用于实现散射介质中时间反演的数字相位共轭的相干性

抑制散射介质对光的散射,调控光在散射介质中的传输,是光通信、生物光子学、光镊等领域的重要课题.设计并实现了基于宽谱光源和数字相位共轭的可调控光在散射介质中传输的时间反演实验系统.实验获取了不同相干长度下物光和参考光束之间的光程差与干涉图样、相位图及时间反演信号之间的关系,分析了光源相干性对调控光在散射介质中传输的影响.实验结果表明,基于宽谱光源的相干特性和数字相位共轭技术,通过调节光程差能选择性获取同一散射角度及相同传输路径的光束的相对相位,再利用空间光调制器对参考光束进行调控,实现光束的反向传播,从而选择性实现对同一散射角度及相同传输路径的光的时间反演.     

Transparency in Bragg scattering and phase conjugation

Reflectionless transmission of light waves with unitary transmittance is shown to occur in a certain class of gain-grating structures and phase-conjugation mirrors in the unstable (above-threshold) regime. Such structures are synthesized by means of the Darboux method developed in the context of supersymmetric relativistic quantum mechanics. Transparency is associated to superluminal pulse transmission.     

Time reversal symmetry in light scattering by excitations in a film

We present a general symmetry argument for light scattering by excitations in an insulator film. The analysis holds for bulk continuum, guided and surface excitations.The main results represent a simplification of the theory and a better understanding for the time reversal properties of the system. Those properties hold both for forward and backward scatering, and are independent of the substrate upon which the film is deposited. We show that the asymmetry in the shape of the cross-section which is common to many experimental situations is in fact a consequence of the time reversal properties of the system.     

Directional scattering and self-pumped phase conjugation in photorefractive crystal

The directional scattering in photorefractive SBN crystal is reported.The self-pumped phase conjugation has been realized with the feedback of this directional scattering andthe performance parameters are compared with those of cat self-pumped phase conjugator.     

Temporal characteristics of phase conjugation fidelity in stimulated Brillouin scattering

The pulse compression and phase conjugation characteristics of stimulated Brillouin scattering (SBS) in CS₂ were investigated in a focusing geometry using an iodine laser as pumping source. The highly compressed front part of the Stokes pulse was found to have better phase conjugation fidelity than the uncompressed tail part. This result is explained by considering the difference in the interaction length and spatial structure of the front and tail parts of the Stokes pulse.     

Low-power phase conjugation based on stimulated Brillouin scattering in fiber amplifiers

Phase-conjugating stimulated Brillouin scattering (SBS) mirrors are frequently used to improve the brightness of solid-state lasers. With a scheme for designing fiber amplifiers as SBS phase conjugators the power requirements can be decreased by orders of magnitude; yet only low coherence is required, and high fidelity is reached. A peak power of 5 W was demonstrated to be sufficient for optical SBS phase conjugation.     

偏振相移与相位共轭结合的散射光聚焦技术研究

散射介质对光的随机散射作用是制约其光学聚焦和成像的重要因素，光学相位共轭技术能够通过对散射光场共轭还原实现透过散射介质的光学聚焦和成像，其中散射光场相位的获取是共轭还原的核心。阐述了偏振相移的基本原理，将偏振相移与相位共轭技术相结合，设计了新的基于偏振相移的数字光学相位共轭系统。采用633 nm的HeNe激光器作为系统光源，毛玻璃作为散射介质开展散射光聚焦实验研究。实验结果表明:该装置能够成功实现透过散射介质的光学聚焦，其中聚焦点与背景光强的比值可达约400倍。     

Time reversal violation in the nuclear interaction and p-3He scattering

Using T-violating boson-exchange interactions T-violating effects in low energy p-³ He scattering are calculated. The results are below 10^?3 even for full strong (not millistrong) T-violation in the nucleon-nucleon system. It is argued, that the smallness of the effects is not a particularity of the p-³He system but a general property of low energy processes.     

Mechanism of phase conjugation via stimulated Brillouin scattering in narrow band gap semiconductors

We develop a theoretical model to study optical phase conjugation via stimulated Brillouin scattering (OPC-SBS) in narrow band gap transversely magnetized semiconductors. Threshold value of pump electric field and reflectivity of the image radiation for the onset of OPC-SBS are estimated. The analysis is applied to both cases viz. centrosymmetric (CS) and non-centrosymmetric (NCS) crystals. Numerical estimates made for n-type InSb crystal at liquid nitrogen temperature duly irradiated by nanosecond pulsed 10.6 μm CO₂ laser shows that high OPC-SBS reflectivity (90%) can be achieved in NCS crystals at moderate pump electric fields if the crystal is used as an optical waveguide with relatively large interaction length (L = 5 mm) which proves its potential in practical applications such as fabrication of phase conjugate mirrors.     

