Light-induced drift of ions in a magnetic field

The effect of a magnetic field on the ion drift in a weakly ionized gas under the combined action of the light-induced drift and light pressure is studied theoretically. It is shown that, under the action of light, a component of ion drift velocity transverse to the direction of propagation of radiation may appear in a weakly ionized gas upon the application of an external magnetic field. It is shown that the Lorentz force acting on ions in the magnetic field radically changes the dependence of the ion drift velocity on the radiation frequency detuning. It is predicted that the ion drift velocity component along the direction of radiation must reverse its sign upon an increase in the magnetic field and an anomalous light-induced drift may be observed.     

Mathematical modeling of random coupling between polarization modes in single-mode optical fibers: VIII. Zero drift in a fiber ring interferometer with a white light source

Different schemes of fiber ring interferometers (FRIs) with a broadband nonmonochromatic radiation source manufactured on the basis of air-silica microstructured single-mode optical fibers (SMOFs) are considered. This source is close in spectral characteristics to a white light source, because the width of its emission spectrum is comparable to the mean wavelength. It is shown that an increase in the width of the spectrum of the radiation source can lead to either a substantial decrease or an increase in the zero drift, depending on the radiation polarization at the FRI entrance. The latter fact has defied explanation within simple phenomenological models of random coupling between polarization modes in SMOFs of an FRI circuit. The observed increase in the zero drift of the FRI can be explained in terms of the dependence of the parameter of polarization conservation (the parameter h) on the light wavelength for highly anisotropic SMOFs. This dependence is weak for nonmonochromatic radiation sources with a relatively small spectral width, for example, superluminescent diodes, which are traditionally used in FRIs. In contrast, for substantially more broadband radiation sources (including air-silica SMOFs), the above dependence is well pronounced and can lead to a number of undesirable effects in FRIs. Different variants of the FRI design are analyzed. It is demonstrated that the zero drift can be noticeably decreased with an increase in the width of the spectrum of the radiation source for an arbitrary radiation polarization at the entrance of an FRI with a depolarizer of nonmonochromatic radiation and a circuit fabricated from a weakly anisotropic SMOF, for which the parameter h does not depend on the light wavelength. The numerical estimates are obtained.     

激发光线宽对原子光致漂移速率的影响

在光致漂移效应的研究中,激发光线宽会改变原子激发的速度选择性,进而影响漂移速率的大小.本文以原子光致漂移速率方程理论为基础,利用强碰撞模型描述原子与缓冲气体的碰撞作用,运用数值方法对速率方程进行求解计算,研究了激发光线宽对原子漂移速率的影响.研究结果表明,其他条件相同时,随着线宽的增大,漂移速率的值呈现先增大后减小的趋势.存在一个最佳的激发光线宽,使得原子的漂移速率达到最大值.最佳线宽与激发光功率密度、温度和缓冲气体压强有关.为了获得最佳的光致漂移效果,激发光应工作在最佳线宽条件下.当激发光线宽在最佳线宽附近波动时,设置激发光线宽略大于最佳线宽可减少线宽波动对漂移速率的影响,对获得较大漂移速率更为有利.     

Anomalous drift of resonant particles in a buffer medium under pressure of light

We study theoretically the drift of resonant particles in a buffer medium when a traveling light wave impinges on the medium, with allowance for the velocity dependence of the transport collision rate. When the pressure of light dominates over the light-induced drift (low pressure of the buffer gas or the drift of conduction electrons in semiconductors), we discover a new sudden transformation of the spectral dependence of the drift velocity of the resonant particles: Instead of the ordinary bell-shaped function representing the velocity spectrum we have a double-humped curve with deep dip at the center of the absorption line. We show that the largest transformation of the drift velocity spectrum occurs in the atmosphere of a heavy buffer gas in the case of Coulomb interaction between the resonant and buffer particles. The transformation effect is caused by the variation of the transport rate of the collisions of the resonant and buffer particles due to the recoil effect in the absorption of radiation. Zh. éksp. Teor. Fiz. 112, 856–868 (September 1997)     

Influence of a magnetic field on the light-induced ion drift

It is shown that the velocity component transverse to the radiation propagation direction can arise in the light-induced ion drift (LIID) after applying an external magnetic field to a weakly ionized gas. It is predicted that the projection of the ion drift velocity onto the radiation direction changes its sign with an increase in the magnetic field, resulting in the anomalous LIID.     

