两种纤维增强复合材料与连续激光耦合规律

 利用双积分球-光电管系统，开展了不同厚度的芳纶纤维/环氧、碳纤维/环氧复合材料在不同强度（低于烧蚀阈值）的1.319 mm连续激光辐照下的能量耦合规律研究。结果表明：芳纶纤维/环氧复合材料的反射率、能量耦合率随材料厚度增加而增大，透射率随材料厚度增加而减小；在材料厚度一定时，反射率、透射率随激光强度增加而增大，能量耦合率随激光强度增加而减小；体吸收系数随材料厚度的增加而减小，激光强度的变化对其没有影响。碳纤维/环氧复合材料的反射率随激光强度增加而增大，能量耦合率随激光强度的增加而减小，材料厚度对反射率和能量耦合率的影响不大。     

两种纤维增强复合材料连续激光烧蚀阈值测量及吸收特性分析

 通过双积分球-光电管测试系统和摄像记录的方法,对芳纶纤维/环氧和碳纤维/环氧两种复合材料在1.319 μm连续激光作用下的烧蚀阈值和烧蚀过程中材料对激光能量的吸收特性进行了实验研究。结果表明：芳纶纤维/环氧复合材料的平均烧蚀阈值随材料厚度增加而降低,碳纤维/环氧复合材料的平均烧蚀阈值不受材料厚度影响,约为70 W/cm²;两种纤维增强复合材料烧蚀前的反射率随激光功率增加而缓慢增大,芳纶纤维/环氧材料从0.40变化到0.45,碳纤维/环氧材料从0.15变化到0.20;当发生烧蚀时,芳纶纤维/环氧材料的反射率急剧下降,吸收率增大,碳纤维/环氧材料的反射率无明显变化,吸收率约为0.80。     

厚度具有线性变化的吸收平板或膜层的非相干透射率和反射率

计算了垂直入射下厚度具有线性变化的吸收平板样品的非相干透射率和反射率（正面和反面），给出了直接确定无基底样品以及透明基底上薄膜能量（强度）系数的精确表达式。     

吸收对垂直腔面发射激光器光学特性的影响

采用光学传输矩阵方法，详细分析了反射镜以及键合界面的吸收对垂直腔面发射激光器光学特性的影响. 结果表明，反射镜以及键合界面的吸收对反射镜和垂直腔面发射激光器的反射率和势透射率有较大影响，而对反射镜中心波长处的反射相移以及垂直腔面发射激光器模式的反射相移和模式位置影响很小. 随着反射镜以及键合界面的吸收增大，反射镜中心波长处的反射率逐渐减小，垂直腔面发射激光器的模式反射率变化则是先急剧减小，达到一个极小值，然后再逐渐增大，而反射镜中心波长处以及垂直腔面发射激光器模式处的势透射率则都是迅速降低的. 此外，将有吸收的键合界面离有源区的距离远一些，有利于提高垂直腔面发射激光器模式处的光输出效率.     

多层介质光学膜系的计算机模拟误差分析

用计算机数值计算方法模拟多层介质光学膜系的实际镀膜工艺过程，分析膜厚误差对λ0／4高反射膜系透射率和反射率的影响，讨论在一定技射率要求条件下不同层数膜系的允许厚度误差。发现当膜层偏离λ膜厚时，光学厚度控制工艺能通过自动调整其后膜层的厚度有效地降低厚误差对膜系透射率和反射率的影响。对于镀制高反射率膜而言，膜厚允许误差仍可有较大的范围。     

飞秒激光诱导锗等离子体辐射光谱特性

由于飞秒激光脉冲宽度小于靶材电子—晶格热弛豫时间,飞秒激光烧蚀靶材过程以及诱导击穿产生的等离子体膨胀动力学过程与纳秒激光作用过程不同,因此研究飞秒激光诱导等离子体发射光谱特性对于研究飞秒激光烧蚀机制以及飞秒激光诱导等离子体的膨胀动力学过程非常重要。Ge材料是一种常用的中远红外探测器以及光学元器件材料,对中心波长为800 nm,脉宽为50 fs的激光脉冲烧蚀空气中Ge靶材产生的等离子体发射光谱强度的时间和空间演化规律研究,并探讨了飞秒激光脉冲能量对等离子体发射光谱强度的影响规律。实验结果表明在等离子体羽膨胀初期,飞秒激光诱导Ge等离子体发射光谱主要由线状光谱和连续光谱构成,在200 ns时间内连续光谱强度逐渐减弱,线状光谱开始占主导地位。通过探测Ge等离子体的时间分辨发射光谱,随着等离子体的快速膨胀,等离子体发射光谱强度随着时间的增加呈现先增加后下降变化,在335 ns达到最大。通过探测Ge等离子体的空间分辨发射光谱,随着距离Ge靶材表面的位置增加,等离子体发射光谱强度随远离Ge靶材表面距离增加呈现先增加后下降变化,在0.8 mm位置达到最大。由于存在等离子体自吸收机制,等离子体发射光谱...     

