一种自聚焦透镜的成像解

自聚焦锥透镜与圆柱透镜相比较，有其特殊之处。柱透镜在近轴平行光入射时，光线的振幅、相位、周期保持不变，而锥透镜的三者均发生变化，而且求解锥透镜的光线方程也比圆柱透镜复杂得多。本文推导了一个锥透镜的光线方程。可据此设计经过锥透镜后的光源大小，这在微系统照明中非常有用。     

平顶光束经表面有缺陷的厚非线性介质后的光强分布

基于非线性薛定谔方程和分步傅里叶算法, 研究了高功率平顶高斯光束经过厚非线性介质后在自由空间传输的光强演变过程, 详细分析了介质表面缺陷对光强演化规律的影响.研究结果表明, 平顶阶数越大, 光束的自聚焦效果越差, 最大光强点越靠近介质后表面.入射光束的初始场强越强, 介质的厚度和非线性系数越大, 光束的自聚焦越强, 且极值点越靠近介质后表面.介质表面的缺陷使得光束在靠近介质后表面有较大的光强. 并且相位调制型缺陷比振幅调制型缺陷对光强的影响更大. 关键词： 平顶光束 自聚焦 缺陷 光强分布     

Self-Focusing of a Light Beam in a Medium with Relativistic Nonlinearity: New Analytical Solutions

In contrast to the existing theories of the relativistic self-focusing of a light beam in a plasma, the problem of a steady self-focusing light beam with a given input Gaussian radial intensity distribution has been analytically solved approximately with the use of a renormalization group approach. Depending on the parameters of the plasma and laser beam, solutions describing its longitudinal–radial waveguide structure have been obtained. These solutions demonstrate three characteristic types of relativistic self-focusing: (i) self-focusing on an axis, (ii) self-focusing in the form of a tubular channel, and (iii) self-trapping distribution.     

Self-focusing of elliptically shaped high-power laser beams in a fully ionized plasma

The effect of self-focusing of a high-power light beam with an elliptical cross-section in a strongly ionised plasma has been evaluated. The mechanism of self-focusing considered is non-uniform heating of plasma by an electromagnetic wave having transverse variation of amplitude along its wave front. The heating causes redistribution of carriers which lead to a nonlinear relationship between the electron density and the electric vector and hence to a field-dependent effective dielectric constant. It is found that the beam gets focused at different focal points in different directions exhibiting the effect of astigmatism. There are several critical powers, below thex=0,y=0 planes; at higher powers one dimension focuses while the other defocusses, an oscillatory waveguide is formed in both dimensions. Above the highest critical power both dimensions self-focus, the dimensionless beam width parametersf ₁ andf ₂ forx andy focusing reach different minima corresponding to different self-focusing distances beyond which both dimensions continue diverging. The effect of energy loss through absorption from the beam has also been considered. It has been observed that absorption brings about a reduction in the extent of self-focusing and favours defocusing of the beam.     

Correction to: Generation of amplitude- and phase-modulated signal beam with a phase-only spatial light modulator and its detection by the transport of intensity equation method for holographic data storage

This study evaluates a novel holographic data storage (HDS) that uses a phase-only spatial light modulator (SLM) for the multilevel complex amplitude modulation of a signal beam and the transport of intensity equation (TIE) method to detect the signal beam without interferometry, to increase the capacity of the HDS, simplifying its optical system, and improving the stability of the signal beam modulation and detection. Both the amplitude and phase of the signal beam were modulated by a computer-generated hologram displayed in a phase-only SLM, a 4-f optical system, and a pinhole placed in the Fourier plane. The complex amplitude-modulated signal beam generated by this scheme does not always perfectly match the target complex amplitude, and deviations from the amplitude and phase of the target complex amplitude may exist. It is unclear whether the TIE method, which is sensitive to the state of the beam intensity and the phase distributions to be detected (such as zero-intensity points and phase discontinuities), can accurately detect a signal beam whose complex amplitude is modulated by the modulation scheme with a phase-only SLM. Here, we demonstrate via numerical simulations and experiments that several methods of complex amplitude generation using a phase-only SLM can achieve multilevel modulation of the amplitude and phase of a signal beam and are suitable for detection by the TIE method in HDS.