On the collisional transfer of nonequilibrium in the velocity distribution of resonant particles in a laser radiation field

The collisional transfer of nonequilibrium in the velocity distribution of resonant particles in a laser radiation field is investigated theoretically. It is shown numerically that the transfer effect is weak. This makes it possible to use simpler approximate one-dimensional quantum kinetic equations instead of three-dimensional equations to solve spectroscopy and light-induced gas kinetics problems, where it is important to take account of the velocity dependence of the collision frequency. It is shown for anomalous light-induced drift, calculations of which are most sensitive to neglecting the transfer effect, that in a wide range of spectroscopy and light-induced gas kinetics problems the transfer of nonequilibrium can be neglected without risking the loss of important fine details of the phenomena being described.     

The possibility for a significant decrease in the zero drift of fiber ring interferometers with a circuit formed by a highly anisotropic optical waveguide and a broadband radiation source

It is shown that the zero drift of interference of counterpropagating waves at the output from a fiber ring interferometer can be completely eliminated in the first order of magnitude and significantly decreased in the second order of magnitude if a sufficiently broadband source of nonmonochromatic radiation is used in a fiber ring interferometer with a circuit formed by a highly anisotropic optical waveguide when the depolarization length of nonmonochromatic radiation in a single-mode fiber-optical waveguide becomes significantly shorter than the correlation length of random inhomogeneities. Numerical estimates are made.     

Heat and mass transfer in a gas in a capillary induced by light with nonuniform intensity distribution over the beam cross section

An analysis is presented of the heat and drift fluxes induced by velocity-selective light absorption in a single-component gas in a capillary tube. The light intensity distribution across the beam is assumed to have a Gaussian profile. Kinetic equations are solved numerically to calculate flux profiles and kinetic coefficients quantifying the contributions of surface and collisional mechanisms to light-induced transfer as functions of the Knudsen number, the ratio of the rate of radiative decay of the exited level and intermolecular collision frequency, accommodation coefficient, and the ratio of the tube radius to the light beam radius.     

激光线宽和光强对同位素原子选择光电离的影响

本文研究了非单色(有限带宽)激光场与同位素原子体系相互作用的激发光电离过程. 采用混沌场随机模型描述激光场,用密度矩阵理论和Fokker-Planck方程方法首次给出了非单色激光场与多能级原子相互作用的激发动力学方程. 针对三能级同位素原子体系,讨论了激光线宽和激光光强对同位素原子电离概率和激光同位素分离过程中分离选择性的影响. 关键词： 激光同位素分离 激发动力学方程 激光线宽 Rabi频率     

Mathematical modeling of random coupling between polarization modes in single-mode optical fibers: VI. Evolution in the degree of polarization of nonmonochromatic radiation traveling in a twisted optical waveguide

An analysis is made of the evolution of polarization of nonmonochromatic radiation travelling through single-mode optical fibers in the presence of random coupling between orthogonally polarized modes, which is caused by the random twist of axes of natural linear birefringence of an SMF, and in the presence of a regular axis twist. It is shown that the depolarization length of nonmonochromatic radiation in an SMF increases with increasing regular SMF twist, regardless of the presence of a random axis twist. Using the Monte Carlo method, the dependences of the mathematical expectation and the mean-square deviation of the degree of polarization of nonmonochromatic radiation on the fiber length for SMFs with different linear birefringences and regular axis twists are calculated. It is shown that the fiber length on which the degree of polarization of nonmonochromatic radiation reaches its limiting value increases and the limiting value itself decreases with an increasing regular twist. It is also shown that an induced SMF twist is able to improve parameters of a fiber interferometer, in particular, to decrease random phase changes and intensity fadings of the interference signal, which are caused by random coupling between polarization modes.     

