Using degenerate parametric interaction of intense acoustic beams to amplify weak signals

The paper considers the degenerate parametric interaction of an intense acoustic pumping beam and a weak signal beam at the subharmonic. The use of a special emitter system with independent signal emission at the harmonic and subharmonic made it possible to study the features of nonlinear interaction both for different amplitude levels and arbitrary phase relations of the fields at these frequencies. Just as predicted in the theory, the experiment showed that signal amplification at the subharmonic hardly occurs at all. It is shown that the use of odd field harmonics, which are absent for a zero amplitude of the signal wave, makes it possible to substantially increase the efficiency of isolating a weak signal wave. The interaction of beams for large and small acoustic Reynolds numbers of the signal wave is studied.     

Tunneling of optical beams through inhomogeneity of a refractive index

We consider the effect of tunneling of optical beams through a narrow induced inhomogeneity in a refractive index. It is shown that under the condition of total internal reflection from the induced channel, part of the signal beam leaks if the channel is narrow. Dependence of the pump beam width at which the tunneling of half signal power occurs is found as a function of the pump intensity and the angle of beam crossing.     

Forward four-wave interactions in bismuth silicon oxide-theory and experiment

The interaction between two strong pump beams and two signal beams is investigated both theoretically and experimentally. It is shown that the frequencies of the beams need to satisfy the relation that the sum of the pump beam frequencies should be equal to the sum of the signal beam frequencies. The gain is shown to depend on the detuning of the pump beams relative to the incident signal beam and on the direction of the signal beam. Maximum gain is obtained when the signal beams bisect the pump beams.     

Photorefractive amplification and generation of light beams in BaTiO3 controlled by the intensity of the pump light

3 by photorefractive forward four-wave mixing. The intensity of a signal beam is amplified by a factor up to 3000. We show that heating of the crystal by absorption of the pump light influences the gain because of the thermo-optic effect. Thus both, amplification and generation can be controlled by the intensities of the pump beams. Received: 23 January 1998     

自散焦介质中光束聚焦的最佳参数选择

对自散焦介质中由交叉相位调制效应引发的光束聚焦过程进行了细致的系统研究,讨论了抽运光初始振幅、抽运光信号光初始间距和抽运光信号光波长比三个参量对信号光聚焦过程的影响.数值模拟结果表明:抽运光越强,信号光聚焦程度越大;抽运光信号光初始间距和抽运光信号光波长比均存在一个最佳值,使得光束聚焦达到最佳效果.通过参数最佳值的选择,可提高光束聚焦的效率 关键词： 非线性折射率系数 交叉相位调制效应 自散焦介质 光束聚焦     

Mathieu-Gauss beams: A thermal consideration

The generation of Helmholtz-Gauss beam families such as Mathieu-Gauss (MG) and Bessel-Gauss (BG) beams can be dramatically suffered from heat-induced thermal effects. This work is devoted to the generation of MG beams under severe induced thermal loads. The output MG beams of a solid-state laser were assumed to pass through an ABCD system matrix. The simulated results which were achieved by characterization of the transfer matrix of the thermally affected inhomogeneous medium report systematic variations of the intensity profile of MG beams from pump power of 1-5 W. It is shown that for high values of pump power, the thermal effects are so influential and the beam intensity profile is so changed that the identification of MG beams is not easily possible at first hand. The results can facilitate this identification and can be utilized for deeper understanding of thermal effects phenomena.     

Photorefractive power transfer from parallel laser diode amplifiers into a single output beam

Photorefractive BaTiO(3):Ce was used to combine coherently a signal beam and two amplified pump beams at lambda=678 nm. The master laser beam was split and then amplified in two semiconductor laser amplifiers with gain of as much as 6. The amplifiers were made from quantum-well laser chips by use of antireflection coating. 40% of the power could be transferred to the signal beam without transferring the phase distortions of the pump beams.     

Phase sensitive parametric amplification of optical vortex beams

Optical vortex beams with different topological charges are amplified in a travelling wave phase sensitive parametric interaction. Amplified beams observed either in the near field or in the far field domains exhibit patterns that depend on the relative phase between the pump and the vortex beams. Experimental results are compatible with the conservation of orbital angular momentum of the input beam whatever phase matching conditions.     

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.     

On observability of the ac Stark effect line splitting in case of beams of finite size

The small signal gain distortions in the field of a Gaussian pump beam were investigated theoretically and experimentally in a Λ-type quantum system depending on the size and power of the pump beam. The strong pump beam induces a change of refraction index of a methanol model material, depending on the frequency of the signal. It leads to a nonlinear active waveguiding effect causing distortion (narrowing and shift) of the signal line shape. In case of Stark splitting, the induced nonlinear active waveguiding effect manifests itself in the disappearance of the line split, for narrow pump beams. Collision induced frequency shift of the signal line depending on the pressure of the methanol gas was observed.     

