Refraction of an astigmatic laser beam in a transition layer of a stratified liquid

Refraction of an astigmatic laser beam in a transition layer formed at the boundary between two liquids with different optical characteristics is studied theoretically and experimentally. An algorithm of calculating a ray trajectory in the transition layer of a stratified liquid is considered. The profile of the refractive index in the medium is modeled for three different distributions: linear, sinusoidal, and tangential. Computer modeling based on the above models allowed obtaining two- and three-dimensional images of the laser beam (refractograms) inside and outside of the medium. The influence of the beam parameters and the experimental layout on the shape of refractograms is studied, and optimal experimental conditions are determined. The experimental arrangement for detection of 2D and 3D refractograms by the laser refractography method is described. An experimental technique is developed, and digital registration of the experimental refractograms is carried out. Adaptation of special software for processing 2D refractograms allowed retrieving the refractive index profile in the transition layer of a liquid. Based on the tangential model of the diffusion layer in a saltstratified liquid, the values of the refractive index were obtained that allowed determining the salinity profile in the given layer.     

Refraction of a cylindrical laser beam in a boundary temperature layer

Refraction of a cylindrical laser beam in the boundary layer near the surface of a heated metal spherical ball in water is investigated theoretically and experimentally. An experimental setup and typical refraction images (refractograms) are described. Refractograms calculated using the proposed geometric-optics model demonstrate the close agreement with experiment. Possibilities of reconstructing the temperature field in the boundary microlayer from experimental results are considered.     

线激光束均匀化整形方法研究

为解决核物理工程、表面热处理、激光加工、激光通信中传输激光束的均匀性问题,提出采用三角楔柱透镜改善线激光均匀性的新方法,并在ZEMAX开发环境下建立了整形光路系统的理论模型。利用所建立的模型,分析了整形柱透镜在不同半径、斜面夹角、透镜介质折射率等参数下光束的均匀性。研究表明：在远场条件下,通过改变整形柱透镜的参数,能够得到能量分布均匀的线激光。在实际应用中,可以采用1个半圆柱和2个相同三角楔胶合成型的简单工艺。进一步模拟发现,两种整形系统所得到的均匀度和线宽一致。     

Characteristics of structural changes in fused silica under the effect of femtosecond laser radiation in the heat accumulation regime

Structural changes induced in fused silica subjected to focused femtosecond laser radiation in the heat accumulation regime are studied. The spatial profile of the induced refractive index and the effect its amplitude has on such effects as the self-focusing of the laser radiation, and thermal and photochemical processes are investigated. Individual contributions from each of these effects are separated and their effect is described on the basis of experimental dependences for the first time. It is also shown that the use of heat accumulation regime substantially increases the induced refractive index, compared to the nonthermal regime.     

Transformation of a Gaussian light beam reflected in the vicinity of the Brewster angle

Variations in the profile and shift of a Gaussian light beam upon reflection from a plane interface between a transparent dielectric and a medium with a complex refractive index are studied. The beam is assumed to be incident at angles close to the Brewster’s angle, where the effects of transformation of the beam profile are most substantial. Based on numerical analysis, the reflected beam profiles are obtained for different properties of the media, parameters of the incident beam, and positions of the plane of observation.     

Theoretical prediction and experimental validation of the angle-of-arrival probability density of a laser beam in a strong plane-flame turbulence

The propagation of a laser beam through a plane turbulent flame is studied using geometrical optics approximation. The random fluctuations of the refractive index caused by a strong thermal turbulence in the flame create random perturbations of the laser beam direction. From the Markovian process model applied along the whole random path of the beam, the theoretical probability density of the laser beam angle-of-arrival is derived from the analytical solution of the Einstein-Fokker-Kolmogorov equation, which we have determined in terms of a series expansion of spherical harmonics. An experimental setup and a method for measuring this probability density are described. The experimental results obtained are shown to agree with the theoretical predictions.     