Impressed by light: Mechanical action of radiation on atomic motion

Intense light can exert a mechanical action on the motion of atoms in two quite distinct ways. The resonance radiation force is based upon the exchange of photon momentum between an atom and the radiation field. The phenomenon of light-induced drift results from the difference in mobility of excited and ground-state atoms in a buffer gas. We derive the relevant equations of motion for the atomic distribution function, by applying a systematic formulation of time-dependent degenerate perturbation theory. For the resonance radiation force, we obtain a Fokker-Planck equation for the velocity distribution of atoms in one or several light beams, while including the degeneracy of the atomic levels. The derivation generalizes previous results, and it clarifies the physical origin of the various terms. Likewise we obtain a diffusion-type equation for the density of atoms which contains light-induced drift, and we obtain explicit solutions in several model cases of practical interest. These solutions demonstrate the front formation that produces the effect of the optical piston in an optically thick system. Finally we discuss the differences and the analogies between the two effects.     

Scanning of a laser beam and purification of materials through the use of light-induced particle drift in semiconductors

The light-induced drift of electrons, light-absorbing impurities, and defects in II-VI semiconductors is investigated experimentally, along with some potential practical applications of the phenomenon. It is shown that the light-induced drift of electrons induces a very pronounced change in the refractive index, |Δn|∼0.01, and can be used to implement effective scanning of nanosecond and picosecond laser pulses through frustration of total internal reflection. The light-induced drift of absorbing particles increases their density in the surface layer of the crystals, and this effect can be exploited in semiconductor technology. Zh. Tekh. Fiz. 68, 121–124 (April 1998)     

Light-induced drift of neon under optical excitation from a metastable state

A running light wave, resonant with the neon transition 1s₅-2p₂, has been found out to affect the spatial distribution of neon atoms in a positive discharge column. The effect has shown an antisymmetric dependence (typical of the light-induced drift) on the tuning out of the radiation frequency relatively the absorption line centre, as well as on the direction of the light wave vector.     

基于光闪烁法的烟气流速反演和路径加权函数的分析

为了对工业管道排放的污染气体的流速进行连续监测,设计了一种双路平行对射式烟气流速测量系统,并对基于光闪烁法测量烟气流速的相关理论进行了研究.利用相位屏技术对湍流介质中的光闪烁现象进行了理论分析,得到了用于工业管道烟气流速反演的光闪烁互相关的表达式.对烟气流速反演过程中平均流速的路径加权问题进行了探讨,给出了路径加权函数,数值模拟表明路径加权函数具有近似高斯线型的轴对称分布特性,反映出管道中心流速对于平均流速的较大贡献.在此基础上,分析了光源光谱对路径权重函数的影响,实验证明由于光源光谱具有一定的带宽,使得路径加权函数值有一定的变化,但加权函数的分布形状保持不变.对于平均流速的路径加权函数的分析为计算工业管道中烟气流速的空间分布提供了依据.     

基于光闪烁法的烟气流速反演和路径加权函数的分析

为了对工业管道排放的污染气体的流速进行连续监测,设计了一种双路平行对射式烟气流速测量系统,并对基于光闪烁法测量烟气流速的相关理论进行了研究.利用相位屏技术对湍流介质中的光闪烁现象进行了理论分析,得到了用于工业管道烟气流速反演的光闪烁互相关的表达式.对烟气流速反演过程中平均流速的路径加权问题进行了探讨,给出了路径加权函数,数值模拟表明路径加权函数具有近似高斯线型的轴对称分布特性,反映出管道中心流速对于平均流速的较大贡献.在此基础上,分析了光源光谱对路径权重函数的影响,实验证明由于光源光谱具有一定的带宽,使得路径加权函数值有一定的变化,但加权函数的分布形状保持不变.对于平均流速的路径加权函数的分析为计算工业管道中烟气流速的空间分布提供了依据.     