Acoustic phase conjugation in a three-phase medium

Acoustic phase conjugation is studied in a sandy marine sediment that contains air bubbles in its fluid fraction. The considered phase conjugation is a four-wave nonlinear parametric sound interaction caused by nonlinear bubble oscillations which are known to be dominant in acoustic nonlinear interactions in three-phase marine sediments. Two various mechanisms of phase conjugation are studied. One of them is based on the stimulated Raman-type sound scattering on resonance bubble oscillations. The other is associated with sound interactions with bubble oscillations whose frequencies are far from resonance bubble frequencies. Nonlinear equations to solve the phase conjugation problem are derived, expressions for acoustic wave amplitudes with a conjugate wave front are obtained and compared for various frequencies of the excited bubble oscillations.     

Wave phase conjugation of the stokes component of Brillouin scattering of ultrasound

Wave phase conjugation of a low-frequency ultrasonic wave due to Brillouin scattering of high-frequency parametrically phase conjugate waves in liquid was theoretically and experimentally studied. It was shown that the envelope of the low-frequency conjugate wave is proportional to the product of envelopes of high-frequency waves. The conjugation efficiency corresponds to the angular diagram of the resonant interaction of ultrasonic beams. Compensation for the phase shift of the plane low-frequency wave due to its wave phase conjugation was demonstrated.     

The energy saturation of the stimulated Raman scattering in quasi-stationary regime and phase conjugation

The energy dependence and the total saturation of the SRS and SBS are investigated in the nanosecond range at different geometries of the illuminated volume. The theoretical explanation of the experimental results for SRS is given, the change of the vibrational level population being taken into account. The solution of the quasi-stationary equations for SRS if shown to have the form of a solitary wave. The results of the work permit to govern the amplitude-phase distribution of the complex light fields at the stimulated scatterings.     

On the possibility of phase conjugation of cw multifrequency HF-laser radiation using Bragg scattering in a HBr-Xe mixture

We consider an approach to phase conjugation (PhC) of multifrequency radiation of a cw HF chemical laser (HF-CCL) using Bragg scattering in a resonantly absorbing medium. The choice of working components of this medium, HBr and Xe, is analyzed at length. Numerical calculations are performed employing a mathematical model presented in this paper; the reflectivity is plotted as a function of the component concentration ratio, the intensities of waves determining the volume hologram in a WFI cell, and the angle of their convergence. We consider the problem of cooling the medium by mixture circulation. The calculations show that the approach suggested is a candidate for implementing the PhC of a high-power multifrequency HF-CCL.     

Holographic phase conjugation through a sub-wavelength hole

Holographic phase conjugation is analyzed as a method to create a photo-refractive lens with high numerical aperture. For this purpose a sub-wavelength hole is drilled into a metal surface directly on top of an iron-doped lithium niobate crystal. An interference pattern generated by the light coming from this point source and a plane reference wave is recorded. By using the phase-conjugated reference wave for read-out, a light wave being focused onto the former point source is reconstructed. In principle, a focusing system close to the theoretical diffraction limit could be implemented by this method. The performance of this arrangement is mainly determined by properties of the lithium niobate crystal, especially the crystal symmetry. Experimentally, the tight holographic focusing is demonstrated. The focus width of the reconstructed wave is shown to be below 1.2 μm, which is our spatial resolution. The diffraction efficiency obtained, however, is just 3×10⁻⁵ compared to 3×10⁻² in the plane-wave case. This can be explained by experimental reasons, the inhomogeneous light intensity and limitations originating from the crystal symmetry. We estimate that the diffraction efficiency for phase conjugation through a sub-wavelength hole can be improved by three to four orders of magnitude by addressing the above-mentioned issues.     

Temporally stable continuous-wave phase conjugation by stimulated Brillouin scattering in optical fiber with cavity feedback

Temporally stable high-fidelity optical phase conjugation by stimulated Brillouin scattering of cw laser radiation is achieved in multimode silica fiber with cavity feedback. A power threshold of approximately 500 mW and a reflectivity of 70% were obtained in a 340-m-long fiber with a cw Nd:YAG laser.     

NN scattering phase shifts; quark-gluon versus meson exchange

Nucleon-nucleon scattering phase shifts are determined from an effective NN interaction provided by the central and spin-spin part of the one-gluon-exchange quark-quark potential in the Born-Oppenheimer approximation. The results are compared with corresponding predictions given by conventional ω, ?-meson exchange taken from a realistic OBE-potential.