis of component thickness and arrangement are given for different control levels of particle sizes. Moreover, the statistical results can also predict the laser beam quality approximately under the existing optics design and environmental cleanliness. The optimized design for final optics assembly based on environmental cleanliness level is useful to prolong the lifetime of optics and enhance the output power of high-power laser facilities.     

Laser-induced damage on large-aperture fused silica gratings

Two sets of laser-damage experiments on large-aperture fused silica optics have been carried out in a high-power laser facility. Severe damage has been found on the grating which presented dense craters on the front surface. This phenomenon is quite different from other fused silica optics, which are damaged on the rear surface. The damage possibility due to the redeposition layer was ruled out by acid-etching the grating’s front surface. The remarkable stimulated Brillouin scattering (SBS) effect was observed in grating and the reason for the front surface damage is thought to be the backward SBS.     

多波段高功率激光扩束系统设计

 对透射式和反射式扩束系统应用于高功率激光扩束的优缺点进行了对比研究。采用次镜为凸抛物面,主镜为凹抛物面的无焦卡塞格林系统,运用ZEMAX光学设计软件,按激光扩束系统的扩束倍率和系统的波像差要求,设计出多波段高功率激光扩束系统。对用于高功率激光反射镜的基底材料进行分析,选用无氧铜作为基底材料;采用金增强的膜系设计,膜系从近红外到远红外宽光谱波段激光的反射率均在98%以上。面形精度均方根值优于λ/40(λ=0.632 8 μm)的平面镜作为基准镜,采用光学干涉方法对设计的激光扩束系统进行检测实验,结果表明：该扩束系统的扩束倍率为3.53,波像差为0.206λ,满足多波段高功率激光光束发射要求。     

Nonlinear diffraction and total internal reflection in optical-beam interaction in defocusing media

We elaborate the theory of reflection of an optical signal beam from an inhomogeneous channel created by a high-power laser beam of different frequency in a nonlinear defocusing medium. We study the effects of total internal reflection and nonlinear diffraction in the interaction of these beams. We derive and analyze the equation for the trajectory of the reflected signal beam and obtain the expression for the critical value of beam-intersection angle below which the total internal reflection occurs. We discuss the results of numerical modeling of nonlinear Schr¨odinger equations for envelopes of the beams in media with quadratic, photorefractive, and thermal nonlinearities. We present experimental data on the interaction of beams of argon and helium–neon lasers in a cuvette with tinted alcohol.     

真空度对强紫外激光在空气中长程传输的影响

 采用瞬态受激旋转拉曼散射(SRRS)模型，并充分考虑非线性介质分子密度、增益系数、泵浦光的衰减、斯托克斯光的非线性放大以及初始的随机斯托克斯散射场等因素，研究了空气真空度对强紫外激光在空气长程传输过程中产生的SRRS效应的影响。结果表明，真空度对SRRS效应的阈值传输距离和Stokes光的转换效率的影响较大，随着真空度的提高，Stokes光的转换效率将降低，产生SRRS效应的阈值传输距离将增大。因此，在实际工作中，可以采用真空度较高的激光传输环境来抑制SRRS效应的产生，以减小入射泵浦光能量的衰减并提高光束质量。