Phase-conjugate Beam Research Articles

Viewing-angle enhancement of speckle-reduced volume holographic three-dimensional display by use of integral imaging.

In a conventional integral imaging system the viewing angle is limited by the f-number of the microlens. To overcome this limitation we employ a phase-conjugate beam to read out elemental images, which are stored in photorefractive volume holographic storage, while the rotating diffuser reduces the speckle noise. In the proposed system the viewing angle can be enhanced over the f-number limitation. Experimental results and discussions of viewing parameters are presented.

Polarization-Dependent Light-Gated Phase Conjugation in Methyl-Orange Doped Polymer Film

We have investigated the polarization state of a gated phase conjugate beam using a methyl-orange doped poly-vinyl alcohol (MO/PVA) film in 16 different combinations of linear and circular polarization states of a probe beam and pump beams. The experimental results agree with the theoretical prediction based on the Jones matrix method. Of the third-order nonlinear susceptibility tensors χ ijkl of the MO/PVA film, the χ 1221 at the 647 nm wavelength in degenerate four-wave mixing under exposure of a gating 515 nm light takes a very small value compared with χ 1111 and χ 1122 . On the other hand, in the configuration of a probe beam and pump beams having linearly crossed polarizations and without gating light, the MO/PVA film generates a strong linearly polarized PC beam, so that χ 1221 takes a large value at 515 nm.

Spatio-temporal nature of the optical return field generated by four-wave mixing in photorefractive and Kerr materials

We have theoretically analyzed the return field produced by an externally pumped Kerr medium, an externally pumped photorefractive medium, and a self-pumped photorefractive medium, for an input field with arbitrary spatial and temporal characteristics. For a stationary beam with spatial structure, all three cases yield a spatial phase-conjugate return beam. For a beam with dynamic fluctuations, each case returns a field with different temporal character. For the case of a self-pumped photorefractor, it is found that the dynamics of the return beam are represented by a conventional reflection rather than a temporal phase conjugate. While this geometry may have been thought of as a simple way to provide phase-conjugate feedback for dynamic stabilization in semiconductor lasers, an in-depth analysis shows that our theoretical findings agree with previously reported experimental results.

OPTICAL INVERSE FILTER FOR HOLOGRAPHIC FILTERING BY PHOTOREFRACTIVE FOUR-WAVE MIXING WITH INCOHERENT ERASURE

We analyzed the phase conjugate reflectivity by photorefractive four-wave mixing exposed to incoherent erasure, and found that the incoherent erasure brings inversion relation in amplitude of the phase conjugate beam with the intensity of the erasure even in a large coupling photorefractive material. Since such inversion relation appears in real-time, this function contributes to put forward holographic filtering to real-time processing. For the sake of confirmation of the analysis, we performed an experiment with employing a 45^{\circ} -cut \hbox{BaTiO}_3 crystal which is known as a crystal providing one of the largest photorefractive beam coupling.

Specular reflection enhancement and reduction from the surface of a photorefractive crystal in the presence of a “pump” beam

We demonstrate the enhancement and reduction of the specular reflection from the surface of a photorefractive crystal in the presence of a “pump” beam. The pump beam could be the phase conjugate beam of the input beam or another laser beam. The contribution of the reflection grating is also demonstrated in a 45°-cut crystal. The change of the specular reflection with respect to the intensity ratio of the input beam to the pump beam is measured.

Secure optical memory system with polarization encryption

A novel secure holographic memory system with polarization encoding is proposed. Two-dimensional original data are encoded as a two-dimensional polarization distribution. The polarization state at each pixel is scrambled by a mask that changes the polarization state into a random state. The mask can rotate the direction of the principal axes of the elliptically polarized light and can change the phase retardation at each pixel. The light with the random polarization state is stored as a hologram that can produce the vector phase-conjugate beam. In the decryption the vector phase-conjugation readout can recover the original polarization state by use of the same mask used in the encryption. Experimental results of encryption and decryption with a bacteriorhodopsin film are presented.

Enhanced performance of a mutually pumped phase-conjugation setup using two photorefractive crystals

A mutually pumped phase-conjugator (MPPC) consisting of two photorefractive crystals is investigated experimentally at a wavelength of about 807 nm. In addition to a Rh:BaTiO3 crystal, which is used in a modified bridge configuration for phase-conjugation, a Co:BaTiO3 crystal is used to amplify the phase-conjugate beam by wave-mixing. The phase-conjugate power is at least doubled compared to that using only one crystal, and the beam quality of the phase-conjugate beam is distinctly improved. Moreover, the temporal stability of the phase-conjugate output using two crystals is much higher than that using only one crystal.

