The effect of optical bias on grating formation dynamics in photorefractive BSO

We present experimental results describing the influence of optical bias on the transient characteristics of gratings formed in photorefractive BSO. A transient peak is evident, and the properties of this peak can be controlled by suitable adjustment of the relative intensities of the optical beams involved.     

Polarization properties in two wave mixing with moving grating in photorefractive BSO crystals. Application to dynamic interferometry

The polarization properties of the volume gratings in photorefractive B₁₂SiO₂₀ crystals permit to increase the contrast and signal to noise ratio in the image plane of a large size vibrating object structure. The experiment is based on the nearly degenerate two wave mixing of a pump beam with a low intensity signal beam due to the object structure (moving grating recording with an externally applied field in the 110 crystallographic direction). For a correct orientation of the incident recording beam polarizations, the pump beam emerging from the crystal is linearly polarized, while the signal, after two beam coupling is elliptically polarized. The selective attenuation of the direct pump and scattered beams is obtained with a polarizer. Improved contrasts in the interferogram plane of the vibrating object are demonstrated.     

Spatially multiplexed phase conjugate imaging and processing in photorefractive BSO

Theoretical considerations and experimental results are presented for simultaneous degenerate multi-wave mixing in photorefractive BSO. Using the same crystal volume and common pump beams, multiple imaging has been achirved in which each image shows no cross-talk. The mutual modification of the multiplexed gratings presents no problem in terms of destructive depletion effects. Simultaneous correlation of two images with the same test image has also been achieved, verifying the independent nature of each grating within the crystal.     

Spinning-top dynamics of photorefractive grating

The photorefractive grating generated by two coupled waves produces a rich spatiotemporal dynamics such as those of solitons and chaos. In this paper, the dynamics of grating along with the slow-varying envelope of two coupled waves is studied where the photorefractive-coupling constant is large or more importantly the dynamics is much faster than the photorefractive response time of the material. Under this condition, new solutions of the dynamic grating equations are presented and analytically solved in the form of the Jacobi’s Elliptic functions. The spatiotemporal grating can result in a periodic space-charge field which in turn generates the space charge waves in the periodic case as a result of the spatiotemporal dynamics of the anisotropic two-wave mixing process. The theoretical work of the limiting case is in good agreement with the experimental results using a BaTiO₃ crystal.     

Space-charge grating kinetics in photorefractive crystals

Equations for the grating space-charge density and concentration of free carriers induced by a standing light wave in an anisotropic photorefractive crystal are derived and solved. The charge density and electric field are found to depend on the orientation of the graing relative to the crystal axes. It is shown that the grating kinetics is determined by the difference between two exponentials with characteristic times the sum of which can be treated as the decay constant of the grating. Three ranges of Maxwell relaxation times are distinguished, where this time constant behaves differently. The grating kinetics for a number of crystals is considered.     

CRT－LCLV and photorefractive crystl BSO based real－time joint transform correlator

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High speed two input real-time optical correlation using photorefractive BSO

Two input real-time optical correlation using photorefractive media is discussed. Consideration is given to the bearing of the input arrangement on the correlator's response time to changes in input. It is demonstrated that previous correlator arrangements using photorefractive Bi₁₂SiO₂₀ (BSO) may be adapted to improve their time response characteristics. Re-allocation of information carrying inputs leads to a two input correlator in which one input may be updated with arbitrary rapidity. The updating of the remaining input is subject to the limits imposed by the dynamics of the photorefractive effect. This is contrasted with previous correlator designs in which the updating of both inputs is subject to such dynamic limitations.     

Anisotropic diffraction of stratified volume holographic grating in photorefractive media

We have studied the anisotropic diffraction properties of the stratified volume holographic gratings recorded in photorefractive media using the anisotropic coupled wave theory. It is shown that the diffraction efficiency of such system exhibit the uniform periodic Bragg selectivity properties. In addition the dependence of the stratified volume holographic optical elements (SVHOEs) diffraction properties on the buffer-layer thickness, grating-layer thickness, number of modulation layers, and total thickness of system are discussed in detail.     

Effect of higher order nonlinear optical processes on optical absorption in the photorefractive BSO and BGO crystals

The processes of nonlinear refraction and nonlinear absorption are studied in the photorefractive Bi₁₂SiO₂₀ (BSO) and Bi₁₂GeO₂₀ (BGO) crystals at a wavelength of a picosecond Nd:YAG laser of 1064 nm. The nonlinear refraction in the crystals is shown to be related to the Kerr effect, and the nonlinear absorption at this wavelength, to three-photon absorption. The three-photon absorption coefficients of the BSO and BGO crystals are equal, respectively, to (2.5±0.8) ×10^?20 and (4.4±1.3) ×10^?20 cm³W^?2.     

Image rotation and amplification based on the photorefractive higher-order grating

In this paper, we reported the properties of the higher-order diffraction images of a photorefractive grating recorded with a large modulation depth. It was shown that the image corresponding to the second order diffraction is magnified and that corresponding to the negative first order is 180° rotated.     

Competition between subharmonic and signal beams for photorefractive gain in BSO with two pump beams

It is shown that two pump beams incident upon a bismuth silicon oxide crystal give rise to a subharmonic beam bisecting the pump beam. Simultaneously a signal beam injected between the pump beams may be strongly amplified at the expense of the pump beams. It is shown that the amplified signal beam grows with a faster time constant than the subharmonic beam and that there is competition for gain in the steady state regime.     