锥形太赫兹量子级联激光器输出功率与光束特性研究

针对锥形太赫兹量子级联激光器, 利用有限差分波束传播法和速率方程法, 建立了准三维的太赫兹有源器件仿真模型, 能够对具有轴向非线性波导结构的激光器进行模拟. 利用此模型, 研究了锥角大小对激光器输出光功率及光束质量的影响. 仿真结果表明, 考虑到器件之间的光耦合效率, 为了达到最大的有效输出光功率, 锥形太赫兹量子级联激光器的锥角存在一个最优值.     

An electrodynamic model of the optical characteristics of blood and capillary blood flow rate

A model for the prediction of the optical characteristics of blood (refractive index and absorption coefficient) and for the determination of the rate of blood flow in the capillary bed under irradiation of a laser beam is proposed. The dependences of the intensity of the laser radiation on the refractive index and absorption coefficient of the system of blood vessels in the upper layer of the dermis and on the rate of blood flow in the capillary bed are obtained.     

Experimental observation and numerical simulation of guided modes in Nd:YLiF4 channel waveguides produced by carbon ion implantation

We report on, to our knowledge the first time, the channel waveguide formation in Nd:YLiF₄ laser crystal produced by 6 MeV carbon ion implantation. The guided modes are observed by using an end‐face arrangement. We construct the two‐dimensional (2D) refractive index profile of the channel waveguide cross section, which is based on the related planar waveguide index distribution as well as the rectangular shape of the waveguide cross sections. The modal intensity distribution is numerically calculated by using the beam propagation method according to the reconstructed index profile, which shows a reasonable agreement with the experimental result. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mutual repulsion of optical beams in media with nonlocal nonlinearity

Specific features of the dynamics of reflection of optical beams in media with defocusing thermal nonlinearity have been studied. It is shown that, upon laser heating of a medium, the induced inhomogeneity profile outside the pump beam is determined by thermodiffusion and heat sink conditions at sidewalls. Signal-wave trajectory equations in a medium with an induced refractive index gradient have been derived and solved. A dependence of the nonlinear total internal reflection angle on the pump power and the initial distance between the beams has been established.     

层状GRIN板折射率分布的自动测量

采用溶液 凝胶扩散共聚法成功地合成了二元体系的甲基烯酸苄酯 甲基烯酸甲酯 (BZMA MMA)层状梯度折射率板 ,折射率分布是其主要性能指标。给出了一种测量该制品折射率分布的简便实验装置和自动实现方法。该测量方法基于激光干涉的原理 ,由计算机自动跟踪干涉条纹的移动来计算梯度折射率材料的折射率分布。实验表明 ,该方法简单易行 ,具有较高的折射率测量精度 ,为科学评价自研制梯度折射率材料的性能提供了保证     

Mode field distribution of an integrated-optical waveguide generated by UV-laser radiation at the surface of a planar polymer chip

An integrated-optical strip-waveguide is directly written into the surface of a planar polymer substrate by an excimer laser beam. The mode field distribution, the surface refractive index and the refractive index depth profile of the waveguide have been examined. The intensity distribution of the mode field strongly depends on the refractive index depth profile, on the light coupling conditions and on the waveguide fabrication process parameters.     

Exploitation of the Gaussian beam for the investigation of the refractive index changes induced in a medium by an intensive light wave

Conclusion It has been shown that the experimental method consisting in the change of parameters of the Gaussian beam by the lens that the beam creates in the layer passing through it is simple and suitable for use. The experimental results obtained through this method by studying the refractive index thermal changes that took place when an He-Ne laser beam was absorbed in a layer are in good agreement with the results the theory of stationary thermal defocusation affords. Comparing the results obtained with the solutions of the organic dye brilliant green in different solvents (C₂H₅OH, C₆H₅NO₂, C₃H₈O₃) the constant dn/d for glycerol has been determined. The method enables studying the physical properties of media which are connected with different mechanisms of the change of the refractive index by an intensive light beam passing through the medium.     

Femtosecond micromachining of symmetric waveguides at 1.5 microm by astigmatic beam focusing

We report on a new spatial beam-shaping approach for fabrication of waveguides with a circular transverse profile by femtosecond laser pulses, using an astigmatic beam and controlling both beam waist and focal position in the tangential and sagittal planes. We apply this technique to write single-mode active waveguides at 1.5microm in Er:Yb-doped glass substrates. The experimental results are well described by a simple nonlinear absorption model.     