沉积温度对热舟蒸发MgF2薄膜性能的影响

 采用热舟蒸发方法沉积了氟化镁(MgF₂)材料的单层膜,沉积温度从200 ℃上升到350 ℃,间隔为50 ℃。测量了样品的透射率和反射率光谱曲线,进行了表面粗糙度的标定,并在此基础上进行了光学损耗及散射损耗的计算。同时对355 nm波长处的激光诱导损伤阈值进行了测量。结果表明:随着沉积温度的升高,光学损耗增加;在短波长范围散射损耗在光学损耗中所占比例很小,光学损耗的增加主要由吸收损耗引起;在355 nm波长处的损伤阈值变化与吸收损耗的变化趋势相关,损伤机制主要是吸收起主导作用。样品的微缺陷密度也是影响损伤阈值的一个重要因素,损伤阈值随缺陷密度的增加而降低。     

含有吸收材料对称结构一维三元光子晶体的透射谱

利用传输矩阵法理论,研究含吸收材料对称结构一维三元光子晶体的光传输特性。结果表明:当各层介质无吸收时,在较宽的禁带范围内出现一条透射率为100%的透射峰;当介质的折射率为带有正虚部的复数,即介质存在吸收时,禁带中的透射峰出现明显的透射衰减,且随着复折射率虚实比β的增大透射率出现单调衰减规律;当单个介质存在吸收时,随着各介质折射率虚实比的增大,透射率的衰减速度存在各异,其中以C介质的折射率虚实比βc对透射峰透射率的影响较大,其次为B,A的影响相对较小;当所有介质同时存在吸收时,透射衰减最为明显。这些特性为光学衰减器的设计提供理论指导意义。     

WS2纳米薄膜厚度依赖的三阶非线性吸收特性研究

利用液相剥离法制备了WS2纳米片,结合真空抽滤技术,控制上清液体积制备了不同厚度的WS2薄膜.在此基础上,使用800 nm飞秒激光的Z扫描技术表征了WS2纳米薄膜的三阶非线性吸收特性.研究发现,制备的不同厚度的WS2都表现出可饱和吸收特性,可饱和吸收主要是由于单光子吸收所引起的泡利阻塞效应引起;随着厚度的增加,饱和强度...     

泡沫镍孔隙尺度光谱辐射特性的实验与数值研究

实验测量了不同厚度的泡沫镍在0.4~2.2μm波长的法向-半球反射率/透射率,采用蒙特卡罗法对泡沫镍的计算机断层扫描结构进行孔隙尺度辐射传输建模,对比研究了泡沫镍辐射特性随入射光谱和样品厚度的变化,计算得到了泡沫镍辐射特性的孔隙尺度分布特征。结果表明:所建立的泡沫镍孔隙尺度辐射传输模型在计算其光谱辐射特性方面具有正确性。波长增长,吸收率逐渐降低,反射率逐渐升高;样品厚度增加,吸收率逐渐升高并趋于稳定,透射率逐渐降低至0。孔隙尺度辐射特性分布强烈依赖于局部纹理结构,波长1.5μm时,泡沫孔隙中的平均吸收率是肋筋上的1.5倍,而肋筋上的平均反射率则达到孔隙中平均反射率的3.7倍。     

Giant nonlinear optical properties of bismuth thin films grown by pulsed laser deposition

Thin films of Bi were grown by pulsed laser deposition on glass substrates at room temperature. The thickness and roughness of the films were characterized by grazing-incidence x-ray reflectivity, and the complex refractive indices were measured in the range from 1.5 to 4 eV by spectroscopic ellipsometry. We performed Z-scan measurements to study the third-order optical nonlinearity of the films. It was found that the Bi films exhibited an unusually large nonlinear refractive coefficient, n(I)~1.24x10(-1) cm(2)/kW and nonlinear absorption coefficient, alpha(I)~-3.97 cm/W , at low laser intensity, ~60 kW/cm(2) . This anomaly is believed to have an origin related to melting of the Bi films at the focus spot by the laser beam.     

Effect of thickness on the optical nonlinearity of gold colloidal nanoparticles prepared by laser ablation

The effect of thickness on the nonlinear optical properties of Au nanoparticles which produced by laser ablation of high purity gold bulk in distilled water were investigated by employing different optical techniques. Experiments were performed on the 0.3, 0.6, and 1 mm thickness cells of Au nanoparticles-fluid. The effect of nonlinear refractive index of samples on the laser beam broadening was observed. The optical limiting behavior of samples is investigated by measuring the transmitted intensity of the laser beam through the samples. The third order nonlinearity of Au nanoparticles-fluid was measured by using Z-scan data. Results show a positive second order refractive index and absorption coefficient for Au nanoparticles-fluid that are increased in due of increasing the thickness of the cells. The magnitudes of both real and imaginary parts of third order susceptibility of samples are increased by increasing the thickness.     