Anisotropic Bragg diffraction of finite-sized volume holographic grating in photorefractive crystals

Anisotropic diffraction of uniform plane wave by finite-sized volume holographic grating in photorefractive crystals is considered. It is found that the anisotropic diffraction can take place when some special conditions are satisfied. The diffracted image is obtained in experiment for the anisotropic Bragg diffraction in Fe:LiNbO₃ crystals. A coupled wave analysis is presented to study the properties of anisotropic diffraction. An analytical integral solution for the amplitudes of the diffracted beams is submitted. A trade off between high diffraction efficiency and the deterioration of reconstruction fidelity is analyzed. Numerical evaluations also show that the finite-sized anisotropic volume grating exhibits strong angular and wavelength selectivity. All the results are useful for the optimizing design of VHOE based on finite-sized volume grating structures.     

Field structure in a photorefractive holographic grating in a planar waveguide

A theoretical investigation is made of the field structure in a holographic grating formed by TE₀ modes in a planar photorefractive waveguide by the photogalvanic recording mechanism. Waveguides with three characteristic refractive index profiles are considered, namely, a step function, a parabola, and a hyperbolic secant squared. A comparison is made between the light field distributions of TE₀ modes in waveguides having the different profiles. The field structure of the space charge in planar holographic gratings is analyzed for their initial formation phase. It is shown that the space charge field has a similar structure for all three waveguide types considered when planar photorefractive gratings are recorded by TE₀ modes.Institute of Automated Control Systems and Radio Electronics, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 85–92, November, 1993.     

Two-wave mixing bistability based on mixed grating formation in photorefractive crystals

1 L) and the relative strength (x) of PCG compared with PRG in two cases: (i) when g₁L>0 and x>1 so that there exists either pure PRG or PRG dominates over PCG, and (ii) when g₁L<0 and x<-1 so that there exists either pure PCG or PCG dominates over PRG. The threshold condition g₁L>(g₁L)_Th=2x(1+1/R) with R>1 and the effective coupling strength, g₂L=g₁L/x>0 with 1<x<-1, need to be satisfied for the bistability. Possible regions of bistability affect the position of the region for the intensity ratio (m) of the incident beams where two branched multiple solutions exist and hysteresis in the gain is expected. Gain solutions are found to be single valued without bistability when g₂L<0. A comparison between the mixed grating gain and pure PRG/PCG gains with the occurance of bistability is presented and interpreted physically. Received: 9 January 1996/Revised version: 17 September 1996     

Charge grating kinetics in photorefractive crystals subjected to an electric field

The kinetics of charge gratings induced by a standing light wave in a photorefractive crystal in the presence of an electric field is described correctly. It obeys an ordinary differential equation of the fourth order that has two pairs of complex conjugate solutions. A general solution to this equation that satisfies a given initial condition is derived. The field-related mechanism causing charge oscillations and extending the time the gratings take to pass into the steady state is revealed. A relation between these oscillations and space-charge waves is found.     

Discrepancy between the times of space charge grating formation and relaxation in photorefractive crystals

A discrepancy between the times of formation and relaxation of space charge gratings is found. This effect is observed when the uniform and nonuniform parts of the optical excitation of the medium appear and disappear simultaneously. It is shown that the formation time of these gratings is always shorter than the relaxation time and the higher the light intensity, the greater the discrepancy.     

Theoretical study on lifetime of thermal-fixed volume holographic phase grating in photorefractive crystals

<正>We review the dark decay of the electronic holographic phase grating before thermal fixing,and deduce the general analytic expression of the lifetime of thermal-fixed ionic holograms in the photorefractive crystal,by means of analogizing.Because the ions are optically inactive,the lifetime of thermal-fixed ionic holograms is only closely relate to the ionic decay rate which is determined by the conductivity of ionic species at a given temperature.We theoretically analyze and numerically simulate the influences on the lifetime of ionic grating from the crucial factors in the experiment and application.The results reveal that low temperature,low ion-concentration,and large grating spacing are advantages for extending the life of the thermal-fixed volume holographic phase grating in photorefractive crystal.     

Interaction of light waves on a reflecting holographic grating in cubic photorefractive crystals

The interaction of two antiparallel light waves on a reflecting grating is analyzed theoretically in arbitrarily oriented optically active cubic crystals of point group 23. The effect of nonunidirectional energy transfer on the interaction efficiency is investigated in the undepleted-pump-power approximation in (100)-, (111)-, and $(11\bar 2)$ -cut Bi₁₂TiO₂₀ crystals.     

Two-wave interaction on a photorefractive grating in cubic gyrotropic crystals in the case of strong coupling

The effect of nonunidirectional energy exchange on the intensity and polarization state of the signal wave is analyzed in the case of symmetric two-wave coupling on a transmission photorefractive grating produced by a diffusion mechanism in a gyrotropic cubic crystal belonging to the point group 23.     

光折变晶体中的体全息光栅光衍射的耦合波理论

本文考虑了晶体的各向异性特点,导出了光折变晶体中的体全息光栅光衍射的耦合波方程,给出了方程的解.用本文的公式还可以解释光线非对称入射时的实验结果.