Below-band-gap waveguiding behaviors of a weakly index-guided GaAs/AlGaAs quantum well laser

We report the reduced waveguiding efficiency for the signals around 1560 nm as the injection current of an GaAs/AlGaAs multiple quantum well laser diode (lasing wavelength at 840 nm) with a ridge-loading waveguide configuration increased. This reduction trend stopped when the injection current reached the threshold condition of the laser diode. The decreased waveguide transmission and the more expanded mode profile indicated the variation of the effective refractive index gradient in the lateral dimension with injection current. This variation was due to the refractive index decrease with increasing carrier density even below band gap. A slab waveguide model was used to simulate the lateral mode profile variation with injection current. The refractive index differences between the guiding layer and claddings in the slab waveguide model provided estimates of refractive index contrasts of the laser diode at a concerned wavelength under various injection conditions.     

Analysis of Photo-Induced Waveguide in Lithium Niobate Crystal

In this letter, we estimate the refractive index distribution in a photo-induced waveguide. The photo-induced waveguide was fabricated by scanning a focused laser beam through a lithium niobate crystal. We made use of a parametric model of refractive index distribution of the photo-induced waveguide. The results of numerical simulation on the near-field patterns of curved waveguides are compared with experimental results.     

硅基VO2纳米薄膜光致绝缘体—金属相变的THz时域频谱研究

利用THz时域频谱技术(THz-TDS)研究了硅基二氧化钒(VO₂)纳米薄膜的光致绝缘体—金属相变特性.在连续光激发下前后,观察到了非常明显的THz透过率变化,并通过薄膜近似计算出了THz波段金属态VO₂薄膜的电导率.根据实验结果建立了金属态VO₂薄膜的等效Drude模型,得到了复电导率,复电容率以及复折射率等相关的基本参数,并通过基于时域有限积分法模拟了THz波穿透硅基金属态VO₂薄膜的过程,验证了所建立的模型的正确 关键词： 二氧化钒 光致相变 Drude模型 THz时域频谱技术     

A conduction model for deep penetration laser welding based on an actual keyhole

Using a specially designed experimental setup and properly choosing the sample material and the process parameters, we obtained a clear stable keyhole with a high-speed camera. On the basis of the actual keyhole profile, a conduction model with a cylindrical surface heat source has been developed under the assumption of the keyhole per thin layer being cylindrical. The model is numerically solved by the finite-difference method, the temperature field around the keyhole and the heat flux lost on the keyhole wall can be obtained. The effects of such factors as the shape and the size of the keyhole, the welding speed on the shape of the melt pool are studied. By comparing the laser intensity absorbed on the keyhole walls with the heat flux lost there, the mechanism of energy balance on the keyhole walls was investigated.     

Methods of ellipsometric analysis of polarization optical properties of inhomogeneous surface layers of optoelectronics elements

The basic regularities of changes in the main ellipsometric parameters of a light beam reflected from inhomogeneous surface layers of silicate glasses have been described on the basis of the Drude-Born theories of polarized light reflection. A method of physicomathematical modeling of the refractive index profile in an inhomogeneous surface layer of silicate glasses is reported, which makes it possible to determine, with the lowest second-kind error probability, a model of an inhomogeneous reflecting system that is adequate to the object of study.     

聚焦光场俘获微球的FDTD分析

基于动量守恒原理,采用2维时域有限差分方法(2D FDTD)建立了激光场对微米量级微球的作用力模型,讨论了入射高斯光场的波长、束腰半径、微球的折射率和半径等对聚焦光场俘获力的影响.结果表明：位于聚焦光场中特定位置的微球可被俘获,当离轴距离增加,俘获力减小.微球所受到的俘获力与微球的折射率有关,当小于环境折射率时(如汽泡),不能形成俘获,而被推离光场.模拟结果与其他文献中报道的实验结果一致. 关键词： 光镊 俘获力 时域差分有限方法(FDTD) 动量守恒