     

Diffraction of a modulated laser beam in 3ω optics system

Based on diffraction optical theory, diffraction of a laser beam with periodic amplitude modulation and phase distortion is derived in 3ω optics system. Influence of defocus distance and focal length of a focusing lens on intensity distribution of diffraction light is investigated by numerical simulation. The results show that appropriate distance away from the focus spot and increase the focal length in final optical systems are beneficial to control the modulation of light intensity fluctuations and reduce the optical components damage caused by small-scale self-focusing effect.     

高功率激光光束经颗粒污染后的近场衍射效应

采用高功率激光驱动器设计软件Laser designer数值研究了激光光束经颗粒污染后的近场衍射特性. 对颗粒污染所产生的调制进行了分类, 并详细分析了真空和石英玻璃中, 振幅调制型和位相调制型颗粒污染对光束近场的影响. 研究结果表明, 无论在真空或石英玻璃中, 位相型颗粒污染对高功率激光光束的近场调制影响较大. 且在石英玻璃中, 位相型硬边颗粒污染所引起的高功率激光光束小尺度自聚焦热像调制现象相当严重. 而由颗粒污染引起光束强区的横向偏移, 却在一定程度对小尺度自聚焦有抑制作用.     

反射式液晶光阀用于激光束空间整形可行性研究

 从理论和实验两方面对一种反射式液晶光阀（LCOS）的调制特性进行了研究。着重研究了在高功率激光系统中,反射式液晶光阀作为空间整形器件面临的一些应用问题,特别是它的各种调制特性、整形能力以及对于光束质量的影响。研究发现,与透射式液晶光阀相比,反射式液晶光阀有着明显的优势,效率达到42%,远远优于透射式液晶光阀10%的效率,从而能够实现系统的大幅度简化,是一种非常有潜力的空间光调制器件。     

反射式液晶光阀用于激光束空间整形可行性研究

从理论和实验两方面对一种反射式液晶光阀（LCOS）的调制特性进行了研究。着重研究了在高功率激光系统中,反射式液晶光阀作为空间整形器件面临的一些应用问题,特别是它的各种调制特性、整形能力以及对于光束质量的影响。研究发现,与透射式液晶光阀相比,反射式液晶光阀有着明显的优势,效率达到42%,远远优于透射式液晶光阀10%的效率,从而能够实现系统的大幅度简化,是一种非常有潜力的空间光调制器件。     

Propagation of cosh Gaussian laser beam in plasma with density ripple in relativistic – ponderomotive regime

Self-focusing of cosh Gaussian laser beam in plasma with periodic density ripple has been investigated. The pondermotive force on electron and the relativistic oscillation of the electron mass causes periodic self-focusing/defocusing of the cosh Gaussian laser beam. The beam converges in the region of high plasma density due to dominance of self-focusing effect over diffraction effect and diverges in the low density region. Non-linear partial differential equation governing the evolution of complex envelope in slowly varying approximation is solved using paraxial ray approximation. The variation of beam-width parameter is studied with distance of propagation for different values of ripple wave number d and decentred parameter b. In order to get strong self-focusing, wavelength and intensity parameters of cosh Gaussian laser beam are optimized.     

All-optical integrated logic gates based on intensity-dependent transverse modal coupling

We present a study of the spatial propagation of light in a third order planar step-index waveguide with a transverse parabolic width film. By using the Lagrangian formalism, the width and phase evolution of a Gaussian beam has been completely described showing both the refractive contributions of the lens-like waveguiding geometry and self-focusing nonlinearity. As a result of this study, we have proposed the design of a new kind of integrated all-optical devices which, by means of the spatial beam power modulation and its influence on the transverse modal coupling can operate as phase-insensitive AND, OR and XOR logic gates.     

Off-axial contribution of beam self-focusing in plasma with density ripple

Off-axial contribution of beam self-focusing in plasma with density ripple is investigated. Apply paraxial ray theory and Wentzel–Krammers–Brillouin approximation, the results shown that, in interaction of laser and plasma with density ripple, beam self-focusing presents some interesting diverse features when off-axial contribution is obvious. In the paper, we find, on the one hand, density ripple can minimize the defocusing and beam still retains a localized profile with an oscillatory self-focusing and defocusing, on the other hand, with the increase of off-axial contribution, laser beams presents four various self-focusing features, which laser beam intensity profile splits into three-splitted with central axial convex profile, three-splitted with equal amplitude profile, three-splitted with central axial concave profile and two-splitted intensity profile.     