位相畸变对强紫外激光在空气中长程传输的影响

 采用瞬态受激旋转拉曼散射（SRRS）模型,详细考虑了激光脉冲的位相变化、时间波形、泵浦光的衰减、斯托克斯光的非线性放大以及初始的随机斯托克斯散射场等诸多因素,研究了空间位相畸变对空气中长程传输过程中产生的SRRS效应的影响。研究结果表明：位相畸变对阈值条件和时间特性影响较小,而对光束质量影响较为明显;随着位相畸变的增大,光束的能量分布变得极不均匀,光束的光强对比度也明显变大,从而使激光光束质量变差,而且当传输距离达到阈值条件后,随着传输距离的增加,光束质量将进一步变差。     

Comparison of SRS thresholds under pumping by Gaussian and Bessel beams

In the paraxial approximation, it has been shown that the quasi-stationary SRS amplification coefficient in the near-threshold region is determined by the product of the pump intensity in the beam center and the overlap integral of the transverse structure of the pump and the Stokes component. Expressions for the overlap integral and the scattering powers have been obtained and analyzed for the following cases: the pump and the Stokes field are Gaussian beams; the pump and the Stokes field are Bessel beams; the pump is a Bessel beam and the Stokes field is an axial Gaussian beam. Relations between the SRS excitation thresholds in different cases have been found.Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 71, No. 6, pp. 778–787, November–December, 2004.This revised version was published online in April 2005 with a corrected cover date.     

Application of focused beams for parametric reflection control

The effect of parametric reflection of a signal beam from a focused reference beam has been investigated. It is shown that focusing reduces a pump power reflection threshold but causes certain distortions due to the reduction of the area of the parametric mirror and occurrence of its longitudinal curvature. These effects limit the width and angle of incidence of the signal beam. To decrease the sum wave power, it is proposed to introduce an additional dispersion mismatch.     

Nonlinear repolarization in optical fibers: polarization attraction with copropagating beams

We propose a type of lossless nonlinear polarizer, novel to our knowledge, a device that transforms any input state of polarization (SOP) of a signal beam into one and the same well-defined SOP toward the output, and perform this without any polarization-dependent losses. At the polarizer output end, the signal SOP appears to be locked to the input pump SOP. The polarizer is based on the nonlinear Kerr interaction of copropagating signal and pump beams in a telecom or randomly birefringent optical fiber.     

Saturated-absorption spectroscopy: eliminating crossover resonances by use of copropagating beams

We demonstrate a new technique for saturated-absorption spectroscopy by use of copropagating beams that does not have the problem of crossover resonances. The pump beam is locked to a transition, and its absorption signal is monitored while the probe beam is scanned. As the probe comes into resonance with another transition, the pump absorption is reduced and the signal shows a Doppler-free dip. We use this technique to measure hyperfine intervals in the D2 line of 85Rb with a precision of 70 kHz and to resolve hyperfine levels in the D2 line of 39K that are less than 10 MHz apart.     

非傍轴截断平顶光束的质量因子

 使用功率密度的二阶矩方法和Li提出的平顶光束模型，推导出了非傍轴截断平顶光束的束腰宽度、远场发散角和光束质量因子的解析表达式。计算结果表明：非傍轴截断平顶光束的质量因子不仅与截断参数、光束阶数有关，而且与初始束腰宽度和波长之比有关；当截断参数趋近于0时，远场发散角趋于渐近值63.435°；对于光束阶数为1和截断参数趋于无穷大的特例，计算结果分别退化为非傍轴截断高斯光束和非傍轴无截断平顶光束的结果；功率密度的二阶矩方法可用于截断光束，并克服了光束质量因子的发散困难。     

Nonlinear interaction and reflection of incoherent light beams

The interaction dynamics between two incoherent light beams in a medium with thermal nonlin-earity is studied both experimentally and theoretically. It is shown that in the mode of a nonlinear reflection the transformation of a spatial structure of a weak signal beam can be theoretically described in the framework of a spatially distributed model for a nonlinear change in the refractive index of the medium.     

Partially coherent nonparaxial beams

The concept of a partially coherent nonparaxial beam is proposed. A closed-form expression for the propagation of nonparaxial Gaussian Schell model (GSM) beams in free space is derived and applied to study the propagation properties of nonparaxial GSM beams. It is shown that for partially coherent nonparaxial beams a new parameter f(sigma) has to be introduced, which together with the parameter f, determines the beam nonparaxiality.     

Method for investigation of parameters of X-ray beams diffracted from a single crystal in the presence of temperature gradient

A method for investigation of the parameters of X-ray beams pumped from the transmission direction into the reflection one depending on the value of applied temperature gradient is proposed. An increase in the integral intensity of diffracted X-ray beam is due to both the overthrowing effect inside the Darwin table and an increase in the angular width of beam. It is shown that it is possible to determine to a high accuracy the value of temperature gradient, beginning with which the angular width of the pumped X-ray beam increases.     

Simultaneous and sequential power transfer between coherent beams by photorefractive coupling

The power of several coherent laser beams can be transferred into a single diffraction-limited signal beam by photorefractive coupling. The efficiency of the power transfer is investigated experimentally in the case of two pump beams. Two different geometrical arrangements, one with overlapping (simultaneous) and the other with separate (sequential) pump beams are compared in a BaTiO₃:Ce crystal, with diffusion-driven photorefraction. We measured about the same power transfer efficiency in both arrangements for strong signal beams, but the efficiency was higher in the sequential arrangement for weak signals. A simple theoretical model of the power transfer process based on the standard linear two-wave-mixing theory is presented and the observed stationary beam-coupling behavior for different pump-to-signal intensity ratios is found to be in qualitative agreement with the theoretical predictions. Received: 10 November 1998 / Revised version: 22 December 1998 / Published online: 24 March 1999