Effect of partial coherence on four-wave mixing in photorefractive materials via reflection grating approximation

We investigate the effect of beam coherence on four-wave mixing via reflection gratings in photorefractive media. For the case of phase conjugation, the results of our theoretical analysis indicate that partial coherence always leads to a drop of signal gain and phase conjugate reflectivity in non-depleted cases. In general, the mutual coherence of the signal beam and the pump beam can be enhanced due to the process of wave mixing. The mutual coherence of the phase conjugate beam and one of the pump beams depends on the beam intensity ratio as well as the optical path difference. This is distinctly different from the four-wave mixing case with a transmission grating.

Secure optical storage that uses fully phase encryption

A secure holographic memory system that uses fully phase encryption is presented. Two-dimensional arrays of data are phase encoded. Each array is then transformed into a stationary white-noise-like pattern by use of a random-phase mask located at the input plane and another at the Fourier plane. This encrypted information is then stored holographically in a photorefractive LiNbO(3):Fe crystal. The original phase-encoded data can be recovered, by use of the two random-phase masks, with a phase-conjugate readout beam. This phase information can then be converted back to intensity information with an interferometer. Recording multiple images by use of angular multiplexing is demonstrated. The influence of a limited system bandwidth on the quality of reconstructed data is evaluated numerically. These computer simulation results show that a fully phase-based encryption system generally performs better than an amplitude-based encryption system when the system bandwidth is limited by a moderate amount.

Edge-enhanced correlation with four-wave mixing in bismuth silicon oxide crystal by use of moving gratings

The nonlinearity of phase-conjugate beam reflection with four-wave mixing in bismuth silicon oxide crystal by use of moving gratings at large fringe modulation formed by the incident-beam ratio is investigated. On the basis of this investigation, the edge enhancement of an object and the edge-enhanced optical correlation with improved discrimination capability are achieved by use of moving gratings at an appropriate fringe velocity. Experimental results are presented.

Improvement of the focusing quality by parametric phase conjugation of ultrasonic beams in a ferrite cylinder with grooved working surface

Results of direct measurements of the acoustic pressure distribution in a phase-conjugate ultrasonic beam in the focus of a converging lens are presented for two types of parametric phase-conjugating elements, namely, with flat and grooved working surfaces. It is demonstrated that grooving noticeably improves the quality of focusing of an ultrasonic beam generated in water by a solid phase-conjugating element.

Bistability control of phase conjugate reflectivity by erase beam in photorefractive four‐wave mixing

When the nonlinear effect of the crystal is strong in four-wave mixing using a photorefractive crystal, the generated phase conjugate light takes multiple values. In this paper, a new method of controlling the bistable values of the phase conjugate reflectivity, that is, the ratio of the phase conjugate beam to the probe beam, via the erase beam is proposed for use in four-wave mixing using the fanning beam generated in the crystal in place of the forward pump beam. It is extremely difficult to solve analytically the problem of four-wave mixing including the fanning beam. By means of a model in which a part of the probe beam is used for fanning and acts as the forward pump beam in the interaction region, the previous method of analysis for four-wave mixing can be used. First, a steady-state analysis of the phase conjugate reflectivity is carried out. It is shown that the phase conjugate reflectivity has bistable values as function of the intensity of the erase beam. Next, by time response analysis, the intensity of the erase beam is varied and the bistable values of the phase conjugate reflectivity are controlled. Finally, the theory is compared with experiments. © 2000 Scripta Technica, Electron Comm Jpn Pt 2, 83(6): 8–14, 2000

Multiplexed phase-conjugate holographic data storage with a buffer hologram

We describe and demonstrate a volume holographic storage system in which a phase-conjugate object beam is reconstructed by the same reference beam that was used for recording. An intermediate hologram is used as a temporary buffer, recorded with its own reference beam and the data-bearing object beam. Reading this buffer hologram with the phase conjugate of its reference beam reconstructs the phase conjugate of the object beam, which can then be recorded into the desired volume hologram for long-term storage. This method combines the immunity to lens aberrations provided by phase-conjugate readout with the simplicity of using the same multiplexed reference beam for both recording and readout. Only a single pair of phase-conjugate reference beams is required. Experimental results are shown with a single LiNbO(3):Fe crystal used as both buffer and storage holograms and a self-pumped phase-conjugate mirror in BaTiO(3) that provides the pair of phase-conjugate reference beams.

Electric-field-induced frequency shift and temporal instability in a self-pumped phase conjugator using BaTiO 3

By externally applying an electric field across a photorefractive BaTiO 3 crystal we experimentally demonstrate that the self-pumped phase conjugator in the `cat' configuration gives a frequency shift between the output phase conjugate beam and the input beam. For large electric fields the conjugator shows temporal instability in its output. The experimental results are in good agreement with the theoretical results.

