Broadband photorefractive properties and self-pumped phase conjugation in Ce-SBN:60

The first use of cerium-doped Sr0.6Ba0.4Nb2O6as a broadband self-pumped phase-conjugate mirror using internal reflection is reported. The phase-conjugate reflectivity at normal incidence ranged from two percent at 442 nm to seven percent at 515 nm and was zero at 633 nm. The electron-hole competition was found to be significant and had a wavelength dependence in one sample but not the other. The charge carrier density wassim 7 times 10^{16}cm^-3and was wavelength in-dependent. The absorption coefficient ranged from 2 cm^-1at the shorter the wavelengths to zero at longer wavelengths. The dispersion in the indexes of refraction was measured and the birefringence was -0.036. The sign of the dominant charge carriers was determined to be negative and the sign of the electrooptic coefficient, r33, was positive. Using the above values, a wavelength dependent coupling coefficient has been determined. The experimental results indicate that the phaseconjugate reflectivity decreases at shorter wavelengths due to increased absorptive losses and experiences a threshold effect at longer wavelengths.     

光致折变晶体中相位光栅的相位移动的物理作用

在光致折变晶体(如SBN,BaTiO_3等)中发生两波混频及能量耦合是由于晶体干涉场内产生了相对于干涉场有相位移动的折射率光栅(相位光栅),这已被实验和理论证实。如果仅有两波混频产生的相位光栅,而此光栅相对于干涉场并无相位移动,或相位移动量较小(不足π/2),此时不会发生波能量耦合或耦合极微弱。1981年A.Yariv用耦合波方程从数学上解决了这个问题。     

用透射光栅光谱仪求解原子的真实发射光谱

本文详细讨论了一种用针孔透射光栅(PTG)光谱仪求解原子真实光谱的方法,并给出了在6路激光系统上得到的镁、铝和硅等离子体的真实软X射线光谱。     

Short-pulse grating formation in photorefractive materials

Solutions are obtained for the refractive index grating in a photorefractive material in which the writing or erasing beams are short-pulse (less than tens of nanoseconds), high-irradiance (greater than kW/cm²) lasers. The response of the photorefractive material is modeled using the continuity equation for the charge carriers, a rate equation for the ions, a current equation, and Poisson's equation for the space charge field. Two cases are discussed in detail: 1) the time for recombination of carriers with ions is much less than the pulse length of the laser; and 2) the recombination, drift, and diffusion times are all much longer than the pulse length. The energy requirements for short-pulse writing and erasing are greater than or equal to those obtained for typical CW lasers in all cases investigated. Application to previous observations in BaTiO3, Bi12SiO20, and LiNbO3is discussed.     

Study of a displacement sensor based on transmission varied-line-space phase grating

A displacement sensor based on transmission varied-line-space （VLS） phase gratings is proposed. The relationship be-tween the properties of the sensor and the parameters of VLS is discussed. Compared with the displacement sensor manu- factured by the reflective VLS grating, this type of sensor contains a grating with simpler structure and high diffraction efficiency. It also has good stability with the change of temperature.     

Study of a displacement sensor based on transmission varied-line-space phase grating

A displacement sensor based on transmission varied-line-space(VLS) phase gratings is proposed. The relationship between the properties of the sensor and the parameters of VLS is discussed. compared with the displacement sensor manufactured by the reflective VLS grating,this type of sensor contains a grating with simpler structure and high diffraction efficiency. It also has good stability with the change of temperature.     

基于相位测量法测量透射式相位光栅的表面形貌

建立了一套基于马赫-曾德尔干涉仪的测量系统,获得了透射式相位光栅的相位信息并推算出光栅的表面形貌。给出了相位测量法的基本原理,运用傅里叶变换法和相位解包裹技术,对实验系统采集的干涉图的相位信息进行处理并计算出光栅的高度、周期和底角等结构参数。结果表明,该测量方法和系统能够精确地测量光学相位元件的表面形貌,具有较高的实用价值。     

自泵浦相位共轭中的温度与混沌态

光折变晶体ＢａＴｉＯ３中实验测量了自泵浦相位共轭输出与温度的关系，发现不发温度超过８２℃后，自泵浦相位共轭输出出现混沌。与已有理论模型的计算机模拟解非常接近，说明在光折变晶体的ＳＰＰＣ中，非随机性混沌行为是温度的函数。     

Elimination of photorefractive grating writing instabilities in iron-doped lithium niobate

Two types of instabilities, one slow (seconds) and one fast (nanoseconds), are observed while writing volume gratings in bulk crystals of iron-doped lithium niobate using contra-directional two-beam coupling along the c axis. A method to eliminate the fast instability was achieved by coating the z surfaces of the crystal with either dielectric (anti-reflection) or transparent conductive coatings. For the second case, electrically shorting the crystal eliminated the slow instability.     

Bragg reflection by a gain medium optically pumped in a grating geometry

The utilization of Bragg reflection caused by a grating-like pump intensity distribution in a laser-pumped laser amplifier is analyzed theoretically. In the amplifier, an incident laser beam is split into a transmitted and reflected beam, both of which are amplified. We explicitly take into account the interference between the transmitted and reflected beam and the effect of the interference on the saturated gain. We determine the reflective conversion efficiency of such an amplifier and show that if the gain grating is accompanied by a pump-heating-induced refractive-index grating, the reflective conversion efficiency may approach 100 percent.     