高斯光束非线性“热像”效应研究

高功率激光系统中非线性“热像”强度可能达到光学元件损伤阀值，从而对系统的安全运行造成威胁.以往的非线性“热像”现象研究都是基于无限大平面光波背景，但实际系统中传输的是有限束宽光束.本文以高斯光束为例，基于无像差自聚焦理论分析了有限束宽光束的“热像”形成规律.结果表明，高斯光束“热像”的强度与位置在光束腰斑较小时与平面光波情况有明显不同，但光束腰斑较衍射物大得多时，高斯光束可以近似当作平面光波处理.     

Determination of thermal effusivity of solids by a photoacoustic scanning technique

A new method is proposed to determine the thermal effusivity of solid samples using a one dimensional photoacoustic scanning technique. The method employs a sample configuration in which the backing for a good light absorber layer is changed from a reference sample to the unknown sample by scanning the absorber surface with an incident modulated light beam. From the measured phase difference or amplitude ratio one can determine the thermal effusivity of the unknown sample, knowing the effusivity of the reference sample. The Rosencwaig-Gersho theory of photoacoustic effect has been extended to the present experimental situation and expressions have been derived for photoacousitc phase difference and amplitude ratio as the backing is changed. Values calculated using these expressions are found to agree well with measured values for different sample combinations except in amplitude ratio values when the thermal effusivities of the samples differ very widely. The reason for this disagreement is discussed.     

Lie-optics, geometrical phase and nonlinear dynamics of self-focusing and soliton evolution in a plasma

It is useful to state propagation laws for a self-focusing laser beam or a soliton in group-theoretical form to be called Lie-optical form for being able to predict self-focusing dynamics conveniently and amongst other things, the geometrical phase. It is shown that the propagation of the gaussian laser beam is governed by a rotation group in a non-absorbing medium and by the Lorentz group in an absorbing medium if the additional symmetry of paraxial propagation is imposed on the laser beam. This latter symmetry, however, needs care in its implementation because the electromagnetic wave of the laser sees a different refractive index profile than the laboratory observer in this approximation. It is explained how to estimate this non-Taylor paraxial power series approximation. The group theoretical laws so-stated are used to predict the geometrical or Berry phase of the laser beam by a technique developed by one of us elsewhere. The group-theoretical Lie-optic (or ABCD) laws are also useful in predicting the laser behavior in a more complex optical arrangement like in a laser cavity etc. The nonlinear dynamical consequences of these laws for long distance (or time) predictions are also dealt with. Ergodic dynamics of an ensemble of laser beams on the torus during absorptionless self-focusing is discussed in this context. From the point of view of new physics concepts, we introduce a stroboscopic invariant torus and a stroboscopic generating function in classical mechanics that is useful for long-distance predictions of absorptionless self-focusing.     

Relativistic nonlinearity and wave-guide propagation of rippled laser beam in plasma

In the present paper we have investigated the self-focusing behaviour of radially symmetrical rippled Gaussian laser beam propagating in a plasma. Considering the nonlinearity to arise from relativistic phenomena and following the approach of Akhmanov et al, which is based on the WKB and paraxial-ray approximation, the self-focusing behaviour has been investigated in some detail. The effect of the position and width of the ripple on the self-focusing of laser beam has been studied for arbitrary large magnitude of nonlinearity. Results indicate that the medium behaves as an oscillatory wave-guide. The self-focusing is found to depend on the position parameter of ripple as well as on the beam width. Values of critical power has been calculated for different values of the position parameter of ripple. Effects of axially and radially inhomogeneous plasma on self-focusing behaviour have been investigated and presented here.     

Displacement and aberration effects in nonlinear focusing

A model for external self-focusing of a laser beam due to the optical phase shift generated in a thin nonlinear layer has been studied. The cubic nonlinear susceptibility used has both absorptive and refractive components. Intensity profiles of the beam in the region of the focal plane of a linear lens have been constructed for various laser input cross-sections, and yield insight into the nonlinear focusing and defocusing mechanisms. The shape of the wings of the input beam has been found to affect the diffraction pattern significantly.     

Evaluation of the decay in temporal coherence of a laser beam propagating in a slowly fluctuating medium

A simple method is described for evaluating the change in temporal coherence which occurs when a monochromatic light wave traverses a medium the properties of which fluctuate randomly. The degree of second-order coherence is formulated in terms of the amplitude and phase autocorrelation functions which are derived from the photocurrent produced in a light beating experiment. This method makes it possible to evaluate even a small deterioration in temporal coherence which is closely related to the frequency fluctuations in the light wave. The change in temporal coherence of a laser beam propagating through a slowly fluctuating gaseous path has been measured.