Generation of Forward Phase-Conjugate Beam by Cu:KNSBN-Cat Type Self-Pumped Phase-Conjugator

A forward phase-conjugate beam was generated using a copper-doped (K0.5Na0.5)0.2(Sr0.61Ba0.39)0.9Nb2O6 (Cu:KNSBN) crystal with Cat type self-pumped phase-conjugator incident geometry. This Cat self-pumped phase-conjugator was formed simply with a small incident beam diameter of 0.4 mm at an incidence angle of 30°. In this configuration, the forward phase-conjugate beam could be generated at the same time with the self-pumped phase-conjugate beam. Furthermore, it was possible to generate the forward phase-conjugate beam while maintaining the high reflectivity of the self-pumped phase-conjugate beam. A forward phase-conjugate reflectivity of 0.9% was achieved at a self-pumped phase-conjugate reflectivity of 12%.

Four-wave mixing with partially coherent waves in photorefractive mediaITransmission grating approximation

We investigate the effect of beam temporal coherence on four-wave mixing via transmission gratings in photorefractive media. We take into account the propagation of mutual coherence and self-coherence in space and the evolution of mutual coherence and self-coherence with time. Analytical solutions for nondepleted-pump cases and numerical results for general cases are obtained. We find that the mutual coherence of the signal beam and the pump beam can be enhanced or reduced, depending on the coupling constant and the beam ratio of the signal beam and the pump beam. A decrease of the initial mutual coherence can lead to a lower phase-conjugate reflectivity in the nondepleted-pump case. Even with low initial mutual coherence, high phase-conjugate reflectivity can still be obtained in the case with pump depletion. As a result of the shift of the grating profile, partial coherence can lead to an enhancement of the phase-conjugate reflectivity when the signal–pump-beam ratio is small. We also found that when the initial value of the phase-conjugate beam is zero, the phase-conjugate beam and the pump beam are in phase during propagation. Results for beam intensity and mutual coherence are presented and discussed to provide an insight into four-wave mixing with partially temporally coherent waves.

Spatial shifts of the conjugate beam generated by a nondegenerate photorefractive phase-conjugate mirror

We demonstrate that the phase conjugation of a beam during nondegenerate four-wave mixing is accompanied by a spatial shift relative to the degenerate conjugate-beam location. Experiments with a photorefractive phase-conjugate mirror reveal that the phase-conjugate beam shifts have a nonmonotonic dependence on the probe's detuning frequency and comprise both lateral displacements of up to 218 mum and angular tilts of up to 34 arc sec. An approximate theory based on spatial dispersion coefficients is in partial agreement with the experimental results.

Observation of the Goos-Hänchen Effect in a Phase-Conjugate Mirror

The Goos-H\anchen shift between a supercritically incident beam and its reflection in a total-internal-reflection mirror comprising a material interface is well established. We experimentally demonstrate that the Goos-H\anchen effect can also occur between the probe beam and the phase-conjugate beam in a phase-conjugate mirror. Controlled lateral displacements of up to $218\ensuremath{\mu}\mathrm{m}$ were measured using photorefractive four-wave mixing with a frequency-detuned Gaussian probe beam.

Influence of multigrating operation on the generation of phase-conjugate beams by four-wave mixing

A numerical simulation for studying the kinetics of phase-conjugate beams generated by multigrating four-wave mixing has been developed. It has been applied to investigate the four-wave mixing process in BaTiO3 by use of realistic coupling coefficients. The role of the distinct gratings has been analyzed. Depending on the experimental conditions, constructive as well as destructive interference of the different contributions was obtained. Moreover, because of grating competition effects, π-phase changes in the phase-conjugate beam can occur during the kinetics. For large values of the coupling strength and specific regions of the physical parameters, unstable four-wave mixing dynamics were obtained. The analysis of the results provides some criteria for predicting the phase conjugator’s behavior in multigrating conditions.

Cross-polarized photorefractive four-wave mixing with extraordinary writing beams and an ordinary reading beam

In parallel-polarized photorefractive four-wave mixing the phase-matching condition can be achieved automatically if the pump beams are allowed to propagate in directions opposite to each other. However, in cross-polarized photorefractive four-wave mixing, the incident angles of the beams should be adjusted to maintain the phase-matching condition. We analyze the phase-matching properties of cross-polarized four-wave mixing with extraordinary writing beams and an ordinary reading beam and obtain the phase-conjugate reflectivity in consideration of phase mismatching. We also perform an experiment in which a phase-conjugate beam is generated in cross-polarized four-wave mixing and confirm the analyses.