真空环境下激光烧蚀铝靶冲量耦合系数的数值模拟

 通过分析不同情况下激光与固态靶、气化物质的作用机理,利用激光体烧蚀模型,采用流体力学理论和1维Lagrange有限差分的计算方法,对真空条件下不同激光参数下气化物质对靶产生冲量的过程进行了数值模拟,模拟计算结果与实验测量结果、Phipps定标关系符合较好。计算结果表明,在等离子体的情况下冲量耦合系数随着激光强度增大而减小。     

Top-hat cw laser induced thermal mirror: a complete model for material characterization

A complete theoretical model is presented for the thermal mirror technique under top-hat laser excitation. Considering the attenuation of the top-hat excitation laser intensity along the thickness of a sample due to its optical absorption coefficient, we calculate the laser-induced temperature and surface deformation profiles. A simplified theoretical model for a high absorption sample is also developed. The center intensity of a probe beam reflected from the thermal mirror at a detector plane is derived. Numerical simulation shows that the thermal mirror under the top-hat laser excitation is as sensitive as that under Gaussian laser excitation. With top-hat laser excitation, the experimental results of thermo-physical properties of opaque samples are found to be well consistent with literature values, validating the theory.     

A novel “sandwich” method for observation of the keyhole in deep penetration laser welding

A sandwich method was used to observe the keyhole in deep penetration laser welding, which provided an effective way to analyze both the Fresnel and inverse Bremsstrahlung absorption. In the transparent metal-analog system, different densities of metal vapor, ionized atoms, and free electrons in the keyhole can be simulated by changing the thickness of aluminum films. The research results show that inverse Bremsstrahlung absorption exerts a tremendous influence on the energy absorption of the laser beam for CO₂ laser welding. Low density of keyhole plasma benefits the incident laser energy coupling to the materials. However, excess density of keyhole plasma baffles the transmission of the incident laser beam to the interior material. By comparing inflow energy and outflow energy, there exits an energy balance on the keyhole wall by balancing the absorbed laser intensity and heat flux on the wall.     

Optical phase conjugation by degenerate four-wave mixing in basic green 1 dye-doped gelatin film using He–Ne laser

Optical phase conjugation in gelatin film doped with basic green 1 has been measured using CW laser radiation (λ=632.8 nm) generated by He–Ne laser of total power 35 mW. The degenerate four-wave mixing (DFWM) experiment allowed for measurement of phase conjugate reflectivity as a function of dye concentration, backward beam intensity, forward beam intensity, probe beam intensity, mean pumping beam intensity and angle between the forward pumping beam and probe beam. For 1 mM concentration of basic green 1-doped gelatin film, 0.1% phase conjugate reflectivity has been observed.     

Oscillation dependence of two-wave mixing gain for unidirectional ring resonator in photorefractive materials

Dependence of the coupling strength of two-wave mixing gain in photorefractive materials for the single unidirectional ring resonator on oscillation conditions has been analyzed in the strong nonlinear regime. In this regime, difference between the frequency of the pump beam and oscillating beam is proportional to the cavity-length detuning, which can be explained in terms of the photorefractive phase-shift. This phase-shift results due to slightly non-degenerate two-wave mixing that compensates for cavity detuning and satisfies the round-trip phase condition for the steady-state oscillation. The presence of such a phase-shift allows the possibility of the nonreciprocal steady-state energy transfer between the pump and oscillating beams. If the gain due to the beam coupling is large enough to overcome the cavity losses then the signal beam is amplified in the presence of material absorption. Such amplification is responsible for the oscillations. For the single unidirectional ring resonator, the effects of cavity-length detuning, energy coupling coefficient, crystal thickness of the material, reflectivity of the cavity mirrors and material's absorption coefficient on the frequency and intensity of oscillations have also been studied in detail. It has been found that for the smaller value of absorption coefficient (α) of the photorefractive crystal, the unidirectional ring resonator can oscillate at almost any cavity-length detuning (ΔΓ) whereas for the larger value of α oscillation occurs only when the cavity-length detuning is limited to small region (around ΔΓ=0). But reverse of the case is found for energy coupling coefficient (γ₀).     

SBS新介质C4Cl6参数确定及性能研究

 从介质对光的吸收机理出发，研究了吸收系数较小的受激布里渊散射（SBS）新介质六氯-1,3-丁二烯（C4Cl6）。通过基本的物理参数，确定了入射光波长为1 064 nm时C₄Cl₆的SBS参数，其吸收系数约为0.003 cm^-1，声子寿命约为0.25 ns，增益系数约为4.9 cm·GW^-1。分析了入射光波长与介质SBS参数的关系，布里渊频移与入射光波长成反比，声子寿命与入射光波长的平方成正比，增益系数一般与入射光波长无关。在单池SBS系统中，利用Nd:YAG调Q激光器研究了C₄Cl₆的SBS性能。实验结果表明：由于其吸收系数小，与具有相同增益系数的H₂O比较，C₄Cl₆具有较高的能量反射率（饱和能量反射率大于55%）和较宽的SBS脉宽；由于其声子寿命短，与具有相同吸收系数的CS₂比较，C₄Cl₆的SBS波形上升时间较短。吸收系数小和声子寿命短的特点，使C₄Cl₆适合作为放大池介质，这对优化SBS介质选取，提高SBS系统性能具有一定意义。