Phase Conjugation In Photorefractive Crystals Research Articles

Crystal–air interface enhanced self-pumped phase conjugation in photorefractive crystals

In Ce:BaTiO 3 photorefractive crystal, self-pumped phase conjugation (SPPC) reflectivity is enhanced by 20–400% and the SPPC build-up time reduced by 50% if the incident laser beam, in a horizontal plane, is moved laterally from halfway between the upper and lower crystal surfaces towards the former. Similar results have been observed using a Cu:KNSBN photorefractive crystal. These phenomena can be explained by the increase of four-wave mixing regions, near the upper surface, by total internal reflections of the fanned beams at the crystal–air interface.

Temporal behavior and instabilities of the self-pumped phase conjugation in photorefractive crystals

The temporal behavior of the self-pumped phase conjugation in a photorefractive crystal with a variety of forms of the seed is carefully studied using a two-dimensional model which we proposed. For seeds of the small retroreflection and reflection, the model predicts that the self-pumped phase conjugator shows stable output, and the steady-state phase conjugate reflectivity versus the incident angle are in agreement with the experimental results. For the seed of the diffuse scattering, the conjugator shows either stable or unstable output, which is dependent on the incident angle. The frequency of the self-pulsation of the reflectivity increases linearly (or sublinearly) with an increase of the total light intensity. The model also predicts that there is no frequency shift between the input beam and the output phase conjugate beam for stable output. For unstable output there exists a frequency shift of \ifmmode\pm\else\textpm\fi{} a few Hz.

Self-pumped phase conjugation in photorefractive crystals: Reflectivity and spatial fidelity

A two-dimensional model for the self-pumped phase conjugation in photorefractive crystals is presented. Numerical simulations of the beam path for beam fanning and the self-pumped phase conjugation within the crystals show good agreement with the experimental observations. We numerically study the dependence of the self-pumped phase conjugation reflectivity and spatial fidelity on the incident angle, the position of the incident beam upon the input face, and the input wavelength, which are in good agreement with the published and our experimental results.

Spatial fidelity of externally pumped phase conjugation in photorefractive crystals

The spatial fidelity and the reflectivity of the externally pumped phase conjugation by four-wave mixing in photorefractive crystals are numerically studied by a three-dimensional analysis. The results are given for both the case that the input one-dimensional rectangular amplitude of the image-bearing beam has a finite extent in the plane of incidence of the four extraordinarily polarized beams and the case that the amplitude has a finite extent in the orthogonal plane. The fidelity and the reflectivity versus the input pump ratio, the moving velocity of the crystal, and the externally applied electric field are analyzed. Although one can enhance the reflectivity by moving the crystal or by applying an electric field, the output phase deviates increasingly from the conjugate phase.

Mechanism of self-pumped phase conjugation in photorefractive crystals

A two-dimensional model for the self-pumped phase conjugation in a photorefractive crystal is presented. In the model, no internal reflections at a corner of the crystal are explicitly considered, aside from some low intensity scattered radiation at the crystal surfaces. The numerical simulations of the curved beam path inside the crystal show agreement with experimental observations. The numerical results of high-reflectivity and high-fidelity phase conjugation are consistent with experimental results.

Numerical studies of externally pumped phase conjugation in photorefractive crystals with applied electric fields

In the configuration of four-wave mixing, the output intensity and output phase of the conjugate beam in a photorefractive BaTiO 3:Ce crystal are numerically studied. The fidelity of the conjugate beam is analyzed. When an external electric field is applied to the crystal, the intensity of the conjugate beam can be enhanced, but its phase is no longer exactly conjugate to the input phase. The phase conjugate fidelity is, therefore, reduced.

A continuous compound four-wave mixing mechanism for self-pumped phase conjugation in BaTiO 3 crystals

A continuous compound four-wave mixing mechanism for self-pumped phase conjugation in photorefractive crystals is presented. The coupled-beam equations of this mechanism are solved numerically when depletion of the forward going beam is neglected. The experiment in a Ce-doped BaTiO3 crystal is performed, with the stable phase conjugate reflectivity of as much as ≥ 80%. The curved high output phase conjugate intensity are explained using the numerical results.

STIMULATED DIFFUSION BACKSCATTERING WITH PHASE CONJUGATION

The advantages and conditions to observe the phase conjugation in photorefractive crystals by degenerate stimulated backscattering via diffusion gratings are discussed. Different version of discrimination physics for self conjugation in selfstarting backscattering are demonstrated. Several possibilities to obtain self- and double-conjugation by means of four-wave mixing on reflecting gratings in photorefractive crystals as well as some related problems are pointed out.

Polarization restoration by phase conjugation in photorefractive crystals

Theoretical and experimental investigations are reported of polarization phase conjugation in a photorefractive LiNbO3 crystal. It is shown that the photogalvanic mechanism of hologram formation by orthogonally polarized beams can ensure complete restoration of the polarization as a result of a degenerate four-wave interaction.

Strange attractor in the reflectivity of a phase-conjugate mirror.

Wave equations describing optical phase conjugation in photorefractive crystals are solved using a technique that transforms a boundary-value problem into an initial-value problem. Boundary conditions are satisfied using an iterative map constructed in the parameter space. Within the iterative procedure unstable situations arise, leading to a chaotic response of the crystal. A strange attractor is discovered in the reflectivity of the crystal, existing in the multiparameter space of the single-interaction-region four-wave-mixing process.

Multi-grating phase conjugation in photorefractive media

The phase conjugation in photorefractive crystals in the case of many operating gratings is studied. The role of the pump beam coupling in this effect is emphasized.