单模石英光纤受激拉曼散射温度特性研究

研究了在不同温度下单模石英光纤的受激拉曼散射光谱,从实验和理论上分析了温度对拉曼散射光谱特性的影响,在脉冲调Q倍频YAG激光的泵浦作用下,获得了石英光纤一级斯托克斯光的拉曼频移、带宽及光强随温度的变化规律。实验表明随着温度的升高,拉曼频移逐渐增大,在一定的温度范围内拉曼频移和温度成线性关系。在相同的泵浦功率作用下,当温度较低时,拉曼光谱的级次较低, 低温对高阶斯托克斯光有抑制作用; 温度越低其阈值越高;而拉曼光谱的谱线宽度随温度的变化不是线性的,存在一个谱线宽度极大值点。理论和实验表明温度对光纤受激拉曼散射的光谱特性有直接的影响。     

Mathematical modeling of random coupling between polarization modes in single-mode optical fibers: XI. Dependence of the zero drift of fiber ring interferometers on the interval of temperature variation of the fiber

The dependence of the value of the zero drift of a fiber ring interferometer (FRI), which is caused by a change in polarization nonreciprocity as a consequence of a change in the random coupling of polarization modes at inhomogeneities of the FRI loop, on the value of the interval of possible temperature variation of the single-mode optical fiber (SMF) of the FRI loop is considered. It is shown that the value of the zero drift of an FRI with a nonmonochromatic radiation source—a superluminescent diode—in a rather broad interval of temperature variations (about 100 K) is proportional to the value of the interval itself and inversely proportional to the linewidth of the radiation source. Numerical estimations are made, and they show that, for practical values of the interval of temperature variation in a room, the value of the zero drift may be much smaller than the values calculated according to the method of averaging over independent realizations of random inhomogeneities in the SMF of an FRI loop. When temperature stabilization is used, the value of the zero drift of an FRI can be additionally reduced by one to two orders of magnitude.     

Light-induced heat and mass transfer in a single-component gas in a capillary

A theoretical analysis is presented of light-induced heat and mass transfer in a single-component gas in a capillary tube at arbitrary Knudsen numbers. Surface and collisional mechanisms of transfer are analyzed, due to differences in accommodation coefficient and collision cross section between excited-and ground-state particles, respectively. Analytical expressions for kinetic coefficients characterizing the gas drift and heat transfer in a capillary tube are obtained in the limits of low and high Knudsen numbers. Numerical computations are performed for intermediate Knudsen numbers. Both drift and heat fluxes are determined as functions of the light beam frequency. In the case of an inhomogeneously broadened absorption line, the light-induced fluxes are found to depend not only on the sign, but also on the amount, of light beam detuning from the absorption line center frequency.     

On a Modification of the Scintillation Method

We solve the problem of diffraction of fluctuating radiation by an optically thin irregular layer (phase screen) with developed turbulent structure. It is shown that in the case of diffraction of radiation with saturated fluctuations and a narrow-band frequency spectrum by a weakly turbulent moving phase screen, the measured frequency spectrum of intensity fluctuations in the observation plane allows one to obtain information on the form of the spectrum of irregularities of an optically thin irregular layer in a wide size range significantly exceeding the size of the first Fresnel zone. Similarly to the well-known phase method of diagnostics of randomly irregular media, the conventional scintillation method modified in such a way yields undistorted information on the form of the irregularity spectrum. However, in contrast to the phase method, it also allows one to obtain data on the drift velocity of irregularities in the studied irregular layer.     

Theoretical study of resistance against light-induced scattering in LiNbO3:M (M = Mg2+, Zn2+, In3+, Sc3+) crystals

Based on a multi-three-wave mixing model, we theoretically study the resistance against light-induced scattering in LiNbO(3):M (M=Mg(2+),Zn(2+),In(3+),Sc(3+)) crystals. We have simulated the intensity angular distribution of light-induced scatterings. We have also shown that the total light-induced scattering will be much less than 1% of the incident light intensity when the photovoltaic field E(ph) is less than 4.0x10(6)V/m .Phase gratings and signal beams can still be formed and amplified effectively in LiNbO(3):M crystals, with E(ph) less than 4.0x10(6)V/m .