Dynamic wavefront correction of Nd:YAG lasers by self-pumped phaseconjugation in photorefractive BaTiO3:Rh

We present an overview of the results we obtained with photorefractive BaTiO₃:Rh in which self-pumped phase conjugation is performed to compensate for dynamic aberrations in Q-switched Nd:YAG lasers. Characterization of the material at 1.06 μm with both cw and nanosecond illumination is detailed. Oxidation of the crystals is proposed to decrease the photorefractive rise time at the expense of absorption. Then an optimized compact ring phase conjugate mirror with high reflectivity and fidelity and a short rise time is described. Finally, such a solid-state mirror is introduced into Q-switched Nd:YAG master oscillator-power amplifier laser sources and provides a diffraction limited beam     

Measurement of phase shift of photorefractive gratings by a novelmethod

A novel and sensitive method to measure the phase shift of the photorefractive phase grating with respect to the interference pattern is demonstrated that uses no moving mechanical parts. Results are presented for the measurement of the electrical field dependence of the phase shift in KNbO₃ crystals with various Fe concentrations. Some experimental results are also presented that show the strong influence of internal photovoltaic fields on the phase shift of the space-charge field     

GaN-based LED基础光栅结构实现光的表面等离极化增强

The transmitting light in GaN-based LED with 30 nm thickness metal film grating is investigated. We proposed a basic grating structure model to enhance light intensity in GaN material, which was simpler and cheaper. We calculated and analyzed the structure with different parameters, and studied the numerical simulation results of Ag-films/Al-films/Au-films. With a simple A1 or Ag basic grating structure, the 7.4-7.6 times intensity of 550 nm light can be obtained easily, and the enhancement efficiency is better than others.     

光折变材料中两暗孤子间距离的演化

本文从光折变晶体中能得出暗孤子所满足的演化方程出发,用粒子分析方法得出了两暗孤子相互作用时,其质心间的距离所满足的演化方程。并得出了两暗孤子相互作用势函数的表达式,得到其数值解,并得出如下结论：当两暗孤子间距离△=0时,其相互作用势函数有最小值,表示在光折变晶体中有稳定的暗孤子对,并且相互作用达到最大;当两暗孤子距离△≥5时,其相互作用势函数几乎为零,表示两暗孤子间相互作用很弱,几乎为零。     

Information transmission in evolution

On the basis of a model previously established for the random-mutation portion of the synthetic theory of evolution, an investigation is made of the information that can be transmitted from the environment to the evolving species. The probability that a given adaptive character is produced decreases with increasing information measure of the adaptive character. The average information transmitted in any one evolutionary step is defined as the product of the information and its probability of transmission. The average information turns out to be very small for a reasonable number of trials and reasonable gene size. This is consistent with the conjecture that evolution proceeds in small steps, and the size of a step can be discussed quantitatively in terms of the amount of adaptive information that is being transmitted into the species.     

Pulse retiming based on XPM using parabolic pulses formed in a fiber Bragg grating

We experimentally demonstrate a novel all-optical all-fiberized pulse retiming scheme incorporating parabolic pulses generated in a linear fashion through pulse shaping in a superstructured fiber Bragg grating. The scheme relies on chirping the signal to be retimed using cross-phase modulation with the broader parabolic clock pulses, and subsequently retiming it through linear propagation in a dispersive medium. We demonstrate the cancellation of up to 4-ps root-mean-square timing jitter for /spl sim/2-ps data.     

Structure of light beams in self-pumped four-wave mixing geometries for phase conjugation and mutual conjugation

A family of self-pumped four-wave mixing-based geometries for conjugation and mutual conjugation of light beams is analyzed in the framework of a three-dimensional theory. Results of an experimental investigation of several such geometries conducted with the aim of verification of this theory's predictions are presented. A correspondence between the three-dimensional theory and a more simplistic (but widely used) one-dimensional theory is established.     

Reflection and transmission coefficients of a thin strip grating for antenna application

Reflection and transmission coefficients of thin strip gratings are discussed for plane wave incidence from arbitrary direction. Upon use of the low-frequency approximation, simple closed-form expressions are obtained. They are applicable for the analysis of grid reflector antennas. Physical interpretations of the phenomena are also given and interesting characteristics of the gratings are pointed out. The validity and the applicability of our results is examined numerically by making use of the point matching method (PMM). It is shown that the accuracy is excellent provided the period of gratings is smaller than about 0.3 wavelength.     

Bragg reflection from a phase grating induced by nonlinear optical effects in liquids

Bragg reflection of light from a phase grating has been used as a technique for the investigation of nonlinear optical phenomena. A continuous argon ion laser was used to probe the structure induced in a liquid by the nonlinear interaction of a high-intensity ruby-laser beam with that liquid. This arrangement had the advantage of allowing direct space and time-resolved measurements of the induced effects.     

Ultrasonic transducer configuration for producing a phase grating of nearly uniform strength

A configuration is given which produces nearly uniform phase retardation of a light beam passing through the near field of an ultrasonic transducer.